HomeMy WebLinkAboutSUP 06-10X2A; AGUA HEDIONDA OUTER LAGOON MAINTENANCE; STUDY OF SEDIMENT TRANSPORT CONDITIONS IN THE VICINITY OF AGUA HEDIONDA LAGOON; 1999-04-15STUDY OF SEDIMENT TRANSPORT CONDITIONS IN THE
VICINITY OF AGUA HEDIONDA LAGOON
Prepared for:
California Coastal Commission
San Diego Gas & Electric
City of Carlsbad
Submitted by:
COAST AL ENVIRONMENTS
2166 Avenida de la Playa, Suite E
La Jolla, CA 92037
January 8, 1999
Revised on April 15, 1999
CE REFERENCE No. 98-11
Study of Sediment Transport Conditions in the
Vicinity of Agua Hedionda Lagoon
Volume I: Technical Report
by
M. Hany S. Elwany
Anne-Lise Lindquist
R. E. Flick
W.C. O'Reilly
J. Reitzel
W.A. Boyd
for
California Coastal Commission
San Diego Gas & Electric
City of Carlsbad
COAST AL ENVIRONMENTS
2166 A venida de la Playa, Suite E
La Jolla, CA 92037
January 8, 1999
Revised on April 15, 1999
CE REFERENCE No. 98-11
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
TABLE OF CONTENTS
TABLE OF CONTENTS ................................................................................................................. i
LIST OF FIGURES ....................................................................................................................... iii
LIST OF TABLES ........................................................................................................................... v
EXECUTIVE SUMMARY ........................................................................................................... vi
1. INTRODUCTION ................................................................................................................. 1-1
2. AGUA HEDIONDA LAGOON ............................................................................................ 2-1
2.1. AREA HISTORY AND GEOLOGY ........................................................................... 2-1
2.2. DESCRIPTION OF AGUA HEDIONDA LAGOON .................................................. 2-1
2.3. OCEANOGRAPHIC CONDITIONS ........................................................................... 2-2
2.3.1. Tides ...................................................................................................................... 2-2
2.3.2. Waves ..................................................................................................................... 2-4
2.4. AGUA HEDIONDA LAGOON DYNAMICS AND SEDIMENTATION ................. 2-8
2. 4.1. Tidal Prism ............................................................................................................ 2-8
2.4.2. Sedimentation ........................................................................................................ 2-8
3. REVIEW OF SEDIMENT TRANSPORT IN THE OCEANSIDE LITTORAL CELL ...... 3-1
3.1. OCEANSIDE LITTORAL CELL ................................................................................ 3-1
3.2. SEDIMENT SOURCES AND SINKS ......................................................................... 3-1
3.2.1. Rivers ..................................................................................................................... 3-1
3.2.2. Cliffs ...................................................................................................................... 3-4
3.2.3. Dredging and By-passing from Oceanside Harbor to Moonlight Beach .............. 3-4
3.2.4. Sedimentation in Carlsbad Submarine Canyon .................................................... 3-4
3.3. LONGSHORE TRANSPORT ALONG THE OCEANSIDE LITTORAL CELL ....... 3-6
3.3.1. Estimates of Longshore Transport Rates from Previous Studies .......................... 3-6
3.3.2. Estimates of Longshore Sediment Transport Rates from Oceanside Wave Data. 3-6
3.4. LONGSHORE SEDIMENT TRANSPORT AT CARLSBAD .................................. 3-13
3.5. CROSS-SHORE SAND TRANSPORT ..................................................................... 3-24
4. SHORELINE AND PROFILE CHANGES FROM OCEANSIDE TO ENCINITAS ......... 4-1
4.1. COASTAL SETTING .................................................................................................. 4-1
4.2. SHORELINE AND BEACH-PROFILE DATA .......................................................... 4-1
4.3. SHORELINE CHANGES ............................................................................................ 4-2
4.3.1. Historical Shoreline Changes (1887-1982) ........................................................ 4-2
4.3.2. Recent Beach Width Changes from Beach-Profile Surveys .................................. 4-4
4.4. SEASONAL BEACH-PROFILE CHANGES ............................................................ 4-13
5. EFFECT OF THE ENCINA POWER PLANT AND AGUA HEDIONDA LAGOON ON
NATURAL SAND TRANSPORT ......................................................................... 5-1
5.1. SEDIMENT TRAPPED BY TIDAL CURRENTS ...................................................... 5-1
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
5.2. SEDIMENT TRAPPING AND DIVERSION BY INLET AND DISCHARGE
JETTIES .......................................................................................................................................................................................................................... 5-2
5.3. SEDIMENT TRAPPING AND DIVERSION BY THE THERMAL DISCHARGE
PLUME ......................................................................................................................... 5-3
6. SAND-PLACEMENT OPTIONS ......................................................................................... 6-1
6.1. SAND MOVEMENT .................................................................................................... 6-1
6.2. PREDICTIONS FOR SAND-DISPOSAL BERA VIOR .............................................. 6-2
6.3. BEACH-PROFILE RESPONSE TO SAND DISPOSAL (DATA) ............................. 6-4
6.3.1. Station CB-0820 (Middle Beach) .......................................................................... 6-4
6.3.2. Station CB-0840 (North Beach) ............................................................................ 6-7
6.3.3. Station OS-1000 located at Oceanside (April-October 1986) ............................ 6-12
6.4. CONCLUSIONS FROM SAND-DISPOSAL PROJECTS IN THE AREA .............. 6-16
6.5. THE EFFECT OF THE ENCINA POWER PLANT OPERATION ON BEACHES 6-17
6.6. EVALUATION OF SAND PLACEMENT OPTIONS .............................................. 6-18
6.6.1. Replenish Sand that the Power Plant is Responsible for Removing (Option 1) .. 6-18
6.6.2. Minimizing the Need to Redredge the Lagoon (Option 2) .................................. 6-18
6.6.3. Maximize Public Recreational Benefits (Option 3) ............................................. 6-19
6. 6. 4. Achieve Most Mitigating Effect to Regional Beach Erosion (Option 4) ............. 6-19
6.7. STABLE DISPOSAL SITES ...................................................................................... 6-20
7. COST-BENEFIT CONSIDERATIONS ................................................................................ 7-1
7.1. INTRODUCTION ........................................................................................................ 7-1
7.2. BENEFIT CONSIDERATIONS .................................................................................. 7-3
7.3. COST CONSIDERATIONS ......................................................................................... 7-4
7.4. COST-BENEFIT EVALUATION ................................................................................ 7-4
8. SUMMARY AND CONCLUSIONS .................................................................................... 8-1
8.1. TECHNICAL TASKS .................................................................................................. 8-1
8.2. SEDIMENT TRANSPORT IN THE VICINITY OF AGUA HEDIONDA LAGOONS-I
8.3. SHORELINE CHANGE RATES ................................................................................. 8-1
8.4. EFFECTS OF THE ENCINA POWER PLANT ON SEDIMENT TRANSPORT ...... 8-2
8.5. SAND DISPOSAL BERA VIOR .................................................................................. 8-2
8.6. SAND PLACEMENT OPTIONS ................................................................................. 8-3
8.7. STABLE DISPOSAL SITES ........................................................................................ 8-4
9. RECOMMENDATION ......................................................................................................... 9-1
10. REFERENCES .................................................................................................................... 10-1
APPENDIX A. SUBTIDAL TOPOGRAPHICAL SURVEYS ...................................................... !
APPENDIX B. WA VE EXPERIMENT ......................................................................................... I
APPENDIX C. MONITORING THE 1997-1998 SAND DISPOSAL ......................................... 1
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
LIST OF FIGURES
Figure 1-1. Location map of the study area from Oceanside Harbor to Moonlight
State Beach, Encinitas ............................................................................................. 1-3
Figure 1-2. Aerial photograph of Agua Hedionda Lagoon ........................................................ 1-4
Figure 2-1. Configuration of the intake and discharge channels of Agua Hedionda Lagoon ... 2-3
Figure 2-2. Wave exposure for Carlsbad illustrating island shadowing effects ........................ 2-5
Figure 2-3. Joint distribution of significant wave height and peak period at Oceanside ........... 2-7
Figure 2-4. Plant inflow rate time history for July 27, 1993 to July 27, 1994 ........................... 2-9
Figure 2-5. Monthly sedimentation rates, northern half of the Outer Basin, March 1955 to
May 1957 .............................................................................................................. 2-12
Figure 3-1. Location map of the three major littoral cells in the San Diego Region ................. 3-2
Figure 3-2. Monthly mean values of longshore transport at Oceanside from available wave
data from December 1978 to October 1994 ............................................................ 3-9
Figure 3-3. Mean longshore transport for winter, summer, and combined winter and summer
data from 1978 to 1994 at Oceanside ................................................................... 3-10
Figure 3-4. Percentage oflongshore transport during the winter, summer, and combined
winter and summer data at Oceanside .................................................................. 3-11
Figure 3-5. Cumulative probability oflongshore transport (Q~) during the winter, summer,
and combined winter and summer data at Oceanside ........................................... 3-12
Figure 3-6. Monthly means and standard deviation of the longshore transport rates for each
month .................................................................................................................... 3-14
Figure 3-7. Location of the PUV wave gauges deployed in Oceanside and Carlsbad ............ 3-16
Figure 3-8. Adjusted Oceanside wave height data relative to Carlsbad and measured
Oceanside data ...................................................................................................... 3-18
Figure 3-9. Radiation stress (Sxy) at Carlsbad compared to Oceanside ................................... 3-19
Figure 3-10. Comparison of monthly daily mean values of longshore transport potential
between Carlsbad and Oceanside from 1978 through 1994 ................................ 3-20
Figure 3-11. Cumulative probability oflongshore transport (Q~) during the winter, summer,
and combined winter and summer data at Carlsbad ............................................. 3-21
Figure 4-1. History of Oceanside Harbor construction and improvements ............................... 4-3
Figure 4-2. Shoreline positions before and after Oceanside Harbor was constructed ............... 4-5
Figure 4-3. Shoreline positions at Agua Hedionda Lagoon for the years 1887/88, 1934,
1972, and 1982 ........................................................................................................ 4-6
Figure 4-4. Enlargement of shoreline positions directly adjacent to Agua Hedionda Lagoon .. 4-7
Figure 4-5. Location of beach-profile ranges along the Oceanside Littoral Cell ...................... 4-9
Figure 4-6. Beach width versus time for profile ranges at North, Middle, and South
Beaches in Carlsbad .............................................................................................. 4-10
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Figure 4-7. Beach width versus time for Oceanside and South Carlsbad profile ranges ........ 4-11
Figure 4-8. Comparison of beach profiles from Oceanside to Del Mar during summer and
winter ................................................................................................................................................................. , .......... 4-14
Figure 4-9. Typical beach profiles showing the seasonal cycles for Oceanside, Encinitas,
and Del Mar .......................................................................................................... 4-15
Figure 4-10. Typical beach profiles showing the seasonal cycles at CB-0850, CB-0830),
CB-0820, and CB-0800 ........................................................................................ 4-16
Figure 6-1. The spread of a rectangular-shaped beach fill in the longshore direction .............. 6-5
Figure 6-2. Sand-disposal project at Middle Beach, April 1991 ............................................... 6-8
Figure 6-3. Sand-disposal project at Middle Beach, April 1993 ............................................... 6-9
Figure 6-4. Summer beach profiles at Middle Beach from 1990 to 1997 ............................... 6-10
Figure 6-5. Sand-disposal project at North Beach, April 1988 ............................................... 6-11
Figure 6-6. Sand-disposal project at North Beach, April 1992 ............................................... 6-13
Figure 6-7. Summer beach profiles from 1987 to 1996 at North Beach (CB-0840) ............... 6-14
Figure 6-8. Sand-disposal project at Oceanside, 1986 ............................................................ 6-15
Figure 6-9. Longshore variations of southward potential longshore-transport rates ............... 6-21
Figure 7-1. Annual attendance at various Oceanside Littoral Cell beaches .............................. 7-2
Figure A-1. Vessel survey tracks on March 12, 1998 ............................................................... A-2
Figure A-2. Bathymetry and substrate exposure near Agua Hedionda Lagoon ........................ A-3
Figure A-3. Sediment thickness and substrate exposure near Agua Hedionda Lagoon ............ A-5
Figure A-4. Location of profiles where sand probing was conducted ....................................... A-6
Figure A-5. Sand thickness along North Beach at CB-0850, CB-0835, and CB-0830 ............. A-7
Figure A-6. Sand thickness along Middle Beach at CB-0825, CB-0820, and CB-0810 ........... A-8
Figure A-7. Sand thickness along South Beach at CB-0805 ..................................................... A-9
Figure B-1. Comparison of wave parameters between Oceanside and Carlsbad ....................... B-4
Figure B-2. Comparison of Oceanside and Carlsbad waves at 14-second period ..................... B-7
Figure B-3. Comparison of Oceanside and Carlsbad waves at 8-second period ....................... B-8
Figure B-4. Ratio of Hs at Carlsbad and Oceanside vs mean wave direction at Oceanside ..... B-10
Figure B-5. Adjusted Oceanside data relative to Carlsbad and original Oceanside data ......... B-11
Figure B-6. Carlsbad Sxy versus Oceanside and adjusted Oceanside Sxy ................................. B-13
Figure B-7. Longshore transport at Carlsbad compared to Oceanside ..................................... B-14
Figure C-1. Survey method of 1998 beach-profile survey program .......................................... C-3
Figure C-2. Beach-profile surveys at CB-0825 .......................................................................... C-4
Figure C-3. Beach-profile surveys at CB-0835 .......................................................................... C-5
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 2-1.
Table 2-2.
Table 2-3.
Table 3-1.
Table 3-2.
Table 3-3.
Table 3-4.
Table 3-5.
Table 3-6.
Table 4-1.
Table 4-2.
Table 7-1.
LIST OF TABLES
Joint distribution of significant wave height and peak wave period for
Oceanside ................................................................................................................ 2-6
Hydraulic evaluation for various stages oflagoon dredging ................................ 2-10
Dredging history for Agua Hedionda Lagoon ...................................................... 2-13
Estimated river coarse sediment yield in the Oceanside Littoral Cell .................... 3-3
Oceanside Harbor dredging history ........................................................................ 3-5
Previous estimates of longshore transport .............................................................. 3-7
Characteristics oflongshore transport at Oceanside from 1978 to 1994 .............. 3-15
Characteristics of longshore transport at Agua Hedionda Lagoon, Carlsbad from
1978 to 1994 ......................................................................................................... 3-22
Comparison between the longshore sand transport at Oceanside and Carlsbad ... 3-23
Shoreline change rates from Oceanside to Encinitas ............................................ 4-12
Characteristics of beach profiles from Camp Pendleton to Del Mar .................... 4-17
Recent sand-disposal volume distribution .............................................................. 7-6
Table C-1. North, Middle, and South Beaches, Carlsbad, Beach-Profile Program .................. C-2
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Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
EXECUTIVE SUMMARY
The primary objective of this study is to analyze sediment transport in the vicinity of Agua
Hedionda Lagoon, located in Carlsbad, California. Results of the analysis are used to evaluate
disposal options for sediments dredged from the lagoon
The SDG&E Encina Power Plant is located adjacent to Agua Hedionda Lagoon. The five
power-generating units take about 635 to 670 million gallons of water per day (mgd) (85 to
89.6 x 106 ft3/day) from the lagoon to the power plant condenser systems for cooling purposes.
The heated water is discharged through a channel across the beach. The process reduces the
volume of water available to flush the lagoon inlet by about 40% for mean tidal ranges. The
reduced flushing results in sand accumulation within the Outer Basin of the lagoon. The average
rate of sand deposition inside the lagoon is 138,000 yd3 /yr.
SDG&E has been dredging portions of Agua Hedionda Lagoon since construction of the
plant in 1954. The dredging operations are usually conducted at two-to three-year intervals to
remove deposited sediments and maintain tidal circulation in the lagoon. Optimal placement of
the sediments dredged from the lagoon is required to maximize the benefits of sand placement,
and enhance recreational opportunities.
Specific technical tasks in this study include: 1) review sediment transport in the vicinity of
Agua Hedionda Lagoon; 2) estimate shoreline erosion rates from Oceanside Harbor to Moonlight
Beach, Encinitas; 3) evaluate the effect of the power plant and Agua Hedionda Lagoon on the
natural transport and deposition of sediment to the shoreline from Oceanside Harbor to
Moonlight Beach, Encinitas; 4) develop four sediment placement options; and 5) identify stable
disposal sites north and south of the lagoon, or if stability is equal in all areas, identify disposal
sites that will provide recreational benefit. The results of the technical tasks are used to develop
a compromise "optimal" disposal strategy for sediments dredged from Agua Hedionda Lagoon.
The data analyzed for this study were obtained from the existing, large data set collected by
the U.S. Army Corps of Engineers (USACE) and other government agencies, including cities.
Bathymetry, side-scan, and sub-bottom surveys and a wave experiment were conducted to fill
gaps in the existing data set. Numerical models were used to determine the effect of Carlsbad
Canyon on wave and sediment regimes, and to compute longshore transport rates at Carlsbad
using the historical wave data existing at Oceanside from 1978 to 1994.
The principal results of the study are that the operation of the power plant has a short-term
effect on local beaches because it alters the natural coastal processes in the vicinity of the power
plant. Analyzing local beach profiles indicates the zone of impact is between Buena Vista
Lagoon to the north, and Batiquitos Lagoon to the south. The local effect of the power plant is at
a maximum near the intake channel and decreases with distance.
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Carlsbad Submarine Canyon alters the local wave regime and decreases the northward
longshore transport at Carlsbad compared with Oceanside. Approximately 80% of the sand
deposited inside the lagoon is extracted from the southward sand transport and 20% from the
northward sand transport. This indicates that the operation of the power plant, which increases
the lagoon sedimentation rate, has a larger erosional effect on Middle and South Beaches than on
North Beach. However, power plant operation does not have a long-term effect on the local
beaches, since the lagoon is routinely dredged, and the dredged material is placed back into the
beach system once every two or three years. Also, there is no offshore loss of sand because of
the intake and discharge jetties, or by the flow of the discharge channel.
Carlsbad beaches are the most popular beaches in San Diego County according to SANDAG
surveys. The need for sand replenishment along Carlsbad beaches is clear. Transportation of
sand from Agua Hedionda Lagoon to beaches farther away is not recommended because of the
cost of transportation, including fuel, travel time, and impacts of labor costs, and effects on the
power plant operation schedule. In addition, North, Middle, and South Beaches are most
affected by power plant operation.
The evaluation of sand-placement options provides the following results: 1) to replenish sand
removed by power plant operation, about 80% of the dredged sand should be placed on Middle
and South Beach, and 20% on North Beach; 2) to minimize the need for re-dredging, the sand
should be placed as far from the intake channel as possible, but within the permitted area and
away from hard substrate. Therefore, for sand placement on North Beach, at least a 2,000-ft
buffer is recommended, and a 500-ft buffer on South Beach is recommended; 3) to maximize
public recreational benefits, the sand should be distributed on all three beaches. A survey of
parking facilities indicates that none of the three beaches can accommodate the number of people
the additional beach area can support; and 4) the average amount of sand dredged from Agua
Hedionda Lagoon (138,000 yd3/yr) cannot address the entire region's beach erosion problems.
However, the dredged sand can be placed to help maintain the three beaches most affected by the
power plant operation.
Natural geological features, beach access, facilities, and parking availability are factors
controlling the usage of North, Middle, and South Beaches. Visitors at the beaches adjacent to
the power plant generally use the stretch of beach north of the intake channel for surfing
(2,000 ft) and use the northern part of North Beach, as well as Middle Beach and South Beach,
for swimming. North Beach, north of Pine A venue, probably provides great recreational benefit
to swimmers and accompanying beach-goers. The residents of Carlsbad are more likely to use
North Beach since this area has more residential property, is near the shopping centers and
restaurants, and has more parking facilities than Middle and South Beaches.
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Maximum recreational benefits can be attained by maintaining all three beaches. Sand losses
from any one of the three beaches, if not replaced, would limit the benefit of the sand dredged
from Agua Hedionda Lagoon. Regular placement of sand on all three beaches, over the long
term, will help maintain these beaches and maximize local benefits. In addition, disposed sand
that moves offshore may then be carried by coastal currents and be redistributed to other regional
beaches.
Based on the results of this study, it is recommended that 30% of the sand dredged from
Agua Hedionda Lagoon be placed on North Beach near Pine A venue and 70% be placed on
Middle and South Beaches. This recommended distribution represents a reasonable compromise ·
between the competing needs for the sand, benefits and costs, and environmental constraints.
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
1. INTRODUCTION
This report presents the results of an analysis of sediment transport in the vicinity of Agua
Hedionda Lagoon. The California Coastal Commission (CCC), San Diego Gas and Electric
Company (SDG&E), and the City of Carlsbad requested this study to evaluate optimal disposal
strategies for sediments dredged from Agua Hedionda Lagoon.
Agua Hedionda Lagoon is located in the City of Carlsbad, San Diego County, within the
Oceanside Littoral Cell (Figure 1-1). The coastline within the City of Carlsbad extends from
Buena Vista Lagoon at the north boundary to Batiquitos Lagoon at the south boundary, a reach
of about 6 miles.
During flood tides, marine sediments suspended by waves are transported and deposited into
Agua Hedionda Lagoon by tidal currents. Most of the intercepted sand is deposited in the Outer
Basin of the lagoon. About 30% of the flood tidal prism is diverted through the power plant for
cooling purposes and returned to the sea via a separate sediment-free discharge channel. The
weaker ebb tidal currents are much less efficient at removing sand from the lagoon.
The lagoon traps about 138,000 cubic yards (yd3) of littoral sediment each year. However,
the regular maintenance dredging conducted by SDG&E returns most of this sediment to the
Oceanside Littoral Cell system. The placement location of the material and time of year for the
placement are key factors to maintaining the beaches in the area and offsetting the effects of the
power plant cooling system, and the intake and discharge channels on local and regional beaches.
SDG&E has been dredging portions of Agua Hedionda Lagoon since 1954. The dredging
operations are usually conducted at two-to three-year intervals to remove sand deposited in the
lagoon to maintain continuous tidal circulation in the lagoon. Continuous tidal circulation is
required to provide the SDG&E Encina Power Plant with adequate seawater supplies for cooling
purposes. Optimal placement of the sediments dredged from the lagoon is required to maximize
the benefits of sand placement and enhance recreational opportunities. According to SDG&E,
all sediment has been deemed suitable for beach disposal. Currently, the following three beaches
are approved to receive dredged sediments (Figure 1-2):
• North jetty to Oak Avenue (known as North Beach)
• Warm Waters Beach to intake (known as Middle Beach)
• Terra Mar to Warm Waters Beach (known as South Beach).
The Encina Power Plant maintenance-dredging program is expected to continue for the life
of the power plant. Therefore, the CCC has required an investigation of the sediment dynamics
of the beaches adjacent to the plant, which, to a lesser extent, would include the littoral processes
as far north as Oceanside, and as far south as Scripps Beach. The analysis requires an
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
understanding of the major forcing functions (primarily waves and tides) and other
oceanographic and coastal processes talcing place in the study area.
The purposes of this study are to:
1) Review sediment transport in the vicinity of Agua Hedionda Lagoon;
2) Estimate shoreline erosion rates for the coastline from Oceanside Harbor to Moonlight
Beach, Encinitas;
3) Evaluate the effects of the Encina Power Plant and Agua Hedionda Lagoon on the natural
transport and deposition of sediment to the shoreline from Oceanside Harbor to
Moonlight Beach, Encinitas;
4) Develop four sediment placement options which address four objectives:
a. Optimal placement to replenish sand which the power plant has removed from
the littoral transport;
b. Optimal placement to minimize the need to re-dredge the lagoon;
c. Optimal placement to maximize public recreational benefits for area beaches;
d. Optimal placement to achieve mitigation to regional beach erosion; and
5) Identify stable disposal sites north and south of the lagoon. If stability is equal in all
areas, identify sites that will provide recreational benefit.
The results of this study are presented in two volumes. Volume I is a technical report.
Volume II is a data report and contains the data collected and analyzed for this study, including
historical beach-profile data and Coastal Environments' beach monitoring of SDG&E's sand
disposal at Middle and South Beaches, including bathymetry, sub-bottom, and substrate survey
results, and the detailed results of three wave instrument deployments at Carlsbad and Oceanside
from 9 July 1998 to 22 September 1998.
In Chapter 1 of Volume I, the problem is defined and an outline of the required tasks is
provided. Chapter 2 discusses the study area and the major physical forces that affect southern
California beaches. Chapter 3 reviews longshore and cross-shore sediment transport in the
Oceanside Littoral Cell with emphasis on sediment transport at Carlsbad. Chapter 4 summarizes
historic shoreline and beach-profile changes in the study area. The effects of the Encina Power
Plant and Agua Hedionda Lagoon on the natural sand transport are evaluated in Chapter 5.
Chapter 6 discusses optimal sand placement locations for the sand dredged from Agua Hedionda
Lagoon. Cost-benefit considerations are discussed in Chapter 7. The summary and conclusions
of this study are given in Chapter 8. Chapter 9 provides recommendations for distributing the
dredged sand, followed by a reference list (Chapter 10) and three appendices. The appendices
give a summary of the results obtained from the fieldwork conducted during this study. All units
of measure presented in this report are in English units. However, when appropriate and for
comparison with other published data, some of the study results are presented in both, English
and SI (metric) units.
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Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Pacific
Ocean 200
300
330
Contours in Feet
San Luis Rey River
N
-t
-----0
0
2 3 4 5
kilometers
2 3 4
miles
Batiquitos
Lagoon
!~\---\ ',
Moonlight State Beach .
Figure 1-1. Location map of the study area from Oceanside Harbor to Moonlight State
Beach, Encinitas.
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Figure 1-2. Aerial photograph of Agua Hedionda Lagoon showing its three basins and the location of North, Middle, and South Beaches.
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
2. AGUA HEDIONDA LAGOON
2.1. AREA HISTORY AND GEOLOGY
The regional geological setting is fundamental to the understanding of the origin and present-
day functioning of Southern California coastal lagoons, including Agua Hedionda Lagoon. The
Pleistocene Age, representing the last two million years, has been marked by large sea level
fluctuations. During periods of glacial advance sea level dropped by up to 450 ft and during
inter-glacial periods it often stood higher than today. During high stands of sea level, wave
action cut prominent marine terraces that are represented now by the extensive uplands bordering
Agua Hedionda Lagoon (Bell and Scott, 1975).
Agua Hedionda Lagoon is the drowned mouth of a river-cut valley, typical of the numerous
lagoons existing along the southern California coast. When sea level was low, a deep valley
eroded into the sediments and formed Agua Hedionda Lagoon. As sea level began to rise after
the last ice age, 18,000 to 20,000 years ago, the advancing seawater filled the valley, forming a
deep, open embayment. Gradually, this embayment filled with sediments from the creek, and
slope-wash from the sides of the valley. Subsequently, wave deposition and longshore sand
transport formed a partial sand barrier across the bay mouth.
According to Ritter (1972) the lagoon was discovered in 1603 by Gaspar de Portola, a
Spanish explorer, who named it San Simeon Lypmaca. Later, the lagoon was renamed Agua
Hedionda (stinking water) because of its odor. During most of historical time, the lagoon has
probably been nearly or completely closed (Bradshaw, 1976).
In 1885, the railroad was constructed along the coast in this area. In 1909-10 the Pacific
Coast Highway was constructed along with a wooden bridge across the spit and the mouth of the
lagoon. The highway and bridge were widened and paved in 1915-16, with a 75-ft wide
concrete bridge replacing the old bridge. Bradshaw (1976) inspected historical maps prepared by
the Coast & Geodetic Survey and the United States Department of Agriculture and found that the
lagoon was closed to tidal flushing in 1887-89 and in 1915. He also reported that heavy rains in
January 1927 opened the lagoon mouth, which then remained open for 5 years. The entrance
was re-opened in 1948 by local residents and was still open in 1954 when SDG&E began
dredging for the construction of the power plant (Bradshaw, 1976).
2.2. DESCRIPTION OF AGUA HEDIONDA LAGOON
Agua Hedionda Lagoon is located within the City of Carlsbad, California. The lagoon is
bounded on the west by the Pacific Coast Highway (Carlsbad Boulevard), on the north by the
City of Carlsbad residential community, and on the east and south by undeveloped hill slopes
Coastal Environments
Reference Number 98-11
2-1 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Car1sbad, CA
and bluffs. Above the bluffs on the south side of the lagoon lie cultivated fields and the Encina
Power Plant operated by SDG&E.
In 1954, SDG&E dredged the entire lagoon to about -8 ft, NGVD and constructed a channel
with two jetties at the lagoon inlet. About 4 x 106 yd3 of sediment were dredged and placed
along the nearby beaches. The water in the lagoon is used for cooling the power plant. The flow
through the intake channels to the plant is about 330 ft3 /sec. The warm-water discharge channel
is protected by two jetties.
· The Santa Fe Railroad bridge and the Interstate 5 freeway (1-5) divide Agua Hedionda
Lagoon into three sections. These are the Inner, Middle, and Outer Basins with areas of 186, 22,
and 50 acres, respectively.
Figure 2-1 shows the configuration of the intake and discharge channels. The intake jetties,
located west of the bridge, have lengths of about 350 ft (northern) and 368 ft (southern). The
distance between the centerline of the two jetties is about 243 ft. The jetties at the discharge
channel are about 327 and 376 ft long, with the south jetty longer than the north jetty. The
distance that the intake and discharge jetties extend from the shoreline varies with the changing
location of the shoreline.
2.3. OCEANOGRAPHIC CONDITIONS
The two main oceanographic forcing functions controlling the lagoon dynamics and
sedimentation in the short term are tides and waves. This section briefly describes the
characteristics of these two parameters along the s.outhern California coast.
2.3.1. Tides
The tide is the change of ocean water level caused by the astronomical forces of the moon
and sun. The tide is predictable and can be decomposed into a set of constituent frequencies near
I and 2 cycles per day, each having a given amplitude and phase at any location. Longer period
fluctuations in amplitude occur at 2 cycles per month, 2 cycles per year, every 4.4 years, and
every 18.6 years.
On the San Diego coast, the tide is mixed with nearly equal semi-daily and daily components
(Zetler and Flick, 1985). The highest monthly tides in the winter and summer are higher than
those tides in the spring and fall as a result of lunar and solar declination effects. Also, the
extreme monthly higher-high tides in the winter tend to occur in the morning.
The tidal fluctuations are superimposed on sea level. Seasonal sea level in the San Diego
area tends to be highest in the fall and lowest in the spring, with differences of about 0.5 ft.
Local wanning or cooling resulting from offshore shifts in water masses can alter the average sea
level by several tenths of a foot over periods of several months (e.g., El Nino years) (Reid and
Mantyla, 1976).
Coastal Environments
Reference Number 98-11
2-2 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Tamarack Parking Lot
Pacific Ocean
0
~ Discharge 'i,
Channel ~
Scale (ft)
500
Dimensions in feet
1000
Agua
Hedionda
Lagoon
Outer Basin
0
0
Figure 2-1. Configuration of the intake and discharge channels of Agua Hedionda Lagoon.
Coastal Environments
Reference Number 98-11
2-3 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Tidal elevations are usually referenced to Mean Lower Low Water (MLL W), which is
defined as the average elevation of the lowest water level readings of each day over a specified
19-year interval. In the study area, the maximum tidal range is about 9 ft (7 .2 ft above MLL W to
1.8 ft below MLL W). Tidal elevations can be converted to other vertical datum using the
appropriate conversion values.
2.3.2. Waves
Waves are the most important factor in the mobilization, transportation, and deposition of
nearshore sediments. Wave-induced sand transport is important both in the longshore and cross-
shore directions. Ocean waves in southern California and in San Diego fall into three main
categories (U.S. Army Corps of Engineers (USACE), 1986):
• Northern Hemisphere Swell-waves generated m the North Pacific Ocean that
propagate into southern California waters,
• Southern Hemisphere Swell-similar waves generated south of the equator, and
• Local Seas-relatively short period waves generated within the Southern California
Bight.
The southern California coastline is sheltered from deep-ocean waves by numerous offshore
islands and shoals (Figure 2-2). Figure 2-2 shows various wave windows for Carlsbad and the
general characteristics of the waves approaching the coast for each window.
Long-term wave measurements made by the Coastal Data Information Program (CDIP,
1992) at, 36-ft (11-m) water depth near Oceanside, characterize the typical wave conditions in
the San Diego region. A summary of 16 years of data (Table 2-1 and Figure 2-3) show two
distinct categories of energetic wave events. The most common wave conditions are northern
and southern hemisphere swell with moderate significant wave heights of 2 to 4 ft (0.5 to 1.0 m)
and relatively long wave periods (13-18 sec). Figure 2-3 shows that although locally generated
seas can be as energetic as swell, swell events dominate the regional wave climate. At
Oceanside, the peak wave period exceeds 5 sec about 80% of the time, and exceeds 5 sec 86% of
the time when the significant wave height is higher than 5 ft (1.5 m).
Coastal Environments
Reference Number 98-11
2-4 Final Report
Study of Sediment Transport Conditions, Agua Hedionda L;,agoon, Carlsbad, CA
San Miguel
Island
~
Santa Rosa
Island
EXTRATROPICALSTORM
SWELL
NOV. -APR.
H5 to 10 m T=12 to 21 sec.
261°
San Nicolas
Island
PACIFIC OCEAN
N
0 20 40 60km
·. /Oceanside
-Carlsbad
LOCAL WAVES
All Year
H1 to 2.5 m T=3 to B sec.
SOUTH and SW SEAS
Nov. -Apr.
H1 to 6 m T =5 to 1 0 sec.
Figure 2-2. Wave exposure for Carlsbad illustrating island shadowing effects. Modified from
USACE (1991).
Coastal Environments
Reference Number 98-11
2-5 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 2-1. Joint distribution of significant wave height (Hs) and peak wave period (Tp) for
Oceanside (number of observations= 15,101). Modified from CDIP (1992).
e u -... .r: C, ·.;
:I:
Cl) > I ...
C CII u i;:::: '2
C, en
481 -510
451-480
421 -450
391 -420
361 -390
331 -360
301 -330
271 -300
241 -270
211 -240
181 -210
151 -180
121 -150
91 -120
61 -90
31-60
1 -30
Coastal Environments
Reference Number 98-11
3
8
5
5
21
Oceanside Array, Energy
Jun. 1976 -Dec. 1991
1 1
1
1 3 3
1 5 5
3 9 6
5 16 22 21
35 104 68 79
108 406 603 299
199 741 1661 1300
74 287 671 1122
1 1 8 11
19 17 15 13
Peak Period (sec)
2-6
1
1
1 2 1
1
2 2
7 11 1
3 14 25 7
1 21 37 17
17 44 102 39
64 117 220 119
179 215 488 364
480 381 845 1527
352 188 285 998
4 2 5 9
11 9 7 5
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Jun 76 -Dec 91 Swell
TP lsec)
Figure 2-3. Joint distribution of significant wave height (H.) and peak period (Tp) at Oceanside.
Modified from CDIP (1992).
Coastal Environments
Reference Number 98-11
2-7 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
2.4. AGUA HEDIONDA LAGOON DYNAMICS AND SEDIMENTATION
2.4.1. Tidal Prism
Tidal prism is defined as the volume of water that flows into a lagoon between low tide and
high tide, or the volume that flows out between high tide and low tide. In a lagoon with sloping
banks, the surface area of water at high tide will be larger than the area at low tide. At elevation
+7.7 ft (MLLW) the lagoon has approximately 388 acres of water surface area, and at elevation -
2.3 ft (MLL W) the surface area is reduced to approximately 296 acres (Bradshaw, 1976).
The final construction report by Ellis (1954) indicates that, initially, the lagoon system had a
mean tidal prism of 55 x 106 ft3 and a maximum diurnal or spring tidal prism of 80 x 106 ft3.
Since that time, five power-generating units have been brought on line, increasing the maximum
possible flow rate diverted from the lagoon through plant condenser systems to up to
108 x I 06 ft3 /day (808 million gallons per day (mgd)). Plant diversion of lagoon waters at this
maximum flow rate reduces the net tidal prism flowing out the ocean inlet during ebb flow by
27.9 x 106 ft3, or 50.8% of the original mean tidal prism and 34.9% of the original maximum
spring prism. Actual plant inflow rates during high-use demand periods are typically 85 to
89.6 x 106 ft3/day (635 to 670 mgd) (Figure 2-4), which reduces the volume of water available to
flush the ocean inlet by 39% to 42% for mean tidal ranges.
In September 1997, SDG&E started to dredge the Outer, Middle, and Inner Basin of the
lagoon to restore the tidal prism to its original design conditions. The hydraulic evaluation for
various stages of the project was completed by Jenkins and Wasyl (1997). A summary of their
results is given in Table 2-2.
2.4.2. Sedimentation
Coastal lagoons are in continual interaction with the littoral drift of beach sand. Sand
suspended in the breakers and bores near the inlet is drawn into the lagoon on every flood tide.
Depending on several factors, the indrawn sand is usually not completely returned to the sea
during the next ebb tide. Lagoons in southern California are small and intermittently open to
tidal flushing. Usually, the entrance channel is gradually constricted by deposition of beach
sand, and the lagoon may close for a time until the channel is cleared by major flood runoff or by
artificial means (Elwany, et al., 1998).
Agua Hedionda is an unusual and special case, because the Encina Power Plant draws its
cooling water from the lagoon and then discharges it directly to the ocean through a separate
discharge channel. Since the flow of cooling water must be subtracted from the outgoing tidal
flow, the incoming flow in the inlet channel is stronger than the outgoing tide, creating a flood-
tide dominated lagoon. The power plant takes in cooling water at a rate of 14 x 106 ft3 per semi
diurnal half-tide, about one-third of the mean tidal prism of the lagoon. The actual asymmetry in
flow rate and water velocity is then about two to one between the flood and the ebb.
Coastal Environments
Reference Number 98-11
2-8 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
1000
-i, 800 s -= 600 ~
It 0 400 ~
! 200
0
0 50 100 150 200 250 300 350
TIME (DAYS)
Figure 2-4. Plant Inflow rate time history for the calendar year July 27, 1993 to July 27, 1994,
from Jenkins and Wasyl (1997).
Coastal Environments
Reference Number 98-11
2-9 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 2-2. Hydraulic evaluation for various stages of lagoon dredging (70% plant activity,
570 mgd flow rate). Modified from Jenkins & Wasyl (1997).
Original Existing
Conditions Construction Nov.1995 Profile (1954) Sounding
Spring Tidal
Prism (ft3) 80,000,000 63,636,515
Mean Tidal 55,000,000 35,541,937 Prism (ft3)
Spring Subtidal 235.0 189.6 Area (acres)
Mean Subtldal 251.0 233.0 Area (acres)
Spring Intertidal 58.1 90.11 Area (acres)
Mean Intertidal 29.6 31.6 Area (acres)
a dredging to take place between 1997 and 1999.
Coastal Environments
Reference Number 98-11
2-10
Mitigated Mitigated Stage 1
Dredging• Stage 3 Stage 4
Dredging• Dredging 1
68,711,404 72,447,428 84,699,845
42,041,784 42,501,913 49,939,802
177.6 197.8 205.9
232.0 213.6 220.2
102.1 77.7 89.3
35.4 43.8 56.7
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Cartsbad, CA
Ritter (1972) studied the rate of sedimentation in Agua Hedionda Lagoon based on data
obtained from SDG&E. Cross-sections of the lagoon were surveyed monthly from March 1955
through May 1957. During this time period, sedimentation occurred only in the Outer Basin.
The average rate of deposition was 11,500 yd3 per month, or 138,000 yd3 per year. Ritter's
2-year record of the monthly rate of sediment deposition in Agua Hedionda Lagoon indicates a
6-month cycle in the rate of sediment accumulation. High rates of accumulation were recorded
in March and August; low rates of accumulation were recorded in May, June, and December.
The monthly sedimentation rates as estimated by Ritter (1972) are shown in Figure 2-5.
The history of dredging at Agua Hedionda Lagoon since 1954 is given in Table 2-3. This
table provides the date of dredging, volume of dredged material, and the location where the spoil
was placed. From Table 2-3, the average rate of dredging over 44 years is calculated at
approximately 138,000 yd3/yr (105,500 m3/yr).
Coastal Environments
Reference Number 98-11
2-11 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
40
35
'iii 'E 30 Ill >,
u ::c 25 :::, u
0 0 o_ 20 ...
K --C 15 'ti 0 GI ; a .!! ....... ........... l :::, 10 Q E ::, u u Ill 5 -C G>
E 0 :s G> (I)
-5
-10.
in in in in II) II) in in '2 in '9 in '9 '9 in in ,!, >, C: '3 CD a. 11 Ill a. Ill :::, :::, Cl) :ii: <( :ii: -, -, <( II) 0
in in ID CD CD ID CD CD 10 II) ~ in 10 .., t '9 > u .6 ,!, ,!, C: 0 ~ Ill Cl) ta a. Ill :::, z -, IJ.. :ii: <( :ii: -,
CD CD ~ "? en :::, :::, -, <(
August
ID '9 a. Cl) II)
Average, 11,500 ylf
per month
ID 10 10 ,.._ ,.._
in in 'Z '9 in ..!. > C: .6 u 0 Cl) Ill a, 0 z 0 -, IJ..
,.._ ,.._
in in ,!, ,!, Ill a. :ii: <(
Figure 2-5. Monthly sedimentation rates, northern half of the Outer Basin, March 1955
to May 1957. Modified from Ritter (1972).
Coastal Environments
Reference Number 98-11
2-12 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 2-3. Dredging history for Agua Hedionda Lagoon.
Year Date
Start Finish
1954 Feb-54 Oct-54
1955 Aug-55 Sep-55
1957 Sep-57 Dec-57
1959-60 Oct-59 Mar-60
1961 Jan-61 Apr-61
1962-63 Sep-62 Mar-63
1964-65 Sep-64 Feb-65
1966-67 Nov-66 Apr-67
1968-69 Jan-68 Mar-69
1972 Jan-72 Feb-72
1974 Oct-74 Dec-74
1976 Oct-76 Dec-76
1979 Feb-79 Apr-79
1981 Feb-81 Apr-81
1983 Feb-83 Mar-83
1985 Oct-85 Dec-85
1988 Feb-88 Apr-88
1990-91 Dec-90 Apr-91
1992 Feb-92 Apr-92
1993 Feb-93 Apr-93
1993-94 Dec-93 Apr-94
1995-96 Nov-95 Apr-96
1997 Sep-97 Nov-97
Dec-97 Feb-98
1998 Feb-98 Jul-98
(1999) (Feb-99) (May-99)
TOTAL
MAINTENANCE TOTAL
MAINTENANCE AVERAGE
(yd3/yr)
Coastal Environments
Reference Number 98-11
Dredging
Volume
(yd3)
4,279,319
90,000
183,000
370,000
227,000
307,000
222,000
159,108
96,740
259,000
341,110
360,981
397,555
292,380
388,200
403,793
333,930
458,973
125,976
115,395
158,996
443,130
197,342
59,072
214,509
(155,000)
10,639,509
5,931,609
137,944
Disposal
Basin Volume Location Comments
dredged (yd3) placed1
Outer, Middle 4,279,319 N,M,S Initial construction
& Inner dredging
Outer 90,000 s Maintenance
Outer 183,000 s Maintenance
Outer 370,000 s Maintenance
Outer 227,000 s Maintenance
Outer 307,000 s Maintenance
Outer 222,000 s Maintenance
Outer 159,108 s Maintenance
Outer 96,740 s Maintenance
Outer 259,000 s Maintenance
Outer 341,110 s Maintenance
Outer 360,981 M Maintenance
Outer 397,555 M Maintenance
Outer 292,380 M Maintenance
Outer 388,200 M Maintenance
Outer 403,793 M Maintenance
Outer 333,930 N,M,S Maintenance
Outer 458,973 M,S Maintenance
Outer 125,976 N Maintenance
Outer 115,395 M Maintenance
74,825 N
Outer 37,761 M Maintenance
46,410 s
106,416 N
Outer 294,312 M Maintenance
42,402 s
Outer 197,342 M Maintenance
Middle 59,072 M Modification dredging
Inner 120,710 M Modification dredging 93,799 s
(Outer) (155,000) (N) (Maintenance)
11 N = North Beach
M = Middle Beach
S = South Beach
( ) indicates planned work
2-13 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carisbad, CA
3. REVIEW OF SEDIMENT TRANSPORT IN THE
OCEANSIDE LITTORAL CELL
3.1. OCEANSIDE LITTORAL CELL
A littoral cell is defined as a geographical area with a complete cycle of littoral sand sources,
transport paths, and sinks (Inman and Frautschy, 1965). The Oceanside Littoral Cell extends
approximately 53 miles from Dana Point to Point La Jolla and the Scripps-La Jolla Submarine
Canyon System (Figure 3-1 ). This littoral cell can be divided into two main segments or sub-
cells, from Dana Point to Oceanside Harbor and from Oceanside Harbor to Point La Jolla.
The headland at Dana Point to the north and the submarine canyon at La Jolla to the south
define the boundaries of the Oceanside Littoral Cell. These boundaries are considered as
complete littoral barriers, where no littoral sediment is exchanged between the two adjacent
littoral cells. The main sources for sand in the Oceanside Littoral Cell include river discharge,
cliff erosion, and, to a smaller extent, artificial beach nourishment (Section 3.2). The sink of the
Oceanside Littoral Cell is the La Jolla Submarine Canyon. Sand may also be lost offshore along
the littoral cell during storms.
The beaches in this area are mainly narrow, backed by high cliffs, and consist of sands and
cobble (Chapter 4). Since the 1930s, the amount of sediment entering the system has decreased
dramatically, largely because of the damming and stabilizing of the rivers (Inman, 1985). Since
less material is entering the littoral system, the beaches, in many locations, are receding.
3.2. SEDIMENT SOURCES AND SINKS
3.2.1. Rivers
Despite damming and stabilization, rivers along the Oceanside Littoral Cell contribute some
sediment to the littoral cell during periods of high rainfall and runoff. The USACE (1987)
estimated that the average present total sediment yield of all the major rivers in the Oceanside
Littoral Cell is about 159,000 yd3/yr. Flick (1993) compiled estimates of the annual sediment
yield of rivers south of Oceanside Harbor to the La Jolla Submarine Canyon as between 12,800
and 68,000 yd3•
Table 3-1 shows the range of estimated volumes of littoral sediments supplied by the major
rivers in the Oceanside Littoral Cell (Zampol, et al., 1997). The total range of sediment volume
entering the cell is estimated at 67,240 to 167,840 yd3 /yr. The volume of sediments entering the
littoral cell from river supplies is greater north of Oceanside Harbor than south, as seen in
Table 3-1.
Coastal Environments
Reference Number 98-11
3-1 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
33"30'
33"15' l
33'00'
0 I 10 II 20 JSk.,
117"30' 117"15' 117"00'
Figure 3-1. Location map of the three major littoral cells in the San Diego Region.
Coastal Environments
Reference Number 98-11
Modified from Inman et al. (1993).
3-2 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 3-1. Estimated river coarse sediment yield in the Oceanside Littoral Cell (yd3/yr).
DNOD1 Brownlie2 lnman3 & Simmons,' Range of
River/Stream (1977) & Taylor Jenkins Li &Assoc. Estimates (1981) (1983) (1988)
San Juan Creek 47,000 I
56,0005 ---34,000 34,000-56,000
San Mateo Creek 32,000 -8,100 8, 100-32,000
San Onofre Creek 5,000 --1,800 1,800-5,000
Las Flores Creek 4,000 ----2,700 2,700--4,000
Aliso Canyon Creek ----900 900
Santa Margarita River 15,000 7,000 9,000 19,000 7,000-19,000
Oceanside Harbor
San Luis Rey River 351,000 18,000 22,000 10,800 10,800-18,000
(351,000)6
Loma Alta Creek7 --940 940
Buena Vista Creek7 -----0 0
Agua Hedionda 14,0007 --0 0-14,0007
Creek7
San Marcos Creek7 --0 -
Encinitas Creek7 ---0 -
Escondido Creek 14,000 ----0 0-14,000
La Orilla Creek ---0 0
San Dieguito River 4,000 1,500 2,000 1,000 1,000--4,000
Carmel Valley Creeks ----0 -
Los Penasquitos
Creek6 -------0 --
Carroll Canyon Creeks -------0 -
TOTAL RANGE 67 ,240-167,840
1. Present sediment production.
2. Period of maximum control or total period of record, if less.
3. Values calculated using the data of Brownlie & Taylor (1981) and assumed to be sand to suspended load ratios
of Inman & Jenkins (1983).
4. Present condition.
5. USGS estimate of sediment discharge reported in DNOD (1977).
6. Extreme value.
7. San Marcos Group of DNOD (1977) assumed to include Loma Vista Creek to Encinitas Creek.
8. DNOD (1977) sediment estimates from these creeks were not directly given (see Table 2, and footnote 5).
Coastal Environments
Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
3.2.2. Cliffs
Since the beaches within the Oceanside Littoral Cell are relatively narrow, the elevated water
level and increased wave action produced by storms commonly reaches the base of the sea cliffs
and may cause severe erosion. Cliff erosion can be increased during rainfall, because of
landslides. Estimates of cliff erosion rates and total yield of sandy sediments are presented by
the USACE (1991, Table 9-6). These estimates are highly variable and depend greatly on the
location of the cliffs. The estimate of total sandy sediment yield from the cliffs north of
Oceanside Harbor, at Camp Pendleton, is 23.6 x 106 yd3 and from the cliffs at Torrey Pines is
4.4 x 106 yd3, over a 79-year period. This represents an overall sediment contribution rate of
approximately 354,430 yd3/yr from the cliffs to the beaches within the Oceanside Littoral Cell.
3.2.3. Dredging and By-passing from Oceanside Harbor to Moonlight Beach
Oceanside Harbor and some of the lagoons within the Oceanside Littoral Cell, such as Agua
Hedionda Lagoon, trap some of the sand moving alongshore. At Oceanside Harbor, an annual
average of 185,000 yd3 /yr of sediments are dredged from the harbor and returned into the
longshore transport zone south of the harbor. SDG&E dredges approximately 250,000 to
300,000 yd3 of beach sand from Agua Hedionda Lagoon every two to three years and places the
material along the beaches near the lagoon. Table 3-2 describes the history of dredging at
Oceanside Harbor and Table 2-3, in the previous chapter, describes the history of dredging at
Agua Hedionda Lagoon including volumes and placement location. In addition, about
1.6 x 106 yd3 of dredged sediment from Batiquitos Lagoon were placed along South Carlsbad
State Beach during 1994 and 1995.
3.2.4. Sedimentation in Carlsbad Submarine Canyon
Several studies have been conducted to assess the effects of Carlsbad Submarine Canyon on
the littoral sand budget. These studies concluded that the canyon does not trap littoral sand and
is, therefore, inactive (Shepard and Emery, 1941; Fischer et al., 1983; Emery, 1960; Inman and
Frautschy, 1965; and USAGE, 1988). These conclusions were based on 1) the deep-canyon
channel was filled with mud, which is not a major component size of the beach material, 2) the
canyon is located at a depth of about 120 ft and approximately 4,450 ft from the shore,
3) evidence of inactive bedforms were found on the seabed during observations and sampling
dives, and 4) bottom observations of these inactive and decaying features made by divers after
two months of diving in October and November of 1984 indicated that severe wave conditions
did not move sand to the head of the canyon.
Coastal Environments
Reference Number 98-11
3-4 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Car1sbad, CA
Table 3-2. Oceanside Harbor dredging history.
Year Amount Material Source Disposal Location Comments Dredged (yd3)
1942 1,500,000 Del Mar Boat Basin Increase grade around Material was not placed
Boat Basin on the beach
1944 200,000 Entrance Channel Upland Material was not placed
on the beach
1955 800,000 Harbor construction Oceanside Beach dredged material
1960 41,000 Entrance Channel Oceanside Beach dredged material
1961 481,000 Channel Oceanside Beach dredged material
1963 3,800,000 Harbor Oceanside Beach 1.4myd3 was new material
1965 111,000 Entrance Channel Oceanside Beach dredged material
1966 684,000 Entrance Channel 3rd St-Wisconsin St dredged material
1967 178,000 Entrance Channel 3rd St-Tyson St dredged material
1968 434,000 Entrance Channel River-Wisconsin St. dredged material
1969 353,000 Entrance Channel River-3rd dredged material
1971 552,000 Entrance Channel 3rd-Wisconsin St. dredged material
1973 434,000 Santa Margarita R. Tyson-Wisconsin St. New material -beach fill
1974 560,000 Entrance Channel Tyson-Whitterby dredged material
1976 550,000 Entrance Channel Tyson-Whitterby dredged material
1977 318,000 Entrance Channel Tyson-Whitterby dredged material
1981 403,000 Entrance Channel 6th St-Buccaneer dredged material
1981 460,000 Offshore Borrow Site Oceanside Beach dredged material
1982 923,000 San Luis Rey R. Oceanside Beach New material -beach fill
1983 475,000 Entrance Channel Tyson Street dredged material
1986 450,000 Entrance Channel Tyson Street dredged material
1988 220,000 Entrance Channel Tyson Street dredged material
1990 250,000 Entrance Channel Tyson Street dredged material
1992 106,700 Bypass System Tyson Street dredged material
1992 187,000 Entrance Channel Tyson Street dredged material
1993 483,000 Modified Entrance Tyson Street dredged material
1994 40,000 Santa Margarita R. Wisconsin St New material -beach fill
1994 161,000 Entrance Channel Nearshore Wisconsin dredged material
1996 162,000 Entrance Channel Nearshore Wisconsin dredged material
1997 a 150,000 Entrance Channel Nearshore Oceanside Blvd dredged material
1997 b 100,000 Entrance Channel Wisconsin St dredged material
15,566,700 Total
178,017 Average (only including maintenance dredging)
Coastal Environments
Reference Number 98-11
3-5 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
3.3. LONGSHORE TRANSPORT ALONG THE OCEANSIDE LITTORAL CELL
The principal mechanism for transporting beach sand along the shore is the longshore or
littoral drift of sand in the surf zone (Inman and Frautschy, 1965; Inman et al., 1968; Komar and
Inman, 1970). The longshore transport moves sand suspended by the turbulence of breaking
waves and carries it along the shore by the longshore current produced by breaking waves
approaching the shore obliquely.
3.3.J. Estimates of Longshore Transport Rates from Previous Studies
A number of studies that reviewed the sediment budget and longshore transport rates within
the Oceanside Littoral Cell are presented in the Coast of California Storm and Tidal Waves
Study (CCSTWS) prepared by the USACE (1991). Since these studies use various sources of
data their estimates of longshore transport vary. Table 3-3 summarizes the estimated longshore
sediment transport rates reported in the literature.
3.3.2. Estimates of Longshore Sediment Transport Rates from Oceanside Wave Data
The longshore sediment transport was computed from long-term directional wave
measurements at Oceanside, California at 36-ft water depth. The data cover the period between
14 December 1978 and 31 December 1994 (16 years of data). Data collected before or after
these dates were not used in this study because of large data gaps, relocation of the wave array,
and uncertainties in the wave array orientation. These data represent the nearest long-term wave
records available to the study site.
The Seymour and Higgins formula (1978) relates the longshore sediment transport rate in the
surf zone to the component of the radiation stress in the longshore direction (Sxy) and significant
wave height (Hs) at the array. This involves two assumptions: the shoreline contours are straight
and parallel so that Sxy is conserved between the array and the breakpoint (Longuet-Higgins,
1970); and the depth at breaking (hb) can be approximated by 1.65 times Hs at the array
(Griswold, 1964). The relation used in the present study:
(3-1)
where Qe is the "at rest" volume transport rate of sand in ycf /yr, Sxy and Hs at the array are
expressed in Jr and .ft, respectively, and the proportionality coefficient, 3.843 x 106, has units
ycf /(yr *ft2·5).
In metric units Equation (3-1) can be written as:
Coastal Environments
Reference Number 98-11
3-6
(3-2)
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 3-3. Previous estimates of longshore transport, yd3/yr.
Source Location of Southward Northward
Estimate Transport• Transportb
Marine Advisors Oceanside 760,000 545,000 1960
Nordstrom & Inman Oceanside 1975 --
Hales Oceanside 643,000 540,000 1978
Inman & Jenkins Oceanside 807,000 553,000 1983
USAGE Oceanside 1991 --
Simpson, et al. Oceanside 350,000-230,000-
1991 380,000 40,000
a. Transport towards the south.
b. Transport towards the north.
c. Sum of the north and south transport.
d. Difference between the south and north transport.
Coastal Environments
Reference Number 98-11
3-7
Gross Net
Transportc Transportd
1,305,000 215,000
(South)
200,000 -300,000 -(South)
1,183,000 103,000
(South)
1,360,000 254,000
(South)
180,000 -230,000 -(South)
390,000-120,000 -340,000
610,000 (South)
Average 178,700-240,300
(South)
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
where Qt is the "at rest" volume transport rate of sand in m3 lyr, Sxy and Hs at the array are
expressed in their CDIP reported units of cm2 and cm, respectively, and the proportionality
coefficient, 980, has units m3/(yr *Cm2·5). Equations (3-1) and (3-2) are based on the Scripps
Institution of Oceanography (SIO) and USACE relation:
I,= K(E Cn sina. cosa.)b (3-3)
Where /1 is the immersed-weight longshore transport rate m the surf zone, K is a
dimensionless coefficient, evaluated by Komar and Inman (1970) as 0.77, Eis the wave energy
per unit surface area and is equal to 1/8 p gH2, where pis the water density, g is gravity, and His
the root-mean-square wave height. Cn is the wave group velocity, a is the wave incidence angle
with the shoreline, and the subscript b indicates that the parameters are evaluated at the breaker
line. Equations (3-1) and (3-2) are considered accurate to within a factor of two, at best.
The results are considered accurate to within a factor of two, at best. This uncertainty is
because of the assumptions and approximations in the transport relations and the estimation of
the coefficient of the transport model. Although, other more complicated approaches for
computing the longshore transport rates are available, they would not improve the estimates of
the longshore transport rate significantly.
Figure 3-2 shows the monthly mean values of longshore transport from the available wave
data at Oceanside from December 1978 to December 1994. The figure indicates a strong
seasonal variation in transport potential. Southward transport prevails during the winter months
of December through March. Upcoast transport (to the north) dominates during the summer
months of June through September. There appear to be two transitional periods, April to May
and October to November, when transport rates are close to zero.
Some inter-annual variability is evident in the data. Figure 3-3 shows the mean longshore
transport during each year (solid circle) for winter, summer, and combined winter and summer
seasons. The number of days used in the calculation is written below or above the solid circle.
For the combined plot, only the years with over 250 days of data are represented. For all years
except in 1987, 1992, 1993, and 1994, the mean longshore transport is to the south. In 1987, a
small mean longshore transport to the south occurs during the winter season. For the combined
data, the winter mean transport is approximately -2,036 yd3/day (-1,557 m3/day) towards the
south, while the summer mean transport is about 1,757 yd3/day (1,343 m3/day) towards the north.
Figure 3-4 displays the percentage of longshore transport during summer, winter, and combined
summer and winter (all) data. The cumulative distribution of the longshore transport rate is
showr1 in Figure 3-5 for the summer and winter seasons and for the combined two seasons. The
figure shows that during the winter season, 65% of the longshore transport is to the south and
35% is to the north. During the summer season, the transport is about 25% to the south and
Coastal Environments
Reference Number 98-11
3-8 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
'S; cu 32. (Y)
~ -6 a. CJ)
C: cu ~
Q) s... 0 .c CJ)
Cl
C .9
5000
4000
3000
2000
1000
0
-1000
-2000
-3000
-4000
---Oceanside
I
78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
Years
t
Figure 3-2. Monthly mean values of longshore transport at Oceanside from available wave data from December 1978 to October 1994.
Coastal Environments
Reference Number 98-11
3-9 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
~00
t
Winter z
i 2000
I 154 • 75 1000 • 8
l 1&:l • 0 ------------------------------------------• l • • •
-1000 61 174 144 116 • li • • • • Cl) • 166 :I 178 172 178 180 180 ,I.
-2000
-~o
78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
Years
~o
00 T Summer • z
l 2000 1ffi 182 169 • • t 181 •
1000 • 31 183 52 177 -2> 147 178 • • • • i • •
0 ------1i -----------------------------g • 177 • F -1000 143 40 j Cl)
,I.
-2000
-~oo
78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
Years
3>00 Yearn > 250 daywyaar of data t
Combined z
l 2000 ~ •
1000 285 343 ], • 252 • • i 0 ----------..------------. ------------------
Ill • ! • • 353 363
-1000 317 356
355
Ii Cl)
:I ,I.
-2000
-~00
78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
Years
Figure 3-3. Mean longshore transport for winter, summer, and combined winter and summer data
from 1978 to 1994 at Oceanside.
Coastal Environments
Reference Number 98-11
3-10 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
35
Winter .. ,.,., 3) .......
25
j 20
n. 15 ......
10
, __
......
..__
5 -.. _ Cl"l'N ---0
,c-D.IS -• -· _, -e -5 -4 -3 -2 _, 0 2 ll •
35
Summer 91.2 ...
3:>
25
I 20 17.-
15 12.-,.__
10 .....
5 .. _ .. _ ....... ....... 0.-0,711l
.. _
0
c-a.s _, -· -1 _, -· -• -3 -2 _, 0 2 a •
35
~ --
25
i 20 g
Q. 15
10
5 -a.111" 0.1111' ~-Cl-ll-0
<-v., -9 -a -1 -e _, -4 -1 -2 _, 0 I
L.ongshat 'lta,sporl, 101 yd1/day N ..
Figure 3-4. Percentage of longshore transport during the winter, summer, and combined winter
and summer data at Oceanside.
Coastal Environments
Reference Number 98-11
3-11 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Winter
i eo
a: -40
20
0
<-10 -e -6 -4 -2 0 2 4 6 8 > 10
100
Summer
i c:.
20
0 ..,..-,---,---,.---r-~-,---,--r---,---,.-,---,--,----,----.-~--,.-.---.----,.J
<-10 -8 -6 -4 -2 0 4 6 B > 10
100
Combined
8J
E oo
~
Cll D.. 40
20
0 l.,---,-...--,,-...----,,----,---.-~--.--,.......-,---,.-~--,----,---,.--,--.,.J
< -10 -e -6 -4 -2 0 2 4 6 8 > 10
.. s Longshore TranBport, 1a3 yd~/day N,.
Figure 3-5. Cumulative probability (expressed as a percentage) of longshore transport (Q.,) during
the winter, summer, and combined winter and summer data at Oceanside, giving the
probability that Q., s some given value, q.
Coastal Environments
Reference Number 98-11
3-12 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Car1sbad, CA
75% to the north. The combined plot shows that the longshore transport is about 54% to the
south and 46% to the north. From the data, the longshore transport in the vicinity of Oceanside
is 320,625 yd3/yr (245,150 m3/yr) to the north and -371,570 yd3/yr (-284,102 m3/yr) to the south.
The total (gross) transport at Oceanside is 692,195 yd3/yr (529,252 m3/yr) and the net transport at
Oceanside is -50,945 yd3/yr (-38,953 m3/yr) to the south.
The monthly mean and standard deviation of the longshore transport rates are shown in
Figure 3-6 and given in Table 3-4, along with the absolute values of the monthly mean longshore
transport. Clearly, the longshore transport from November through April is generally to the
south; whereas, from June through September the longshore transport is to the north. During
May and October the longshore transport is at their minimum values.
Since 1990, the wave climate in the Southern California Bight has changed. The prevailing
northwesterly winter waves changed to waves approaching from the west, and the previous
southern hemisphere swell waves of summer have been replaced by tropical storm waves from
the waters off Central America. The net result appears to be a decrease of the southerly
component of the net longshore transport of sand that prevailed during the preceding 30 years as
summarized in Table 3-3. Figure 3-3 depicts these observed changes in the wave climate in the
mean values of the longshore transport during the 1990s.
3.4. LONGSHORE SEDIMENT TRANSPORT AT CARLSBAD
Prior to this study, there were no wave measurements at Carlsbad from which estimates of
the longshore sand transport at Agua Hedionda Lagoon could be calculated. For this study, a
wave experiment was conducted between 9 July and 22 September, 1998 (76 days) to determine
the relationship, between the wave regime at Carlsbad and at Oceanside (Appendix B). The
objective was to utilize the historical wave measurements at Oceanside to obtain longshore sand
transport statistics at Carlsbad.
During this wave experiment, wave measurements were obtained in water depth of 33 ft just
offshore from both, Agua Hedionda Lagoon and Oceanside. The Carlsbad station was located
1,000 ft south of the intake channel and the Oceanside station was located at the former CDIP
wave gauging station. InterOcean pressure/horizontal velocity sensors (PUV instruments) were
deployed at each station. The PUV instruments record data to estimate both wave energy and
some basic properties of the local directional wave spectrum, such as the mean wave direction
and the longshore component of radiation stress, Sxy· The locations of the PUV gauges are
shown in Figure 3-7.
The data from the wave experiment were analyzed and are discussed in Section B.1.1. The
data show that south swell wave heights and radiation stresses are larger at Oceanside than ·
Carlsbad, but west-sea conditions at the two sites were similar. The simultaneous measurements
at Oceanside and Carlsbad only covered south swell and west-sea conditions.
Coastal Environments
Reference Number 98-11
3-13 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
s; cu ~ ~ ~
g_
(/J
C ~ F
fa
4000
3000
2000
1000
0
-1000
~ -2000
-3000
11
12
Mean and Standard Deviation
I I I I
I I I
I 10 I
10 l I ----+-r
11
11 12
10 12
I
------------------r-1 I I I I I I I I
11
11
___ 1 ___ ~3 ----
t
i I 0 z
i i :J
0
(/)
l
-4000 ..,_-....---.-----.---...._-,----r-----.--.....---.-----r---'-----r----r----,----,.,
Coastal Environments
Reference Number 98-11
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
Figure 3-6. Monthly means and standard deviation of the longshore transport rates for each month.
3-14 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 3-4. Characteristics of longshore transport at Oceanside from 1978 to 1994, yd3/day (m3/day).
North•
Season
Mean Std Ne
Summer 1,863 2,066 1348 (1,424) (1,580)
Winter 1,582 2,705 815 (1,210) (2,068)
Combined 1,757 2,331 2163 (1,343) (1,782)
Transport to the North per year
Transport to the South per year
Gross Transport per year
Net Transport per year
Coastal Environments
Reference Number 98-11
Southb
Mean Std
-1,589 2,609
(-1,215) (1,995)
-2,263 4,469
(-1,730) (3,417)
-2,036 3,954
(-1,557) (3,023)
320,625
(245,150)
-371,570
(-284,102)
692,195
(529,252302)
-50,945
(-38,953)
Totalc Absoluted
N Mean Std N Mean Std
575 832 2,744 1923 1,781 2,246
(636) (2,098) (1,362) (1,717)
1131 -652 4,275 1946 1,978 3,845
(-499) (3,269) (1,512) (2,940)
1706 84 3,671 3869 1,880 3,154
(64) (2,807) (1,437) (2,412)
a -Longshore transport to the north
b -Longshore transport to the south
c -Combined north and south longshore transport
d -Absolute value of longshore transport
e -N = Number of days
3-15
N
1923
1946
3869
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
San Luis Rey River
Oceanside Pier
Oceanside PUV
-e-l
Pacific Ocean
Scale (ft)
0 2000 4000 6000
Carlsbad PUV
Location of Directional Wave Gauges.
Contours intervals are 3 ft and represent depth below NGVD 1929.
Figure 3-7. Location of the PUV wave gauges deployed in Oceanside and Carlsbad.
Coastal Environments
Reference Number 98-11
3-16 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Wave model simulations (Appendix B, Section B.1.2) were performed for the two sites to
better understand the differences in south-swell measurements and similarities in west-sea
conditions between Carlsbad and Oceanside. Also, the wave model simulations were used to
make judgements about possible differences for local seas from the south and winter swell
arriving from the west (wave conditions that were not observed during the instrument
deployment period). The model-simulation results (Figures B-3 and B-4) suggest that
differences between the sites are primarily a function of the deep-water wave direction, rather
than the wave period. Specifically, Carlsbad Submarine Canyon shelters Carlsbad from south
swell and seas. Therefore, it is assumed that observed differences between Oceanside and
Carlsbad for south swell can also be applied to local seas from the south and observed
similarities between the sites for west seas are also true for west swell.
The measurements were then used to derive empirical formulas to adjust Oceanside
significant wave heights (Hs) and total radiation stresses (Sxy) to be representative of Carlsbad
conditions. The derivation of the relationship between wave conditions at Oceanside and
Carlsbad is given in Appendix B, Section B.l .2. Figures 3-8 and 3-9 show a comparison
between Oceanside, adjusted Oceanside, and Carlsbad for Hs and Sxy, respectively.
The formulas relate Hs and Sxy at Oceanside and Carlsbad, and were then used to adjust the
long historical record of wave measurements at Oceanside for sediment transport calculations at
Carlsbad. The longshore transport at Carlsbad was calculated with Equation (3-1) for values of
Hs and Sxy at Carlsbad. Estimates of the longshore-transport potential for Carlsba_d calculated
from 1978 through 1994, along with the estimates for Oceanside are shown in Figure 3-10. The
figure shows the large differences between northern transport rates at Oceanside and Carlsbad.
The cumulative distribution for longshore sand transport for summer, winter, and combined
seasons are shown in Figure 3-11. The figure shows that during the winter, 90% of the
longshore transport is to the south and 10% to the north. During the summer, the transport is
about 63% to the south and 37% to the north. The combined plot shows that transport is 80% to
the south and 20% to the north. From the adjusted, historical wave data measured at Oceanside,
the longshore transport at Carlsbad is 113,059 yd3/yr (86,445 m3/yr) to the north and
-425,316 yd3/yr (-325,197 m3/yr) to the south. The total (gross) transport at Carlsbad is
538,375 yd3/yr (41l,642 m3/yr) and the net longshore transport at Carlsbad is -312,257 yd3/yr
(-238,752 m3/yr). The estimate of the net longshore transport is in agreement with the USACE
(1994) estimates of270,000 yd3/yr (206,442 m3/yr).
Table 3-5 gives a summary of the longshore transport in the vicinity of Agua Hedionda
Lagoon. A comparison between longshore-transport statistics at Oceanside and Carlsbad is
given in Table 3-6.
Coastal Environments
Reference Number 98-11
3-17 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
6
5
}
4 ..,~. -\,
E ~ U)
J: 3
2
1
• • • • • •
Oceanside
Adjusted Oceanside
Carlsbad
,, JI
01.---.....i..--------.....L.----------'-----------L---------'".___ ___ ...J 7.5
Coastal Environments
Reference Number 98-11
8 8.5 9 9.5
Month of 1998
Figure 3-8. Adjusted Oceanside wave height data relative to Carlsbad and measured Oceanside data.
(Early July {month 7) to late September (month 9))
3-18 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
0.3
0.25 I ,,
0.2 Northward Transport I ~I
I
0.15 ,,
~ I
0.1 ~
~
I
-0.05
-0.1 ••
-0.15 Southward Transport
-0.2
7.5 8
• • • • •
8.5
Month of 1998
•
9
Oceanside
Adjusted Oceanside
Carlsbad
9.5
•
Figure 3-9. Radiation stress (Sxy) at Carlsbad compared to Oceanside. (Early July (month 7) to late September (month 9))
Coastal Environments
Reference Number 98-11
3-19 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
5000
4000
$; 3000 I
-<U 11
32: 11
11
ct) 2000 11
11 -g_ 11
I I -( I
1000 I
t! ~ I ,, I 0 11 I I I I I a. I! / 1', ' : I
I
(I) 0 C ---T-r•,r~-~r---------
Jg I I 1. I I I I
1I I I II If I I II
-1000 I V
Q) I I ' •• -,,
0 11 .c ,,
(I) -2000 -I 0)
C .9 -3000
-4000
-5000
'1 ,,
i J.
,, ,,
11
'1
'1 ,, ,, ,,
1,
~ I I I
l ,,
,, ' I 1' I
I I '1 ~ ~ 11 .. ,, 1111
1' I I 1 '11
/ I t I I '1 I
I \ : \ I \ I I I I I I
I \ : ~ I
I --1 r--:
• I : .! Ii 1, I /1, ,. I
I •I ~I I
\
I I
I
I I ~
11
,1 ,,
:: l ,, ,,,
I 111
I 1111
I I II ~~
I I
I I \-----1-
1 I
Carlsbad
Oceanside
l
' -II
II
I 11
• : ~i
II I I
II I I
II I I II I II I I
II ! I
I I I , , ,
I 't
I II
I 11
I II _ _J
I
I
t
78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
Figure 3-10. Comparison of monthly daily mean values of longshore transport potential between Carlsbad and Oceanside from
1978 through 1994.
Coastal Environments
Reference Number 98-11
3-20 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
100
Winter
eo
20
0 -------.---.---.-...-....-..--..---....-...-........ ,......, ....... --.-,.__,. .............. ....,..--.--.---.--
<-10 -s -e -7 -6 -s -4 -3 -2 -1 o 2 3 4
Summer
j eo
0 ...., _____ ..,.... ....................................... ..---....--...-........ ,......,-.--.----,.---r---r ............. --.--
<-10 -s -e -7 -6 -s -4 -3 -2 -, o 2 3 4
100
Combined
E ED
~ (I) a.. 40
20
0
< -10 -9 -B -7 -6 -5 -4 -3 -2 -1 0 2 3 4
.. s Longshore Transport, 108 yd3/day N ..
Figure 3-11. Cumulative probability (expressed as a percentage) of longshore transport (QJ
during the winter, summer, and combined winter and summer data at Carlsbad,
giving the probability that Q, s some given value, q.
Coastal Environments
Reference Number 98-11
3-21 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 3-5. Characteristics of longshore transport at Agua Hedionda Lagoon, Carlsbad from 1978 to 1994, yd3/day (m3/day).
North1
Season
Mean Std N'
Summer 765 667 952 (585) (510)
Winter 825 1,041 428 (631) (796)
Combined 784 802 1,380 (599) (613)
Transport to the North per year
Transport to the South per year
Gross Transport per year
Net Transport per year
Coastal Environments
Reference Number 98-11
Southb
Mean Std
-1,398 2,155
(-1,069) (1,648)
-2,163 3,829
(-1,654) (2,928)
-1,865 3,300
(-1,426) (2,523)
113,059
(86,445)
-425,316
(-325,197)
538,375
(411,641)
-312,257
(-238,752)
Totalc Absoluted
N Mean Std N Mean Std N
966 -323 1,928 1,923 1,081 1,629 1,923 (-247) (1,474) (827) (1,246)
1,513 -1,500 3,629 1,946 1,863 3,457 1,946 (-1,147) (2,775) (1,424) (2,643)
2,479 -915 2,970 3,869 1,475 2,736 3,869 (-700) (2,271) (1,128) (1,092) .
a -Longshore transport to the north
b -Longshore transport to the south
c -Combined north and south longshore transport
d -Absolute value of longshore transport
e -N = Number of days
3-22 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 3-6. Comparison between the longshore sand transport at Oceanside and Carlsbad.
Variable
Mean north transport per day
Mean south transport per day
Transport to the north per year
Transport to the south per year
Gross transport per year
Net transport per year
Coastal Environments
Reference Number 98-11
Oceanside
yd3 m3
1,757 1,343
-2,036 -1,557
320,625 245,150
-371,570 -284,102
692,195 529,252
-50,945 -38,953
3-23
Carlsbad
yd3 m3
784 599
-1,865 -1,426
113,059 86,445
-425,316 -325, 197
538,375 411,642
-312,257 -238,752
Final Report
Study of Sediment Transport Conditions, Agua Hedlonda Lagoon, Carlsbad, CA
3.5. CROSS-SHORE SAND TRANSPORT
There is also a seasonal transport of sand on-and offshore, with sand moving offshore from
the beach to the bar in the winter and returning to the beach in summer. The magnitude of this
migration at Torrey Pines, as estimated from beach-profile data, is 110 yd3/yd (90 m3/m)
(Nordstrom and Inman, 1975). The sand in the offshore surf zone, at depths of about 9 to 30 ft
(3 to 10 m), is subject to re-suspension by waves and can be transported by longshore currents
(coastal currents). Therefore, the sand may not return to the beach in the same location from
which it left.
Coastal Environments
Reference Number 98-11
3-24 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
4. SHORELINE AND PROFILE CHANGES FROM
OCEANSIDE TO ENCINITAS
4.1. COASTAL SETTING
The beaches along the north San Diego County coast are relatively narrow, and mostly
backed by cliffs. The narrow beaches provide little protection to the cliffs, which are subject to
:wave action. The cliffs are the seaward part of the coastal marine terraces (mesas), raised by
tectonic uplift and eroded by wave forces that attack the base of the cliffs. Gaps occur in the
cliffed coast at lowlands that are predominately located in the areas of estuaries or lagoons. The
lowlands along the study area (Oceanside to Moonlight Beach, Encinitas) are located at the San
Luis Rey River, Alta Loma Creek, Buena Vista Lagoon, Agua Hedionda Lagoon, and Batiquitos
Lagoon.
Cliffs back most of the beaches from Oceanside to Moonlight Beach. Cliff heights range
from about 15 ft to over 100 ft (USACE, 1987a). These cliffs are composed of a wide variety of
material, including sandstone, mudstone, and shale. This soft material composition makes the
cliffs susceptible to erosion and collapse from wave action.
During the winter, steep waves erode the beaches and in the summer long waves restore the
beaches by returning the sand removed during the previous winter. The local beaches contain
both sand and cobbles, which along most of the shoreline, underlie the sand. The cobbles are
brought to the beach by river discharge or by cliff erosion. The cobbles are often exposed during
the winter when the beach sand moves to the offshore bar.
4.2. SHORELINE AND BEACH-PROFILE DATA
Historical shoreline data and recent beach-profile data were analyzed for this study. The
historical data cover the time period between 1887/88 and 1982. The more recent data cover the
time period of the 1980s and 1990s. Historical shoreline positions were compiled and mapped in
a joint effort between the USACE and the National Oceanic Atmospheric Administration
(NOAA) (NOAA/NOS-COE/LAD, 1985). In this series of maps, the shoreline is defined as the
Mean High Water Line (MHWL). The map, covering about 95 years of shoreline changes
contains five surveys during 1887, 1934, 1960, 1972, and March 1982. The first four surveys
were field surveys and the fifth survey, in 1982, was estimated from aerial photography. The
objective of the NOAA/NOS maps was to give an overall view of the regional shoreline changes,
rather than specific site changes. The data are limited in application to coastal reaches where
shoreline position changes were large (greater than 150 ft). The limitations of this valuable
information are discussed by the USA CE (1987b ).
Coastal Environments
Reference Number 98-11
4-1 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Shoreline changes have been documented through beach-profile surveys conducted by the
USACE, the City of Carlsbad, SANDAG, and SIO from 1934 until present. The profiles prior to
1982 were sparse in time and space and of limited use for assessing shoreline changes. Beach
profiles after 1984 have been surveyed more frequently and regularly, and provide adequate data
for assessing more recent shoreline change. Therefore, key beach-profile survey data from 1982
to 1998 from Oceanside to Encinitas were analyzed. The relative beach width at each station
was determined by calculating the distance from the benchmark to the 0-ft NGVD position (0 ft
NGVD = +2.56 ft MLLW referenced to the La Jolla tide gauge). The benchmarks are not
necessarily located just at the inland boundary of the beach however, the distance from the
benchmark to O ft NGVD was referred to as the "beach width" in this study.
4.3. SHORELINE CHANGES
4.3.J. Historical Shoreline Changes (1887-1982)
Prior to the 1920s and 1930s, the rivers and streams along the Oceanside Littoral Cell
coastline provided a significant source of sediment to the local beaches (Inman and Jenkins,
1983; Inman, 1985). These rivers supplied beaches with sand on an irregular, episodic basis,
primarily during wet time periods. Since then, many of the major rivers have been dammed and
urbanization has limited erosion. These factors have combined to reduce the amount of sediment
reaching the coast.
Inspection of the historical shoreline positions of 1887 /88, 1934, 1960, 1972, and March
1982 from the NOAA/NOS map (1985) show that the largest shoreline changes within the study
area were in the vicinity of Oceanside Harbor and the Encina Power Plant. Changes in other
areas were smaller. The errors associated with areas of small shoreline change may be larger
than the observed change, making it difficult to determine actual shoreline advance or recession.
City of Oceanside (Santa Margarita River to Buena Vista Lagoon)
Oceanside Harbor is a major coastal structure that intercepts the longshore sand transport and
diverts the flow of sand offshore. The major structures of Oceanside Harbor (basins; north and
south breakwaters) were constructed between 1942 and 1962 (Figure 4-1). During this period,
the beach north of the harbor accreted and the beach south of the harbor eroded.
Comparing the 1887/88 survey to the 1934 survey, presented in part in Figure 4-2, the shoreline
immediately north of the harbor advanced about 200 ft, tapering to a 50-ft advance 15,000 ft
north of the harbor. The following survey in 1960 shows further shoreline advance (over 400 ft)
just north of the harbor location and about a 50-ft advance at 15,000 ft north. The 1972 and 1982
surveys show minimal, if any, change in the shoreline position north of the harbor compared to
the 1960 shoreline position.
Coastal Environments
Reference Number 98-11
4-2 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Camp Pendleton
North
Fillet Beach ·-······ ..
N
Del Mar
Boat Basin
"' I
North Breakwater
Extension
Pacific Ocean
I
1000 0 1000 ft
~ -----
... Cll > ir
>. Cll a:
en ·:5
_J
c:: tlS Cl)
South Groin
1942
1957/58
1961
1962
1968
1973
1976
1994
Figure 4-1. History of Oceanside Harbor construction and Improvements.
Modified from Inman and Jenkins (1983).
Coastal Environments
Reference Number 98-11
4-3 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
South of Harbor Beach (Figure 4-2) to about 8,500 ft south, the shoreline has advanced from
its 1887/88 position. From 1887/88 to 1934, the shoreline in this area advanced about 200 ft. By
1960, the shoreline had receded 200 ft, back to the 1887/88 position, except near the pier where
the shoreline receded only about 100 ft. This large change in shoreline position is probably
because of the extension of the north breakwater in 1958. By 1982, the shoreline advanced to
the seaward-most position and was generally at the same location as the 1934 shoreline in
response to the sand bypassing and depositing of the dredged material to the south. From
8,500 ft south of Oceanside Harbor to Buena Vista Lagoon, shoreline changes were small with
slight advance in some areas between 1887 /88 and 1982.
Carlsbad (Buena Vista Lagoon to Batiguitos Lagoon)
From Buena Vista Lagoon to about 1,000 ft north of Agua Hedionda Lagoon, the shoreline
generally remained at the same location from 1887/88 to 1982. From the north jetty at Agua
Hedionda Lagoon to about 1,000 ft north there are small changes in the shoreline position. From
1887 /88 to 1934, the shoreline position receded about 100 ft. The construction of the jetties at
the inlet and discharge channel in 1954 stabilized North, Middle, and South Beaches as shown in
Figure 4-3 and 4-4. From 1934 to 1960 the shoreline advanced about 100 to 200 ft and remained
in about the same position through 1982. South of Agua Hedionda Lagoon to Terra Mar Point,
the shoreline has consistently advanced from 1887/88 to 1982 (about 200 ft total). South of
Terra Mar Point to Batiquitos Lagoon, the shoreline has not significantly advanced or receded
over the 1887 /88 to 1982 period of record.
Encinitas (Batiquitos Lagoon to Moonlight Beach, Encinitas)
Changes in the MHWL in this reach were within the practical uncertainty bounds of the
NOAA/NOS map. This segment of shoreline likely exhibited small shoreline changes between
these historical surveys. The entire shoreline along this reach is backed by cliffs. The
NOAA/NOS map shows the 1982 shoreline position about 100 ft seaward of the 1887/88
position. Tue intermittent surveys show some advance and some recession along different
segments of this reach.
4.3.2. Recent Beach Width Changes from Beach-Profile Surveys
Beach profiles are measurements of distance and depth starting at a fixed point or benchmark
located on the beach and extend perpendicular offshore. Beach profiles provide information
about bathymetry changes. Beach profiles vary with location (spatial) and with time (temporal).
The spatial changes depend on the distribution of wave heights, the geology of the area, shelf
width, grain size of the sand or cobbles, and sand supplies. The temporal changes are either
inter-annual or seasonal (winter and summer profiles) and depend mostly on wave height.
Coastal Environments
Reference Number 98-11
4-4 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Oceanside
Harbor
N
~
500
400
:::
fl) 300
.5 e 0 200 &:; CJ)
100
0
-100 ,..___.__..._..__......._ ......... ___. ___ .....__._ __
-6 -5 -4 -3 ·2 -1 0 2 3 4
North Distance, mi South
March 1982 NOS Aerial Photography
---1972 Field Survey
1960 Field Survey
1934 Field Survey
• •• •• ·. • •• ••• • 1887-88 Field Survey
Oceanside Pier
0 1500 ft
0 soom
Figure 4-2. Shoreline positions before and after Oceanside Harbor was constructed. Modified
from NOAA/NOS map (1985).
Coastal Environments
Reference Number 98-11
4-5 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Pacific
Ocean
March 1982 NOS
1972 Field Survey
1960 Field Survey
1934 Field Survey
1887/88 Field Survey
0 1500 ft
0 500m
21'
CARLSBAD
N
t
117° 20'
09'
08'
33° 07' 30"
Figure 4-3. Shoreline positions at Agua Hedionda Lagoon for the years 1887/88, 1934, 1972,
and 1982. Modified from NOAA/NOS map (1985).
Coastal Environments
Reference Number 98-11
4-6 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
-------------------------------------, 09'
0
0
Pacific
Ocean
March 1982 NOS
1972 Field Survey
1960 Field Survey
1934 Field Survey
1887/88 Field Survey
1500 ft
500 m
N
t
21 ' 117° 20'
33° 08'
Figure 4-4. Enlargement of shoreline positions directly adjacent to Agua Hedionda Lagoon.
Coastal Environments
Reference Number 98-11
4-7 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Shoreline change rates were calculated from available beach-profile data collected by the
USACE, the City of Carlsbad, SANDAG, and Scripps Institution of Oceanography. The
locations of the beach-profile stations used in this sect.ion are shown in Figure 4-5, presented as
dashed lines. The rate of shoreline change is estimated with a regression analysis of beach width
versus time. Figures 4-6 and 4-7 show plots of beach width versus time and the best-fit line. A
statistical significance test was carried out for each slope value. The null hypothesis is that the
slopes are equal to zero (H0: slope = 0). The resulting p-value represents the probability of
rejecting the null hypothesis when it is true. The results of the statistical tests are presented in
Table 4-1, which lists the shoreline-change rates for various ranges, start and end years of the
time period of the available data, and the number of profiles used in the analysis. The only
statistically significant result obtained from the regression test was at OS-1030 in Oceanside,
which shows that the area located south of Oceanside Harbor is eroding at a rate of -5.1 ft/yr.
Oceanside (Santa Margarita River to Buena Vista Lagoon)
The beach profiles analyzed within this reach are from 1982 to 1998 at ranges OS-1030,
OS-1000, and OS-0930. The average rate of shoreline change for this reach is -2.94 ft/yr, where
the negative sign indicates an erosion rate.
Carlsbad (Buena Vista Lagoon to Batiquitos Lagoon)
Beach-profile survey data within this reach were collected at ranges CB-0880, CB-0850,
CB-0840, CB-0830, CB-0820, CB-0800, and CB-0760. Each of these ranges has beach-profile
data from 1987 through 1994 or 1997. Range CB-0880 is located just south of Buena Vista
Lagoon. Ranges CB-0850 through CB-0830 are located north of Agua Hedionda Lagoon, along
North Beach. The beach-profile data at these ranges exhibit similar trends in shoreline change
with an average erosional rate of about -0.45 ft/yr.
Range CB-0820 is located on Middle Beach, between the intake and discharge channels of Agua
Hedionda Lagoon. Sediment dredged from the lagoon has been regularly placed on Middle
Beach, which has accreted at a steady rate. As shown in Figure 4-6, the NGVD shoreline
position has been advancing since 1987. Due to the effects of both the jetties and the fairly
consistent sand placement, the rate of shoreline change at Middle Beach is +5.8 ft/yr (marginally
statistically significant). Farther south of the discharge channel is range CB-0800, located on
Terra Mar Point. This range has been advancing at a rate of+ 1.2 ft/yr since 1970.
Station CB-0760 is located north of Batiquitos Lagoon. From 1984 through 1993, the
shoreline has been relatively stable with a slight advance rate of less than +o.23 ft/yr. In 1994,
the shoreline advanced over 150 ft from the 1993-shoreline position in response to the placement
of dredged material from Batiquitos Lagoon in this area. Shoreline change rate at CB-07 40 and
CB-0720 were +3.68 and -2.98 ft/yr, respectively.
Coastal Environments
Reference Number 98-11
4-8 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Pacific
Ocean
Profiles used
in this study
W443 W445
DM590
TP520
W450 W460
N
0 20,000 ft
0 5,000 m
Figure 4-5. Location of beach-profile ranges along the Oceanside Littoral Cell. Solid lines
represent ranges with available beach-profile data and the dashed lines represent
the ranges analyzed for this study.
Coastal Environments
Reference Number 98-11
4-9 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
CB 0850
300
200
100
S' ...., 0
G) •
C 400 ~ CB 0830 L.
0 300 .c u, •
'O 200
C al 100
~
•--------~~------------~
al 0 E .c 0 400
C a, CB 0820
m 300
C G) 200 I 100 .a
G) 0 0 C s 400
ID ·-CB 0800
0 300
200
100 -____ ,,,,.,._ -----·-------------------------------~ .... --··-·--;:...--, ------·
0
B2
Figure 4-6. Beach width versus time for profile ranges at North, Middle, and South Beaches in
Carlsbad. The best-fit line represents the rate of shoreline change.
Coastal Environments
Reference Number 98-11
4-10 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
400
300 ~ .... v .. ~ :;.1.:........ .. . . ······ .. ··-'~
200
100
...... E o......,.....,,....._..--.-_,----.--.---.--.---.----------.....-----'
Q)
C: ·-e
0 .c
CJ)
"C ;
~ L.
a1 E .c 0
C: G) m
C:
(D
i .0
B C: lll ..
.! C
400
OS 0930
300
200 \y .. ~·······················
100
0-------....--------....---------------' 82
CB 0720
300
200
100
0"'""""-....---.---..-----.--.--........ --.---..----.---.--...----,..----------
400---------------------------
CB 0670
300
200
100
V--------" r _____ .. __________________________________ ---. "'./ v-. --=---..,...
0',--.----.--..----...----------.-........ --.---T---.---..----,1
82
Figure 4-7. Beach width versus time for Oceanside and South Carlsbad profile ranges.
The best-fit line represents the rate of shoreline change.
Coastal Environments
Reference Number 98-11
4-11 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 4-1. Shoreline change rates from Oceanside to Encinitas.
Profile Location Dates of #of Rate
Name Surveys Surveys (ft/yr)
OS-1030 Oceanside 1982-1998 15 -5.11
OS-1000 Oceanside 1982-1998 17 -3.48
OS-0930 Oceanside 1983-1998 26 0.14
CB-0880 North Carlsbad 1987-1998 18 -0.25
CB-0850 North Beach 1987-1997 17 -1.04
CB-0840 North Beach 1987-1997 18 -0.1
CB-0830 North Beach 1984-1998 28 -0.4
CB-0820 Middle Beach 1987-1997 18 5.82
CB-0800 South Beach 1982-1997 12 1.17
CB-0760b South Carlsbad 1983-1998 27 0.23
CB-0740c South Carlsbad 1987-1997 18 3.68
CB-0720c South Carlsbad 1982-1998 29 -2.98
SD-0670 Encinitas 1983-1989 14 -3.06
a. P-value less than 0.05 indicates statistically significant shoreline change rates.
b. Data collected from 1994 onward are not included in the regression analysis
(sand added to the beach).
c. Data collected from 1995 onward are not included in the regression analysis
(sand added to the beach).
Coastal Environments
Reference Number 98-11
4-12
P-valuea
0.0300
0.1693
-0.9420
0.9050
0.6434
0.9591
0.7924
0.0611
0.2927
0.8553
0.1327
0.1252
0.1067
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Encinitas {Batiquitos Lagoon to Moonlight Beach)
Beach-profile stations within this reach include CB-0720 and SD-0670. CB-0720 is located
just south of Batiquitos Lagoon. Since 1981, the shoreline at this range has been eroding at a
rate of -2.98 ft/yr. In 1995, some of the dredged sand from Batiquitos Lagoon was placed at this
range (Figure 4-7) resulting in significant beach advance of over 150 ft. Station SD-0670 is
located at Moonlight Beach. Based on the available data at SD-0670, the rate of shoreline
change is about -3.06 ft/yr.
4.4. SEASONAL BEACH-PROFILE CHANGES
Figure 4-8 shows a comparison between profiles taken at various locations from Oceanside
to Del Mar during summer (upper plot) and winter (lower plot) seasons. The differences in the
shape of the profiles, offshore of the closure depth (about 30-ft water depth), are because of the
variation of the shelf width from one location to another. Carlsbad, Encinitas, and Del Mar
beaches have a narrower shelf than Oceanside beaches. Typical seasonal cycles for Oceanside,
Encinitas, and Del Mar beaches are shown in Figure 4-9. Seasonal cycles on North, Middle, and
South Beaches are shown in Figure 4-10. Notice that ranges CB-0830 and CB-0850 are wider in
the winter than in the summer. This is because of the direction of the longshore transport. In the
winter, the longshore transport is to the south (sand accumulates at range CB-0830) and in the
summer, the longshore sand transport is to the north (sand moves north from range CB-0830).
Estimates of berm height, beach slope, beach width. ( defined as the distance between the
benchmark and the 0-ft NGVD elevation), and the seasonal cycle are estimated from the
available beach-profile data. The seasonal cycles for those beaches are also shown in
Figures 4-6 and 4-7 as the beach-width variable fluctuates above and below the best-fit line. The
results are summarized in Table 4-2 along with the median grain-size value. The grain-size data
were obtained from Woodward-Clyde Consultants (1996a, 1996b).
Coastal Environments
Reference Number 98-11
4-13 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
,------------------------------------------T20 SUMMER
_Oceanside (OS-1030 Se,>-a7)
.• _ •.•. South Oceanside (OS-0930 Sep-87)
--Ca~sbad Mid Bch (CB-0820 Oct-92)
____ Batiqultos Lagoon (CB-0720 Ocl-92)
_.....__Moonlight Beach (S0-0670 Sep-87)
__ Del Mar (DM-0560 Sep-87)
10
0
-10
-20
-30
Q > Cl z
£
C ~ II > .!! w
1---.----.-..--,---;1---,---.--,---,--t-..---.---,-...,....--+--..-.....---.--..--+---.-...--.--.--+---.--..----+-SD
3,000 2,500 2,000 1,500 1,000 500 0
,---===.,.------------------------------------,-2D WINTER
_ Oceanside (OS-1030 Apr-87)
..•••.. South Oceana Ide (OS-0930 Apr-87)
__ Carlsbad Mid Bch (CB-0820 Apr-92)
____ Batiquttos Lagoon (CB-0720 Apr-92)
_~_Moonlight Beach (SD-0670 Apr-87)
__ Del Mar (OM-0560 Apr-87)
1D
0
-1D i;
z e:
~ -20 •
-30
: iii
1-----.--..... -f--.----,----.--1--.----.----1---..... --.....---+-..-----.---+-..------+ -50
3,000 2,500 2,000 1,500 1,000 500 0
Distance from Benc:hmark (ft)
Figure 4-8. Comparison of beach profiles from Oceanside to Del Mar during summer (top) and
winter (bottom).
Coastal Environments
Reference Number 98-11
4-14 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Car1sbad, CA
20
OCEANSIDE (OS-1030)
1117
10
__ SUMMER
•• __ WINTER I c > Cl .,o z
£
C ~ -20 .. > • w
-30
-·~-~
-40
-50
3,000 2,500 1,000 1,!DO 1,000 500 0
20
ENCINITAS (SD-0670)
1987
ID
__ SUMMER
••• _WINTER • c > CJ -10 z
~
C 0
-20 i II iii
-30
-40
-50
3,000 2,500 2,000 1,500 1,000 500 0
20
DEL MAR (DM-0580)
1117
10
__ SUMMER
•••• WINTER 0
Q > Cl -10 z
~
C 0
-20 ;,
!: • w
-30
-40
-50
3,000 Z,500 2,000 1,IOO 1,000 500 0
Distance from Banchmark (fl)
Figure 4-9. Typical beach profiles showing the seasonal cycles for Oceanside (top), Encinitas
(middle), and Del Mar {bottom).
Coastal Environments 4-15 Final Report
Reference Number 98-11
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Car1sbad, CA
2D
CB-0150 (1910)
10 • • • Winier
-summer
0
-10 ~ z
£
-20 & i . .. w -30
•40
·50
3,000 2,500 2,000 1,500 1,000 500
20
CB-OUO (1990)
• • • Winter 10
--Summer D
Q >
·10 " z
£ C -20 0 j
·30 w
-40
3,000 2,500 2,000 1,500 1,000 50D
20
CB-0120 (1990)
10 • • • Winier
-summer
g
-1D Cl z
£ C ·2D 0 = j
·30 "'
·4D
-SD
3,000 2,500 2,000 1,500 1,000 500 0
20
C&•0I00 (1917)
10 • • • Winter
-summer 0 g
·10 Cl z
£
-20 & j
-30 ~
·40 ..
-50
3,000 2,500 2,000 1 ,5D0 1,000 50D a
Figure 4-10. Typical beach profiles showing the seasonal cycles at Carlsbad stations CB-0850
(North Beach), CB-0830 (North Beach), CB-0820 (Middle Beach), and CB-0800
(south of South Beach).
Coastal Environments 4-16 Final Report
Reference Number 98-11
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 4-2. Characteristics of beach profiles from Camp Pendleton to Del Mar.
Profile Name Location
PN-111'0 Camp Pendleton
OS-1030 Oceanside
OS-1000 Oceanside
OS-0930 Oceanside
CB-0880 North Carlsbad
CB-0850 North Beach
CB-0840 North Beach
CB-0830 North Beach
CB-0820 Middle Beach
CB-0800 South of
South Beach
CB-0760 South Carlsbad
CB-0740 South Carlsbad
CB-0720 South Carlsbad
SD-0670 Encinitas
DM-580 Del Mar
a -µ is equal to one micron
b -missing value
c -number of surveys
d -no clearly defined value
Coastal Environments
Reference Number 98-11
Berm
Height
(ft, NGVD)
10.2
9.4
8.4
..
7.0
9.8
..
7.4
6.8
..
..
..
..
7.4
7.5
Mean Beach Beach Width (ft)
Grain Size
{µ)a Slope Max Min Ave NC
b 1:20 830.9 293.9 528.8 27
230 1:15.2 461.6 208.5 289.0 29
.. 307.6 142.4 209.7 17
1:19.9 286.8 · 142.7 212.5 26
1:12.5 189.1 90.3 144.4 18
240 1 :11.1 308.7 188.6 277.5 17
240 1:14.3 150.3 68.4 108.6 18
240 1:12.5 221.2 62.7 110.6 28
260 1:14.9 254.2 111.8 177.6 18
.. 106.4 52.9 83.8 12
300 1:20.3 276.8 99.9 143.8 27
.. 264.6 130.4 180.0 18
250 1:14.3 371.6 162.9 211.2 29
460 1:16 199.5 85.25 134.6 14
200 1:18 215 30 150 80
4-17
Seasonal
Cycle (ft)
d ..
40.4
66.1
38.6
43.1
37.2
34.6
..
..
17.8
26.6
18.2
49.8
50.8
140
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
5. EFFECT OF THE ENCINA POWER PLANT AND AGUA HEDIONDA
LAGOON ON NATURAL SAND TRANSPORT
This chapter discusses the effect of the Encina Power Plant and Agua Hedionda Lagoon on
the natural sediment transport and on the local beaches. Two main studies prepared for SDG&E
that have investigated Agua Hedionda Lagoon and the effects on local coastal processes were
reviewed and compared with data acquired for this study. These two studies are Jenkins and
Skelly (1988) and Jenkins, et al. (1989). The 1988 study addresses two aspects of the lagoon's
effects on littoral transport: trapping the beach sediment from the tidal currents flowing into the
lagoon and trapping sediment at the intake jetties. The second study by Jenkins, et al. (1989)
addresses the potential of the thermal discharge plume flowing through the discharge channel to
transport littoral sediment offshore.
The studies concluded that 1) the jetties have a minimal trapping effect on sediment located
immediately adjacent to the jetties on the updrift side because there is no evidence of a sand fillet
formation similar to the fillet north of Oceanside Harbor; and 2) there is no offshore sand
diversion by the jetties or the thermal discharge plume based on the historical bathymetry
surveys conducted in the area and current measurements made at the intake channel.
5.1. SEDIMENT TRAPPED BY TIDAL CURRENTS
The actual asymmetry in the transport of sand into and out of Agua Hedionda Lagoon
resulting from tidal currents and diversion of seawater by the power plant can be estimated from
sand-volume measurements made during dredging of the lagoon basins. The lagoon has been
dredged about every two to three years to keep the lagoon open and to provide adequate flow for
the power plant cooling system.
The dredging history of Agua Hedionda Lagoon is given in Table 2-3. From this table, the
average annual rate of dredging over the 43 years from 1954 to 1997 is about 138,000 yd3/yr.
This average must correspond closely to the average net amount of sand that enters the lagoon
per year, even if the dredging did not exactly match the inflow year by year, otherwise the
lagoon would be significantly overfilling or dredged deeper than the original design depth. In
1998, the Middle and Inner Basins were re-dredged to the original design depth. Approximately
270,000 yd3 were dredged from these basins. This dredged sediment was deposited in the
Middle and Inner Basins from sediment runoff from Agua Hedionda Creek, local runoff from
surrounding land, and sand entering from the littoral transport from the Outer Basin. The
dredged volume is equivalent to about 6,000 yd3/yr since 1954 or about 4% of the average
annual dredging rate and is considered insignificant. The material dredged from the Outer Basin
of the lagoon has been found to be composed of well-sorted beach sand, which suggests that all
the material dredged from the Outer Basin had entered during the tidal inflow.
Coastal Environments
Reference Number 98-11
5-1 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, C1;1rlsbad, CA
The mean value of the absolute longshore transport (without regard to direction) was
calculated to be 538,000 yd3/yr (411,350 m3/yr). Therefore, 25.6% of the total longshore sand
transport moving past the lagoon entrance is trapped inside the lagoon. An equal proportion of
this sand is taken from longshore sand transport moving both upcoast (north) and downcoast
(south), depending on the time of year. The trapped sand is dredged from the lagoon and placed
on the local beaches in discrete quantities of 200,000 to 400,000 yd3 every two to three years
instead of as a continuous flow. If the material were not returned to the beach system, the effects
of continuous trapping of sand would probably appear, if at all, as the narrowing of beaches
downcoast and/or upcoast from the lagoon entrance. The effect of the power plant operation and
subsequent lagoon sedimentation on the beach is reduced with distance from the intake channel
as discussed in Section 6.2.
5.2. SEDIMENT TRAPPING AND DIVERSION BY INLET AND DISCHARGE JETTIES
In 1954, about 4,279,000 yd3 of material was dredged from Agua Hedionda Lagoon and
deposited on the beach directly opposite the lagoon, initially extending the beach seaward by
about 395 ft. Jetties were built across this deposit on either side of the natural entrance, and on
either side of the new discharge channel from the power plant. These jetties were later shortened
as the sediment deposit slowly receded. The present dimensions of the intake and discharge
jetties are shown in Figure 2-1.
According to SDG&E (Jenkins and Skelly, 1988, Figures 10 and 11 ), the intake jetties extend
228 ft and 205 ft beyond the "high-water mark," and the discharge jetties extend 270 ft and
227 ft beyond the "high-water mark." However, because of the natural advance or recession of
the mean high-water position (e.g., from seasonal changes or storms) the length that the jetties
extend offshore from the mean high-water shoreline position varies from 145 ft to 228 ft. The
discharge channel jetties can vary from 170 ft to 270 ft offshore from the mean high-water
shoreline. Beach profiles at Carlsbad show that the surf zone is about 300 ft wide, measured
seaward from the 0-ft NGVD shoreline.
Jenkins and Skelly (1988) have considered different types of evidence to determine whether
the intake and discharge jetties interfere substantially with the longshore transport of sand, and
they conclude that there is no measurable effect. Their principal argument is that the jetties are
short and do not extend beyond the surf zone at those times of comparatively high waves when
the majority oflongshore transport occurs.
Jenkins and Skelly estimate the width of the surf zone(£) from the following formula:
f = Hsf(0. 78 tan p ), (6-1)
where Hs is the significant wave height and tan P is the slope of the shore. This equation can
also be written as:
Coastal Environments
Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
!. == 46. lHs, (6-2)
for the local foreshore slope tan p = 0.0278 (Osborne, et al., 1983). By this reckoning, the surf
zone will extend seaward of the longer intake jetty when the breaker height exceeds 5.9 ft, and
beyond the longer discharge jetty for breaker heights exceeding 4.8 ft.
Estimates of sediment transport by Seymour, et al. (1982-88) are cited by Jenkins and Skelly
to show that more than 85% of longshore sand transport occurs when the wave height is 5. 9 ft or
greater. The wave heights reported by Seymour et al. (1982-88) refer to a water depth of about
32 ft rather than referencing the breaker heights. Wave heights increase as waves approach the
shore. The increase of wave height from the 32-ft to the 6.5-ft water depth is period dependent.
For waves with a period greater than 7 seconds, the wave height at a water depth of 32 ft is less
than the wave height at the wave break point. This implies that the jetties are not long enough to
intercept the majority of the longshore sand transport.
Jenkins and Skelly (1988) cited evidence from sand-tracer experiments in I 979 showing that
about 470 yd3/day (360 m3/day) of sand bypassed the inlet jetties from August to December.
During this time, the longshore transport changes direction from the northward summer transport
to the southward winter transport. Also, from analyzing historical bathymetric surveys, beach
profiles, and vibracore samples, they showed the absence of notable sediment accumulation off
the jetties. While neither of these lines of evidence are strong, they substantiate the main
argument that most of the total longshore transport bypasses the jetties because the length of the
jetties are less than the surf-zone width. For these reasons, the effect of the jetties on the
longshore transport is small.
5.3. SEDIMENT TRAPPING AND DIVERSION BY THE THERMAL DISCHARGE
PLUME
The discharge plume will entrain and divert the longshore sand transport offshore only if the
discharge has a large enough momentum. Jenkins, et al. (1989) investigated the current and
temperature fields around the discharge jetties, to see if the discharge plume might carry sand
offshore.
Two seven-hour current-meter deployments on two separate dates (28 February 1989 and
7 March 1989) were conducted at stations located a distance of 374 to 748 ft (114 to 228 m)
directly offshore from the discharge. The deployments recorded currents ranging from 0.56 to
1.21 ft/sec (17 to 3 7 cm/sec), directed essentially down coast ( southward), with a small cross-
shore component directed shoreward.
Jenkins et al. (1989) mapped the heated discharge plume on the same two dates with a chain
of thermistors at the surface and at depths 2 ft, 4 ft, and 6 ft, together with measurements on the
bottom. On 28 February 1989, the 60.8-°F (16-°C) surface temperature contour enclosed an area
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Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda lagoon, Cartsbad, CA
of about 0.19 mi2 (0.5 km2), with lobes offshore and downcoast. The maps at depths 2 ft and 4 ft
were like the surface map, but at the 6-:ft depth, the 60.8-°F (16-°C) contour enclosed only about
0.054 mi2 (0.14 km2), and on the bottom only 0.019 m.i2 (0.05 km2) near the discharge. The
contours on 7 March 1989 were generally similar, but more complicated, showing, like those of
28 February 1989, a discharge in the form of a buoyant plume (original T = 50 °F (10 °C))
spreading on the surface, and interacting minimally with the waters close to the bottom. From
these direct observations of essentially no offshore velocity and no temperature rise at the bottom
beyond about 394 ft (120 m) from the discharge, Jenkins et al. (1989) concluded reasonably that
the discharge thermal plume does not extend deep enough to carry near-bottom suspended.
sediments offshore, and that it does not extend far enough to carry offshore momentum beyond
the surf zone.
Coastal Environments
Reference Number 98-11
5-4 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Car1sbad, CA
6. SAND-PLACEMENT OPTIONS
Southern California sandy beaches have a large recreational value and serve as a protective
barrier from waves that actively erode and flood the backshore. Sand is a valuable commodity in
southern California Beach-quality sand is not readily accessible in large quantities and when it
is found, transportation and placement costs are relatively high. The cost to place sand from an
offshore borrow area on the beach is approximately $8 per cubic yard, whereas to place sand
from an inland source, the cost ranges from $10 to $16 per cubic yard, depending on the
locations of the source and the placement site. Careful design of beach fills are needed to
minimize sand losses and maximize the benefits. Understanding the dynamics of beaches and
the movement of the sand are important to assess the best placement option(s) for the sand
dredged from Agua Hedionda Lagoon on the stretch of coast between Oceanside Harbor and
Moonlight Beach.
In Sections 6.1 and 6.2, sand movement and predictions along the coast are discussed. In ..
Sections 6.3 and 6.4, previous sand-placement projects conducted by SDG&E are reviewed to
evaluate the response of the beaches to the added beach sand, the fate of the sand, and the
longevity of placed sand. Section 6.5 discusses the effects of the operation of Encina Power
Plant on the beaches. Various options for placement of the dredged sand from Agua Hedionda
Lagoon on the beaches between Oceanside Harbor and Moonlight Beach are presented in
Section 6.6. Section 6-7 discusses stable disposal sites along the study area.
6.1. SAND MOVEMENT
Nearshore sand movement is complicated and is not fully understood (Hicks and Inman,
1987; White, 1987; Inman, 1993; and Conley and Inman, 1994). Sand on the beach moves in
both the longshore and cross-shore directions. Sand movement depends on many factors
including waves, currents, grain size, and beach slope. · Each of these factors vary spatially with
distance from the shoreline. Sediment transport mechanisms vary depending on whether sand is
inside the surf zone, just offshore of the surf zone, or farther offshore. The surf zone is defined
as the area between the wave breakpoint and the highest run-up of water on the beach. Along
southern California, the surf-zone width is usually between 300 and 600 ft. It can be narrower
during calm wave conditions and wider during large wave storms. Also, the width of the surf
zone depends on the tides, beach slope, storm surge, and mean sea level. For the study area, the
water depth at the offshore end of the surf zone can be up to 10 ft.
Inside the surf zone, the predominant forcing functions for sand movement are the waves and
wave-induced longshore current. Just offshore from the surf zone, sand movement is controlled
by waves, wave-induced current, and coastal currents. Farther offshore from the surf zone (at
15-ft water depth) to the closure depth (30 ft), waves and coastal currents are the main factors
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
responsible for sand movement. Therefore, there is a transition area offshore of the surf zone
where Jongshore current produced by waves still plays a role in sediment movement.
Water is transported onshore by breaking waves. It moves with the longshore current for
some distance along the beach and returns offshore through narrow zones called rip currents.
The rip currents extend from the surf zone through the breaking waves to a short distance
offshore. Rip currents are one mechanism by which fine beach sand suspended in the water
column can be moved offshore of the surf zone to areas where coastal currents are the
predominant factor for moving sand along and across the shore.
Different equations are required to compute longshore and cross-shore sand movement, both
inshore and offshore of the surf zone, as discussed by White (1987) and Inman (1994).
Longshore sand transport can be computed with reasonable accuracy with the Komar and Inman
(1970) approach (discussed in Section 3.1) only if the bathymetry and wave climates are known.
Usually, the uncertainties in computing the longshore sediment transport offshore of the surf
zone and cross-shore sand transport are large. Due to the complexity of the coupling between
longshore and cross-shore sand transport (Section 6-2), shoreline models treat these two types of
movement separately. This introduces further errors in the results of the numerical models. All
of these errors limit the capabilities of the shoreline numerical modeling to quantify shoreline
changes.
6.2. PREDICTIONS FOR SAND-DISPOSAL BEHAVIOR
Several solutions are available for mathematically modeling sand-disposal behavior. These
techniques are: 1) analytical solutions for the governing differential equation for the system
(Work and Dean, 1995; Dean, 1996); 2) one-dimensional numerical models based on the
sediment budget of sand moving into and out of a cell (Hanson and Kraus, 1989); and 3) beach-
profile response modeling, which employs governing equations to describe cross-shore sediment
transport rates (Kriebel and Dean, 1985; Hanson and Kraus, 1989; Larson and Kraus, 1989) to
investigate beach-profile evolution.
Analytical solutions typically have restrictive assumptions, such as idealized initial
conditions and wave climate that do not vary temporally or spatially. The one-line model
neglects cross-shore sediment transport and sediment transport offshore of the surf zone. In this
type of model, when the cell length is large and the results are averaged over a long time period,
the errors may be small ( cell length on the order of thousand of yards, and time periods of 3 to 12
months). When the cell length is small and the results are presented over short time periods (cell
length on the order of hundreds of yards and time period of one or two months), errors can be
large. The one-dimensional model depends on estimates of the gradients of longshore transport.
Such gradients are hard to estimate accurately, unless a comprehensive spatial and temporal
measurement was made for the directional wave climate and the bathymetry.
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Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Calculation of the derivatives of the longshore transport to estimate shoreline changes can
produce large spikes because of noise in the longshore data. Models for beach-profile response
assume negligible longshore gradients, so that all profile changes are a result of cross-shore
sediment transport. Nevertheless, numerical models can give some insight to the fate of sand
placed on beaches as demonstrated by Work and Dean (1995), Dean (1996), and Houston
(1996).
Pelnard Consic:Uire (1956) combined the linearized equation of longshore sediment transport
with the continuity equation to yield
By a2y -=G-ar ax 2
(6-1)
in which y represents the shoreline displacement, x is the longshore coordinate, and t is time.
The quantity G is the so-called longshore diffusivity and depends primarily on the wave height
and secondarily on sediment characteristics. The solution for the case of an initially rectangular
sand placement planform is
(6-2)
where w is the cross-shore width of the beach fill, f. is the length of the beach fill, "er/' is the
error function, and G may be considered a "longshore-diffusivity" parameter, where
KH;'2 lg/
G= . 'VIK (6-3)
8(s-1)(1-p)(h. + B)
in which K is the empirical longshore-sediment transport coefficient, Hb is the wave height, g is
the acceleration of gravity, K is the ratio of wave height to water depth at breaking, s is the
sediment specific gravity, p is the sediment porosity, h• is the maximum depth affected by
longshore-sediment transport, and Bis the berm height. Equation (6-2) can be integrated to yield
the proportion of material remaining within the placement region, M(t), as a function of a single
parameter, Gt/f.
Coastal Environments
Reference Number 98-11
M(t) = .J4Gt (e ~y~)2 -1J + erf(-f-) i..fi .J4Gt
6-3
(6-4)
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Figure 6-1 illustrates, in non-dimensional form, the solution of Equation (6-2). The figure
shows the spread of a rectangular-shaped beach fill in the longshore direction for various values
of~-For the case
-.!Gt
(6-5)
the fill width is about 0.28w and it spreads about 2.5£. Substituting in Equation (6-3) K = 0.78,
Hb = 3.3 ft, g = 32 fu'sec, ,c= 1, s = 2.65,p = 0.4, h• = 33 ft, and B = 9 ft, gives G = 0.029 yd2/sec.
If the initial fill length, R, is 500 yd and the width, w, is 60 ft, then from Equation (6-5)
t = 100 days for the beach fill to spread 1,250 yd on both sides of the original fill. The fill width
after 100 days would be 16.8 ft. This example demonstrates the value of an analytical solution,
which provides approximate values for the longevity and spread of the beach fill. The analytical
solution presented here, however, does not take into consideration the offshore (cross-shore)
movement of sand and assumes the waves are perpendicular to the shoreline.
6.3. BEACH-PROFILE RESPONSE TO SAND DISPOSAL (DATA)
Beach-profile data from stations CB-0820, CB-0840, and OS-I 000 were analyzed to
determine the response of local beach profiles to sand-disposal activity. The stations CB-0820
and CB-0840 are located at Middle Beach and North Beach, respectively (Figure 4-5). The
response of South Beach to sand disposal is not discussed in this section because beach-profile
data are not available. However, the response of South Beach, in the cross-shore direction, to
sand disposal should not be significantly different from Middle Beach. Profile OS-I 000 is
located south of Oceanside Beach (Figure 4-5) and is reviewed to compare Oceanside beach
response to sand disposal to that at Carlsbad.
The longevity of the sand disposal is defined as the time period between the end of sand
placement and the time for which the profile returned to its pre-fill condition (±10 yd3/yd of
change above 0-ft NGVD between the. pre-and post-surveys).
6.3.1. Station CB-0820 (Middle Beach)
Station CB-0820 is located on Middle Beach, about midway between the intake and
discharge channels. At this station, two sand-di~posal events will be discussed. In April 1991,
about 461 yd3 per yard-length of beach (yd3/yd) were placed on Middle Beach (459,000 yd3
total) and in 1993 about 155 yd3/yd were placed on Middle Beach. The profiles responded
differently for these two sand disposals. Along this stretch of coast about 446 yd3 /yd of sand is
required to extend the beach width (b) 100 ft (33.34 yd). This is computed from Bruun's
equation (1954):
Coastal Environments
Reference Number 98-11
V= b (B + he) (6-6)
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
1.6
1.4
~ ">, 1.2
.c::: -"C 1.0 ~
en ~ 0.8
C
.Q
~ 0.6
Q)
E b 0.4
0.2
0-4
-··
RI/Gt-+oo
--····· 100
-----10
5
• -------1 •
' ,. ,
I• ,\ •• ; •••••••••••••••••• I~
I • '\ I : ' \ f I : \
I : ' \ / I : \ ~ t I '
__ ... ------··· ----···
··--.... __ _
-3 -2 -1 0 1 . 2
Dimensionless Distance, x/(.f/2)
........... _ -----..
3 4
Figure 6-1. The spread of a rectangular-shaped beach fill in the longshore direction for
various values of _£_ • ./Gt
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Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
where Vis the required volwne (yd3 /yd) to be placed on the beach, B is the berm height (8 ft or
2.7 yd), and he is the closure depth, defined as the point where minimal movement of sediment
by wave action occurs. For this coast, he is about 30 ft (10 yd).
1991 Sand Placement
In April 1991, 459,000 yd3 of beach sand were dredged from Agua Hedionda Lagoon and
placed on Middle and South Beaches. The available profiles at CB-0820 that bracket the
disposal are October 1990 (6 months before), April 1991 (immediately after sand disposal),
October 1991 (6 months after), and October 1992 (18 months after). Figure 6-2 shows plots of
these profiles.
The pre-fill profile of October 1990 was steep and narrow, with a slope of about 1:5. About
461 yd3 /yd were placed on the beach between December 1990 and April 1991. This volume was
computed from the volume change between October 1990 and April 1991. During the winter of
1990 to 1991, the sand placed on the beach moved offshore up to -27 ft NGVD water depth as
shown in Figure 6-2. By April 1991, the beach width increased about 90 ft from the pre-fill
survey and created a gentle-sloped beach with a slope 1: 16. Volume calculations show that
40.5 yd3/yd remained on the beach above Oft NGVD. This is about 8.8% of the total volume of
sand placed on the beach.
The six-month post-fill survey (October 1991) compared with the April 1991 profile shows
that 34 yd3 /yd returned to the bar-berm ( defined as the part of profile from the berm crest to the
break-point bar, from about + 10 to -6 ft NGVD). However, the shorerise (extending from the
break-point bar to a depth at least 30 ft below NGVD), from about -6 to -30 ft NGVD shows
losses of 105 yd3 /yd. This sand likely moved from the profile area along the shore to nearby
offshore areas. Some of this sand may have impinged on the shoreline at some distance away
from the profile (Inman et al., 1991) causing it to accrete.
The 18-month post-fill survey (October 1992) compared with the previous surveys shows a
well-established berm at an elevation of +8 ft NGVD. This profile also shows a 25-ft increase in
the beach width from the October 1991 survey. The volume change between October 1991 and
October 1992 is about 15 yd3 /yd in the bar-berm area and 22 yd3 /yd in the shorerise area.
Therefore, the disposed sand remained for at least 18 months on Middle Beach.
1993 Sand Placement
From February 1993 to April 1993, 115,395 yd3 of dredged material were placed on Middle
Beach. The profiles that bracket the sand disposal are October 1992 (6 months before), April
1993 (at the end of the disposal), October 1993 (6 months after), and October 1994 (1R months
after). The pre-fill profile (October 1992) was fairly wide and shallow with a slope of about
1: 16. Most of the sand placed on the beach moved from the beach to the bar, as seen on
Figure 6-3. The beach width decreased about 25 ft from the pre-fill survey. Our estimate of the
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Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
sand volume placed on the beach is between 120 and 190 yd3 /yd. This estimate was obtained by
dividing the total volume of sand placed on the beach by the assumed disposal length of 600 to
900 yd. The disposal length cannot be more than the length of Middle Beach, which is 900 yd.
The October 1993 profile shows an increase in the volume of the sand on the beach above
0 ft NGVD. About 21 yd3/yd returned to the beach. However, about 85 yd3/yd of sand did not
return to the beach. This sand was naturally removed to build up the offshore part of the profile,
which eroded during the winter of 1992-93.
The October 1994 profile shows a considerable amount of erosion throughout the entire
profile to the -22-ft NGVD position compared to the October 1992 and 1993 profiles. The
beach width decreased by about 70 feet. Although the April 1993 sand disposal did not stay on
the beach more than 6 months at most, the beach in 1994 was wider than the beach was four
years earlier because of the continuous placement of the sand on the beach. Figure 6-4 contains
plots of summer profiles at Middle Beach showing how the continuing sand-disposal projects
have continued to increase the beach width.
6.3.2. Station CB-0840 (North Beach)
Station CB-0840 is located on North Beach at Acadia Avenue. Two sand-disposal events at
this location (1988 and 1992) will be reviewed.
February to April 1988 Sand Placement
From February through April 1988, 347,782 yd3 of dredged material were placed on North,
Middle, and South Beaches. The exact volume placed on each beach is not known. However,
we estimate from the beach-profile data that about 40 yd3/yd were placed on North Beach. The
available profiles bracketing the disposal are September 1987 (7 months prior), April 1988 Gust
after), October 1988 (6 months after), and October 1989 (18 months after). The profile plots are
shown in Figure 6-5.
The pre-fill profile (September 1987) has a steep back beach with a 1: 11 slope. The sand
placement elevated the berm to + 11 ft NGVD and increased the beach width about 25 ft. The
six-month post-fill survey (October 1988) compared with the April 1988 survey shows that ~he
upper berm eroded, returning the slope and the beach width to the pre-fill (September 1987)
location. The 18-month post-fill survey (October 1989) shows continuing erosion throughout
most of the profile. Therefore, the longevity of the sand fill was no more than 6 months.
Coastal Environments
Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Station CB-0820
(Middle Beach)
-oct-9o
--Apr-91
-· -• Oct-91
• • • Oct-92
1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0
Distance (ft)
Figure 6"2. Sand-disposal project at Middle Beach (CB-0820), where 600 yd3/yd of sand were placed on the beach between
February and April 1991. Total volume of material placed on the both Middle and South Beaches is 458,793 yd3•
Coastal Environments
Reference Number 98-11
6-8
20
15
10
5
-C 0 > C) z
-5 £
C 0 i, Ill
-10 t w
-15
-20
-25
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
20
15
10
Station CB-0820 5
(Middle Beach) 0 0 > -oct-92 c:, z
--Apr-93 ¢!
-5 ---· -· Oct-93 C 0 ':;:I
· · · · · · Oct-94 ca > -10 .! w
-15
-20
-25
1-,--,-.,.....,,..........,...T'"'T-,-.,....,.....,..-,-...-,.-,-.,...., ........ -,-,....,.-,-.,......--.--,--,-,e-r-.....-..--.-.-.,....,r--r-....--.,.....,-.--r-....,.....-...-,,....,.-,-.,....,-,--,-.,.....-,-.....,e-,--+ -30
1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0
Distance (ft)
Figure 6-3. Sand.-disposal project at Middle Beach (CB-0820), where about 190 yrf/yd of sand were ~laced on the beach between
February and April 1993. Total volume of material placed on Middle Beach is 115,395 yd3•
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Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
1500
Station CB-0820
(Middle Beach)
-Oct-90
--Oct-94
• --Oct-96
-·· -Oct-97
1400 1300 1200 1100 1000 900 800 700
Distance (ft)
20
15
10
5
0
-5
-10
-15
-20
-25
-30
600 500 400 300 200 100 0
Figure 6-4. Summer beach profiles at Middle Beach from 1990 to 1997 showing the positive effect of continuous sand disposal
on the Middle Beach.
-C > C, z
4:! -C 0 i > Cl)
ii.i
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Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
..----------------------------------------------15
10
5
Station CB-0840 c (North Beach) >
0 (!) z -Sep-87 £ __ Apr-88 C
_ . _ . _ Oct-88 0
.:J
-S C'CI
•••• Oct-89 > CP jjj
)t_
-10
-15
----------.--...-.--....... --T---.--.--....... --T---r-.---.---.-........ -....--....... -.----,--.--...---.----.--.--....... -.--...-...---...... --20
900 800 700 600 500 400 300 200 100 0
Distance (ft)
Figure 6-5. Sand-disposal project at North Beach (CB-0840), where about 40 yd3/yd of sand were placed on the beach February and April
1988. Total volume of material placed on North, Middle, and South Beaches is 347,782 yd 3• .
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Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
April 1992 Sand Placement
In April 1992, 125,976 yd3 of sand were placed on North Beach. The profiles that bracket
the disposal are October 1991 (6 months prior), April 1992 (during), October 1992 (6 months
after), and October 1993 (18 months after). The proftle plots are shown in Figure 6-6. The April
1992 profile does not clearly show the sand disposal, but shows a large offshore bar, which is
probably the fill sand that moved offshore to form a typical winter offshore bar. From this
profile, the volume of sand placed on the beach is estimated to have been about 50 yd3 /yd. There
were no noticeable changes in beach width from the pre-fill survey.
The October 1992 profile shows that none of the nourished material deposited in April
returned to the upper beach during summer conditions. The bar-berm did not significantly
change from April 1992, but the profile eroded slightly below the pre-fill (October 1991)
conditions. None of the sand that had moved offshore appears to have returned to the beach area
above Oft NGVD. By October 1993, 18 months after the sand disposal, the upper portion of the
profile continued to recede. From these observations, longevity of this sand-disposal project was
0 to 6 months.
Figure 6-7 shows summer profiles on North Beach (Range CB-0840) for 1987, 1989, 1991,
1993, 1994, and 1996. The beach was most narrow and steep in October 1993. There have been
two other sand disposals on North Beach since 1992. In April 1994, 158,996 yd3 and in April
1996, 443,130 yd3 were placed on North, Middle, and South Beaches. Both of these sand
disposals helped the beach recover from the 1992-93 winter wave storms.
6.3.3. Station OS-1000 located at Oceanside (April-October 1986)
Station OS-1000 is located in Oceanside about 7,500 ft south of Oceanside Harbor. In 1986,
450,000 yd3 were placed along the stretch of shoreline north and south of this profile. The sand
was dredged from the harbor and placed on the beach near Tyson Street
The April 1986 profile represents pre-fill conditions, and October 1986, September 1987, and
January 1988 represent the post-fill conditions. These profile plots are shown in Figure 6-8.
Volume changes between the pre-and post-fill surveys are about 300 yd3/yd. This represent the
volume of sand placed on the beach between +8.0 and -8.0 ft NGVD. In response to the sand
disposal, the beach width increased by 180 ft. By April 1987, 160 yd3 /yd of the sand on the
beach eroded from the bar berm and about 40 yd3 /yd eroded from the shorerise, returning the
profile to the April 1986 (pre-fill) configuration.
By September 1987, the bar berm and shorerise accreted by about 107 yd3/yd and 40 yd3/yd,
respectively. However, in January 1988, a large wave storm struck southern California causing
sand to erode from the bar-berm area to the shorerise area (200 yd3 /yd) as shown in Figure 6-8.
Coastal Environments
Reference Number 98-11
6-12 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
.----------------------------------------------15
10
5 -Station CB-0840 ~ CJ
(North Beach) z = 0 --Oct-91 , C , , 0
--Apr-92 , ~ , , > -·-·-Oct-92 , Cl> ----" -.. -... -5 iii -• -• Oct-93
-10
-15
1--...-----,----,.-....--r---,----,.-,---....... -r-----,--.--...----r-----r-.--...----r----,.-,----,----,----,.-,---,-.....,..---,.-..----,----,---,-T--,---,---.---+ -20
900 800 700 600 500 400 300 200 100 ' 0
Distance (ft)
Figure 6-6. Sand-disposal project at North Beach (CB-0840), where about 50 yd3/yd of sand were placed on the beach between February
and April 1992. Total volume of material placed on North Beach is 125,976 yd3•
Coastal Environments
Reference Number 98-11
6-13 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
.----------------------------------------------, 15
Station CB-0840
(North Beach)
-Sep-87
--oct-89
--Oct-91
-· -· Oct-93
-·• -Oct-94
-• -Oct-96
.. ----
10
0
' /
' -·--5
-10
-15
J---.---.----r-r---.---.---.-~.....----r--.-r---.---.---,.-r--....----r--r-~-,---.---.-...---,----.--.--...---.---T-.--.....--.---,.-r--+ -20
900 800 700 600 500 400 300 200 100 0
Distance (ft)
Figure 6-7. Summer beach profiles from 1987 to 1996 at North Beach (CB-0840).
-C > C) z
~ -C 0 ';I
ftl > CD iii
Coastal Environments
Reference Number 98-11
6-14 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
....-------------------------------------------------.-15
Station OS-1000
(Oceanside)
.,.. -·-~....... ,
' I I ,
' I
/
I
;
I ,.
10
5
1------------------------------------~~-,.,....---:--7"-----TO 0 --Apr-86 . ..-.,.. ""' >
.,
--~ ~ / ; z -· -• Oct-86 .,,... • ..,, , , e
,,. --.... ~ ,, -5 ----Sep-87 ..!!-• ..... • '"_,,·--,-.--c .,...-;:-# .. -·· 0
/ ~ · · ----Jan-88 , " ~
;
.. .. . .
. , CD
-10 W
-15
-20
+-,...-,,....,.-+.....,..-.-....--t--.--r-,.-+-,,.......--,.-T-.--.-....--+-..-,.-.--,1--,-,.--,.-t--.-.....--.-+--.-...-...-11-,,.......--,.-t--,--.-...--+-...--r-T--+-,,.......--,.-f,. -25
1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0
Distance (ft)
Figure 6-8. Sand-disposal project at Oceanside (OS-1000), where about 300 yd3/yd of sand were placed on the beach in 1986. Total
volume of material placed on the beach is 450,000 yd3•
Coastal Environments
Reference Number 98-11
6-15 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
6.4. CONCLUSIONS FROM SAND-DISPOSAL PROJECTS IN THE AREA
The evaluation of the beach profiles in relation to the sand-disposal events described above
provides the following insights:
J) When a relatively large volume of sand was placed on Middle Beach ( 460 yd3 /yd) during
the winter of 1990-91, the sand moved offshore to build the underwater portion of the
profile. Both the bar berm and shorerise responded to the sand disposal. The beach
profile reached its summer equilibrium shape by October 1991 (Figure 6-2) and the beach
width was 90 ft wider than October 1990. Only 40 yd3 /yd of the volume of sand placed
on the beach remained on the beach (about 8.8% of the total volume). By October 1991,
an additional 27 yd3/yd of sand returned to the beach (berm to 0-ft NGVD). Therefore,
the entire profile lost 105 yd3 /yd sand, or about 22% of the total volume of sand placed
on the beach. The sand-volume loss was small (about 37 yd3/yd) between October 1991
and October 1992. An additional 115,395 yd3 of sand was placed on Middle Beach
between February and April 1993, which maintained the beach in good condition through
October 1994 (Figure 6-3). The continuing sand-disposal projects from 1994 to 1998
have increased the beach width about 100 ft.
2) When small amounts of sand (about 40 and 50 yd3/yd) were placed on North Beach in the
winters of 1988 and 1992, respectively, the sand disappeared from the beach within 6
months. The beach profile prior to sand disposal was in a dis-equilibrium condition
I
covered by a thin veneer layer of sand.
3) Monitoring sand disposal at Oceanside (OS-1000) shows observable changes in the
beach-profile shape between summer (October) and winter (April). The beach width
increased between April 1987 and September 1987 without significant changes in the
offshore profile. This indicates the importance of the longshore sand movement in
shoreline changes. The effect of large wave storms (such as the January 1988 wave
storm) on the beach is significant. This storm moved 200 yd3/yd from the bar-berm to
the shorerise illustrating how storms can affect the longevity of sand disposal on a beach.
4) Theoretical consideration of the evolution of sand disposals (Section 6.2) shows that the
longevity of the sand depends on the project size. Larger sand disposals will usually
remain on the beach for a longer duration than shorter projects, even when the volume of
sand per unit length of beach is equal. In addition, sand spreads on both sides of the
disposal area, and the width of the sand fill decreases with time (Figure 6-1).
Coastal Environments
Reference Number 98-11
6-16 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
6.5. THE EFFECT OF THE ENCi NA POWER PLANT OPERATION ON BEACHES
The Encina Power Plant does have an effect on the local beaches since its operation and the
jetties alter the natural coastal processes in the vicinity of the plant, which may cause erosion or
accretion to nearby beaches. However, most of the local effects are short-term and can be
minimized with proper placement of dredged sand. Over the long-term, the power plant
operation does not significantly alter the beaches since the lagoon is routinely dredged and the
dredged material is placed back into the beach system once every two or three years.
Carlsbad Submarine Canyon alters the local wave regime and decreases the northward
longshore transport along Carlsbad compared to Oceanside. Figure 3-8 shows that
approximately 80% of the cumulative longshore transport at Carlsbad is towards the south and
about 20% is towards the north. Therefore, approximately 80% of the sand deposited inside the
lagoon is from southward sand transport and 20% from northward transport. The material that is
trapped in the lagoon from the southerly longshore transport never reaches Middle and South
Beaches. Similarly, the material that is trapped in the lagoon from the northerly transport never
reaches North Beach.
The local effect of the power plant operation is at a maximum near the intake channel and
decreases with distance. From analyzing local beach-profile data, the zone of impact can be
identified as being between Buena Vista Lagoon to the north and Batiquitos Lagoon to the south.
The beach widths at both OS-0930 (located north of the power plant at Buena Vista Lagoon) and
CB-0760 (located north of Batiquitos Lagoon at Encinas Creek) have been stable for at least 15
years (1982 to 1997). This stability indicates that the power plant does not have an effect at
these locations and the effects must be limited to the area between Buena Vista Lagoon and
Batiquitos Lagoon.
During the summer season, North Beach just north of the intake channel, loses sand and the
shoreline recedes since the longshore transport of sand is moving to the north. However, this
observation is largely a natural coastal process from the effects of Carlsbad Canyon on
southwesterly and southerly swells. The volume of sand entering North Beach from the southern
end (Q1) is less than the volume of sand leaving North Beach from the northern end (Q2). The
difference between Q2 and Q1, divided by the length of the beach, gives the divergence of the
drift. When the divergence of the drift is positive, the beach loses sand and erodes. Figure 5-1
shows that during the summer season, the northern longshore transport is greater at Oceanside
(representing Q2) than in Carlsbad (representing Qi), causing North Beach near the jetty to
recede in the summer.
Coastal Environments
Reference Number 98-11
6-17 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Beach profiles conducted by Coastal Environments along Carlsbad beaches from May
through September 1997 show similar trends along North Beach (Vol. II). The intake channel
jetties have a positive impact on North Beach during the winter season, by stabilizing the
southern end of North Beach and impounding some of the sand moving towards the south.
6.6. EVALUATION OF SAND PLACEMENT OPTIONS
In this section, four placement options will be discussed and evaluated. These options are
evaluated in light of the existing permit limitations on the area available for sand disposal. The
area currently permitted for sand disposal is between Oak Avenue to the north and Terra Mar to
the south. If the dredged sand is to be placed in any other location a modification to the permit
would be required. For each option, the best location for the placement of the dredged material
will be selected based on consideration of three factors; 1) economics, 2) public use, and 3)
minimization of re-dredging.
6.6.1. Replenish Sand that the Power Plant is Responsible for Removing (Option 1)
The power plant operation increases the amount of littoral sand deposited inside the Outer
Basin of Agua Hedionda Lagoon. Averaged over a typical year, about 80% of the sand
deposited inside the lagoon is southerly-moving littoral material and about 20% is littoral
material moving northward, as described in Section 6.5. This deposited sand never reaches its
destination. Therefore, to satisfy the requirements for this option, 80% of the dredged sand
should be placed on Middle and South Beaches and 20% on North Beach.
6.6.2. Minimizing the Need to Redredge the Lagoon (Option 2)
In order to minimize redredging of Agua Hedionda Lagoon, the dredged sand should be
placed as far as possible from the intake channel within certain constraints. Since the dredged
material should be placed on the three beaches most affected by the power plant (North, Middle,
and South) an adequate buffer region is required between the sand placed on the beach and the
intake channel. Because of the effect of Carlsbad Canyon on the wave regime, the northward
sand transport during the summer season is much smaller in magnitude than the south transport
during the winter season (Figure 3-7). Therefore, a smaller buffer is required on the south side
of the intake channel than on the north side.
A 500-ft buffer area south of the intake channel is adequate to minimize the amount of placed
sand that re-enters the lagoon, since the northward sand transport during the summer is
considerably reduced. On North Beach, at least a 2,000-ft buffer is required to minimize the
amount of sand re-entering the lagoon. The 2,000-ft buffer is based on the spreading rate
predictions of disposed sand on North Beach for various disposai scenarios (Section 6.2), and the
predominant longshore transport direction between May and October is to the north. Also, a
hard-substrate area exists north and south of the intake channel (Figure A-2). The hard substrate
Coastal Environments
Reference Number 98-11
6-18 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
extends about 2,000 ft north and about 500 ft south of the intake channel. The area north of the
intake jetty is a popular area for surfing. Lifeguards discourage swimmers from using this area
and direct them farther north. As shown in Section 6.6.3 (Option 3), the farther north the sand is
placed on North Beach (towards the end of the seawall) the more recreational benefit results.
6.6.3. Maximize Public Recreational Benefits (Option 3)
Review of previous sand placement projects on North Beach shows that about 150 to
200 yd3 /yd is required to maintain an adequate beach width in order to increase recreational
benefits. Smaller volumes placed on the beach will disappear more quickly and will not
significantly increase recreational benefits. Assuming about 300,000 yd3 of sand will be dredged
from Agua Hedionda Lagoon every two years the beach area will increase by 23,600 yd2, using
Equation (6-6). SANDAG (1993) estimated 100 ft2 per person as the minimum space necessary
to accommodate beach users. Based on this number, approximately 2,130 people may be
accommodated on the beach resulting from this disposal volume.
Neither South, Middle, nor North Beach can accommodate this number of people since the
area has limited parking facilities. There are only about 30 parking spaces at South Beach, 90 at
Middle Beach, and 211 along North Beach. Based on the parking facilities, to maximize the
public use, the sand should be distributed on the three beaches, such that the amount of sand
placed on the beach is at least 150 yd3/yd of beach.
6.6.4. Achieve Most Mitigating Effect to Regional Beach Erosion (Option 4)
The amount of sand dredged from Agua Hedionda Lagoon is on average 138,000 yd3/yr.
This is a small volume of sand relative to what is needed to solve the regional beach problems.
In a recent study, SANDAG (1993) estimated the initial volume of sand needed to restore the
beaches from Oceanside Harbor south to La Jolla is 25 x 106 yd3 and an additional
320,000 yd3 /yr would be needed to maintain the restored beach. This volume greatly exceeds the
volume available from Agua Hedionda Lagoon annually.
Although the regional erosion is a significant problem, Carlsbad beaches adjacent to Agua
Hedionda Lagoon (North, Middle, and South Beaches) are most affected by the operation of the
power plant and are in need of the dredged sand for beach maintenance. Placement of the
dredged material from the lagoon on Carlsbad beaches has minimized the effects of power plant
operation on these beaches to non-significant levels. The portion of the disposed sand that
moves offshore may then be carried by coastal currents, which may be redistributed to other
regional beaches.
Coastal Environments
Reference Number 98-11
6-19 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Cartsbad, CA
6. 7. STABLE DISPOSAL SITES
Stable disposal sites are defined as those sites that can retain sand for longer periods relative
to other nearby beaches. Those sites that are in equilibrium with natural forces induced by
waves are likely to be stable disposal sites. The shorerise portion of a stable beach profile has
already been built and, therefore, sand losses from the bar-berm to the offshore area is minimal.
Middle Beach and, to some extent, South Beach are stable beaches, because the dredged sand
has been placed on these beaches fairly regularly. The continuous supply of sand to these two
beaches has helped to build the offshore portion of Middle Beach. Table 4-1 and Figure 4-6
show that Middle Beach is accreting at a rate of +5.8 ft/yr from 1987 to 1997. It should be
pointed out that Middle Beach would probably not be a stable beach if dredged material had not
been placed on it.
The area of North Beach near Pine Avenue is also likely to be a stable area because the
divergence of the longshore sediment transport in this area is small (Figure 6-9; from Jenkins and
Wasyl, 1997). It is likely that after several years of depositing sand along North Beach near Oak
or Pine A venues, this beach will retain the placed sand in a manner similar to Middle Beach.
Coastal Environments
Reference Number 98-11
6-20 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
J:l ......
S 3.0 -------------~------------,
' C")
E 2.5.
2.0.
1.5
1.0
0.5
.... 4 •• LOWER WEST WINDOW, 261°
·-....... MIDDLE WEST WINDOW, 270°
······ UPPER WEST WINDOW, 277.5°
-LOWER NORTH WINDOW, 296°
-UPPER NORTH WINDOW, 310°
0.0 -1------1----+----+---+-"'-----1:-t----+----t--J
33.06 33.08 .33.10 33.12 33.14 33.16 33.18 33.20
SHORELINE POSITION, DECIMAL DEGREES OF LATITUDE
Figure 6-9. Longshore variations of southward directed potential longshore-transport rates, from Jenkins and Wasyl (1994).
Coastal Environments
Reference Number 98-11
6-21 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
7. COST-BENEFIT CONSIDERATIONS
7 .1. INTRODUCTION
The objective of this analysis is to maximize the value of sand dredged from Agua Hedionda
Lagoon by assessing the best placement location between Oceanside and Encinitas and to
evaluate other cost-benefit considerations associated with sand placement. The net value of the
sand is a function of the benefits gained by placing the sand on the beaches and the costs
associated with its placement on the beach. Benefits include recreation, structure and prqperty
protection, and tourism. Beach recreation includes, for example, swimming, bathing, angling,
pleasure boating (motorized), jet skiing, kayaking, sailing, informal recreation (volleyball,
squash ball, football, baseball), and passive/observational uses. The costs of placing sand on the
beach include dredging and transportation costs.
According to a survey made by State and City Departments of Parks and Recreation, La Jolla
Shores City Beach and Carlsbad State Beach are the most popular beach destinations in San
Diego County (SANDAG, 1993). Nearly two million people visit Carlsbad State Beach per year
as shown in Figure 7-1, indicating its popularity and economic value.
The need for regular sand replenishment along Carlsbad beaches is clear. SANDAG (1993)
anticipates that, without replenishment, the beach width will decline at a rate of approximately
1 to 2 ft/yr. According to SANDAG, the average current beach width at Carlsbad is
approximately 105 ft and, by the year 2010, without intervention, it will decline to approximately
90 ft. Conversely, an anticipated increase in beach use is expected to create a requirement for a
beach width of approximately 145 ft by the year 2010. This difference between anticipated
demand and anticipated beach width is substantial, and poses a long-term threat to the local
economy.
Present sand replenishment efforts in Carlsbad consist of the placement of sand dredged from
Agua Hedionda Lagoon by SDG&E, primarily from the Outer Basin near the inlet. This
dredging and placement activity has been conducted since 1954 as a necessary maintenance
operation and cost of doing business in order to maintain a sufficient water intake for the
SDG&E power plant.
Between 1954 and 1998, dredging operations at Agua Hedionda Lagoon have been
conducted on two-to three-year intervals, with an annual dredging average of approximately
138,000 yd3/yr (maintenance dredging). The SDG&E dredging operation, in addition to being
necessary for operation of the power plant, represents a significant overall benefit to the region
and to the effected coastal communities, regardless of where the sand is deposited within the
region. For example, the cost of importing inland sand from more remote areas could range up
Coastal Environments
Reference Number 98-11
7-1 Final Report
,-----
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
2,000,000
1,800,000
1,600,000
1,400,000
1,200,000
GI u C l'I ,,
C 1,000,000
~ <
800,000
600,000
400,000
200,000
0
Coastal Environments
Reference Number 98-11
a Annual Attendance
x Beach Length (linear feet)
---------------------------------ii-+ 20,000
X
15,000 g
.r:. -CJ)
C Ill ..J .r:. u l'I
10,000
CD UI
C X 0 0 Cl) ns
ftl "0 C 0 :s
::c
ftl ::,
Cl <
Figure 7-1. Annual attendance at various Oceanside Littoral Cell beaches, from SANDAG (1993).
7-2 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
to $10-16/yd3 according to survey made by Elwany in 1998. The SDG&E operations have, over
the last 45 years, deposited an average of 138,000 yd3/yr on area beaches, representing a current
value of approximately $1,794,000 annually, based on an average cost of $13/yd3•
In order to determine how to maximize the value and utility of distributing dredged sand on
beach areas between Encinitas and Oceanside, a preliminary screening was conducted to
evaluate the relative cost of transporting dredged sand from Agua Hedionda Lagoon to Encinitas
and Oceanside and comparing these costs to more local placement along North, Middle, and
South Beaches.
The analysis indicated that transportation distance would have a profound impact on costs.
This impact is derived not only from the cost of fuel, but from travel time and its impact on labor
costs, effects on the schedule of operations, effects on total dredging operational time, and
effects on total operation and maintenance costs. These cumulative costs would be excessive
and are certain to skew any economic comparison. The analysis was, therefore, narrowed to
consideration of North, Middle, and South Beaches only.
7.2. BENEFIT CONSIDERATIONS
In the Shoreline Preservation Strategy (SPS) for the San Diego Region (SANDAG, 1993) it
is estimated that the annual value of a full-scale beach building to the region's economy would
amount to approximately $8 million in property protection, and $45 million in recreational
revenues. North, Middle, and South Beaches (9,000 ft total length) represent approximately
3.7% of the total length of beaches along San Diego. A gross estimate of the present annual
value of these three segments, as a ratio of the total estimated value of San Diego beaches
described above, could be placed at around $2,000,000. To further distribute the implied
benefits among the three beaches, North Beach is approximately 50% of the total beach length of
9,000 ft, while Middle and South Beaches represent 30% and 20%, respectively.
The deposition of 300,000 yd3 every two years results in an increase of approximately
213,000 ft2 of additional beach area during the summer use period. The additional area was
calculated from Equation (6-6) with B +he= 38 ft (12.7 yd). Based on the SPS estimate that the
average requirement for each beach visitor is 100 fr, this increase represents additional beach
area for approximately 2,130 visitors. The available parking facilities along each of the three
beaches cannot accommodate this many visitors. There are only about 30 parking spaces at
South Beach, 90 at Middle Beach, and 211 along North Beach (Section 6.6.3). Assuming an
average of 3 people per vehicle, then about 1,000 people can visit the beaches (by car) at any
given time. However, it should be noted that one parking space might be used several times
within a day. Residents of Carlsbad are more likely to use North Beach since that area has more
residential property. Hence, the number of users on North Beach may be 100% higher than the
number of users on Middle and South Beaches.
Coastal Environments
Reference Number 98-11
7-3 Final Report
Study of Sediment Transport Conditions. Agua Hedionda Lagoon, Carlsbad, CA
7 .3. COST CONSIDERATIONS
The costs associated with depositing sand along North, Middle, and South Beaches are not
the same for each location. It is slightly more expensive to place sand on North Beach since the
pumping distance is greater. However, sand should not be placed immediately north of the
intake channel because the dominant longshore sanq. transport at Carlsbad is from the north and
the sand may return directly back into the lagoon. The best location for sand placement along
North Beach is farther north of the entrance, near Pine Avenue (Section 6.6.2.). This location
represents an additional cost to the utility of about $0.50/yd3 (William Dyson, personal
communication, 1998).
Table 7-1 reflects the recent dredging and placement history among the three beach segments
(North, Middle, and South). It is evident that the operationally preferred deposition site is
Middle Beach, representing about 66% of the total 5-year deposition. Placement on Middle
Beach represents the shortest distance to a deposition site and the most cost-effective alternative
from the perspective of dredge operations. Approximately 17% of the total dredged sand has
been deposited on North Beach and 17% on South Beach. It is important to note that if the sand
is regularly placed on only one of the three beaches, then the other two beaches will most
probably erode. There are costs associated with lost beaches such as costs of protective
structures and loss of recreational benefit.
7 .4. COST-BENEFIT EVALUATION
While the sand distribution represented in Table 7-1 is the most cost-effective approach from
an operational perspective, it may not represent the optimum ratio from the perspective of overall
benefits. This is because the use-characteristics of the three beaches differ considerably. First,
North Beach, at 4,500 ft in length, represents approximately 50% of the available recreational
use area. Middle Beach, at 3,000 ft in length, represents 33% of the available beach area, and
South Beach, at 1,500 ft, represents approximately 17% of the total. In addition, access and
facilities, two influential factors involved in selecting a recreational site, tend to channel beach
users to certain areas (North Beach), and discourage use of other areas (South Beach).
Natural geological features also affect use patterns. A near-shore reef located immediately
north of the intake channel produces large waves, which draw significant numbers of surfers.
The area farther to the north is immediately accessible to a large residential population and
provides parking, easy access, nearby restaurants, and other facilities. The total residential
recreational catchment area for the southern segment of North Beach is quite limited, is further
restricted by virtue of its slightly higher erosional rate (and therefore narrower beaches), but
offers some access to facilities. Middle Beach, on the other hand, has extremely limited adjacent
parking, relatively restricted access, and no adjacent or easily accessible facilities. These facts,
however, may be desired by those who seek a less populated, or more remote or more isolated
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Reference Number 98-11
7-4 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
beach. Middle Beach also tends to be wider, is easily observed and evaluated from the coastal
road, and, therefore, relatively popular for non-local recreational users.
Finally, the effect of sand deposition must be considered. As indicated in Figure 7-1, overall
use of Carlsbad State Beach has declined from its peak of 253,230 users in August 1995 to
183,229 in August 1996, and to 85,582 in August 1997. This coincides with the fact that the last
beach deposition on North Beach took place in 1996 (106,416 yd3). The limited parking spaces,
and possible beach losses if the dredged sand were only placed on one or two of the beaches,
support the argument that distributing the sand between North, Middle, and South Beaches
would create, in the long-term, the greatest benefits for the project.
Coastal Environments
Reference Number 98-11
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table 7-1. Recent sand-disposal volume distribution (yd3).
Disposal Area
1994
South Beach 46,410
Middle Beach 37,761
North Beach 74,825
Total Cubic Yards 158,996
a no sand placement on the beach
Coastal Environments
Reference Number 98-11
1995
• .
.
.
.
Year
1996 1997 1998
42,402 . 93,799
294,312 197,342 179,782
106,416 . .
443,130 197,342 273,581
7-6
Totals
182,611
709,197
181,241
1,073,049
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
8. SUMMARY AND CONCLUSIONS
This chapter summarizes this study of the sediment-transport conditions in the vicinity of
Agua Hedionda Lagoon
8.1. TECHNICAL TASKS
Five major technical tasks were completed for this study, as follows:
1. Review coastal processes and sediment transport in the vicinity of Agua Hedionda
Lagoon;
2. Estimate shoreline-change rates from Oceanside Harbor to Moonlight Beach, Encinitas;
3. Evaluate the effects of the Encina Power Plant and Agua Hedionda Lagoon on the
transport and deposition of sediment along this reach;
4. Develop and evaluate four dredged-sediment placement options;
5. Identify areas north and south of the lagoon that provide stable sand deposition sites; or if
stability is equal, identify sites that will increase recreational benefit.
The following sections provide a summary of the task results and the study conclusions.
8.2. SEDIMENT TRANSPORT IN THE VICINITY OF AGUA HEDIONDA LAGOON
Long-term wave measurements at Oceanside from 1978 to 1994 were collected and analyzed
to estimate the longshore sediment transport in the Oceanside Littoral Cell. These estimates
show that the average annual longshore transport is about 372,000 yd3 (284,400 m3) to the south
(or 54% of the total) and 321,000 yd3 (245,400 m3) to the north (46%).
For this study, a wave experiment was conducted to determine the relationship between the
wave regime at Oceanside and at Carlsbad. Two wave gauges were deployed at Carlsbad near
the intake channel of Agua Hedionda Lagoon and at Oceanside at the former CDIP station
location. The results show that Carlsbad Submarine Canyon shelters Carlsbad beaches from
Southern Hemisphere Swell, creating a less effective wave climate for transporting sand to the
north during the summer. Consequently, at Carlsbad, about 80% of the longshore sand transport
moves to the south, and 20% to the north.
8.3. SHORELINE CHANGE RA TES
Inspection of historical shoreline maps and recent beach-profile data indicate the largest
shoreline changes within the study area were at Oceanside Harbor and Agua Hedionda Lagoon
during and after their construction. Large shoreline advances near Oceanside Harbor are related
to construction of the harbor breakwaters and the placement of 13.6 x 106 yd3 of sand dredged
from the boat basins. Sand dredged from Oceanside Harbor during routine maintenance is
Coastal Environments
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
placed in shallow, nearshore waters south of the harbor. This practice has minimized the
erosional effects of the harbor on the beaches to the south.
Construction of the Agua Hedionda channel jetties and the dredging and placement of
4.3 x 106 yd3 of sand in 1954 resulted in shoreline advance of between 100 and 200 ft by 1982.
Since then, the shoreline along North Beach has remained in essentially the same position, and
Middle and South Beaches have advanced from the continuing sand disposal. For example,
between 1987 and 1997, Middle Beach accreted at the rate of +5.8 ft/yr.
8.4. EFFECTS OF THE ENCINA POWER PLANT ON SEDIMENT TRANSPORT
The operation of the Encina Power Plant affects the local beaches because it alters the natural
sand transport and distribution of sand in the vicinity of the lagoon. This alteration mainly
occurs in two ways. Over the long term, the lagoon jetties have stabilized the shoreline at a
mean position seaward of its natural location (before the structures were constructed). Also, over
the short term, the operation of the power plant increases the sedimentation rate in the lagoon by
depositing littoral sediments in the Outer Basin.
From 1954, when the jetties at Agua Hedionda Lagoon were constructed, to 1982, the jetties
and initial sand placement caused an advance in the natural shoreline position through accretion
and sand retention, Figure 4-4. From 1982 to present, the shoreline has generally remained in
the same position.
Operation of the cooling system increases the sedimentation rate in the lagoon, causing about
26% of the total longshore-sediment transport to be intercepted and temporarily deposited in
Agua Hedionda Lagoon. This amounts to an average of about 138,000 yd3 of littoral sand each
year. The material is then dredged from the Outer Basin and returned to the beaches every two
to three years. Power plant operation can, therefore, cause short-term narrowing or widening of
nearby beaches at different times, depending on which part of the dredging cycle is considered.
North, Middle, and South Beaches are all affected by the operation of the power plant.
While it is not possible to precisely quantify the short-term local effects of the plant operation, its
influence does decrease with distance, both up and down coast. From analysis of local beach-
profile data, the zone of impact has been identified as being between Buena Vista Lagoon to the
north and Batiquitos Lagoon to the south.
8.5. SAND DISPOSAL BEHAVIOR
Sand on the beach moves in the cross-shore direction, as well as up-and down-coast. Large
waves (in winter) tend to transport sand from the beach face to deeper water, decreasing the
overall beach slope. Smaller waves (in the summer) tend to transport sand back up the beach
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slope, widening the beach. Along the Oceanside Littoral Cell, the closure depth, defined as the
maximum depth that cross-shore sand transport tends to occur, is about -30 ft.
Beach-profile data from North and Middle Beaches were analyzed to determine the longevity
of the sand disposal on the beaches. The data show that sand placed on Middle Beach generally
stays for about 6 to 18 months. while sand placed on North Beach only remains for about
6 months or less.
The longevity of sand placed on a beach depends on wave co~ditions after the placement, as
well as the volume and frequency of the sand deposits. Sand dredged from Agua Hedionda
Lagoon has been deposited on Middle Beach fairly regularly since 1954. These regular sand
placements have nourished and maintained the offshore portion of the beach profile and,
therefore, resulted in better sand retention.
8.6. SAND PLACEMENT OPTIONS
Four sand-placement options were developed and evaluated. These options were selected to
satisfy four different needs, as follows:
I. To replenish sand the power plant operation is responsible for removing;
2. To minimize the need to re-dredge the lagoon;
3. To maximize public recreational benefits; and
4. To optimally mitigate regional beach erosion.
The area currently permitted for sand disposal is between Oak Street to the north, and Terra
Mar to the south. Transporting sand out of this area would greatly increase disposal costs.
Therefore, the sand-disposal options were evaluated within the constraints of the permitted area,
and specifically for North, Middle, and South Beaches.
The results of the disposal-option analysis show the following:
1. To replenish sand the power plant operation is responsible for removing, about 80% of
the dredged sand should be placed on Middle and South Beaches, and 20% on North
Beach.
2. To minimize the need to re-dredge the lagoon, the dredged sand should be placed as far
I
from the intake channel as possible, within certain constraints. These constraints include
' ' the extent of the permitted area, the location of hard substrate, and the longevity of the
placed sand on each beach. For sand placement on North Beach, at least a 2,000-ft buffer
from the intake channel will provide an adequate distance to minimize re-dredging and
along South Beach a 500-ft buffer is recommended.
3. To maximize public recreational benefits, the sand should be distributed on the three
beaches, perhaps proportional to beach length available for beach users for each of the
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three beaches. The conclusion to distribute the sand on the three beaches is based on
limited parking facilities near Agua Hedionda Lagoon. Neither North, Middle, nor South
Beach can accommodate the expected increase in the additional number of 2,130 visitors
resulted from placing the dredged sand on only one of these beaches. North Beach, north
of Pine A venue, provides great recreational benefit to swimmers and accompanying
beach-goers. The residents of Carlsbad are more likely to use North Beach since this .area
has more residential property, is near the shopping centers and restaurants, and has more
parking facilities than Middle and South Beaches.
4. To optimally mitigate regional beach erosion, sand should be disposed on the three
beaches near Agua Hedionda Lagoon. The average amount of sand dredged from Agua
Hedionda Lagoon (138,000 yd3/yr) is a small fraction of the initial amount of sand
needed to restore the beaches between Oceanside and La Jolla, which has been estimated
by SANDAG (1993) to be about 25 x 106 yd3. However, the 138,000 yd3/yr represents
about 43% of the annual amount needed for maintenance, which was estimated to be
320,000 yd3/yr. However, the cost constraints on placement, and the fact that the
influence of this relatively small amount of sand is limited to the area between Buena
Vista Lagoon and Batiquitos Lagoon, suggests that the dredged sand from Agua
Hedionda Lagoon cannot address the entire region's beach erosion problems.
8.7. STABLE DISPOSAL SITES
The study defined Middle Beach, South Beach, and North Beach near Pine A venue as stable
disposal areas. These sites can retain sand for a longer duration relative to other nearby beaches.
Based on the response of North and Middle Beaches to sand disposal, the study recommends
sand placement volume between 150 and 200 yd3 /yd in order to prolong the longevity of the sand
on the beach (Section 6.4).
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Cartsbad, CA
9. RECOMMENDATION
Based on the results of this study, which are summarized above, it is recommended that 30%
of the sand dredged from Agua Hedionda Lagoon be placed on North Beach near Pine A venue
and 70% be placed on Middle and South Beaches. This recommended distribution represents a
reasonable compromise between the competing needs for the sand, benefits and costs, and
environmental constraints on its placement.
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
10. REFERENCES
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Bhogal, V.K., and S.L. Costa, 1989. Modeling Flood Tidal Deposits in Tidal Inlets. Oceans 89:
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Boyd, W., 1998. Underwater visual observations of reef north of intake jetty, Agua Hedionda
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Brownlie, W.R. and B.D. Taylor, 1981. Sediment Management of Southern California
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Bruun, P., 1954. Coastal Erosion and Development of Beach Profiles. U.S. Army Beach
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CDIP, 1992. Coastal Data Information Program, Multi-year report, Vol. I, 1975-1991. SIO
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Conley, D.C. and D.L. Inman, 1994. Ventilated oscillatory boundary layers. Journal of Fluid
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San Diego Gas & Electric Co., Carlsbad, California, 43 pp.
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Report. U.S. Army Corps of Engineers, Waterways Experiment Station, Misc. Paper H-78-8,
Vicksburg, MS, 464 pp.
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Technical Report CERC-89-19, U. S. Army Engineer Waterways Experiment Station,
Coastal Engineering Research Center, Vicksburg, MS, 185 pp.
Hicks, D.M. and D.L. Inman, 1987. Sand Dispersion from an Ephemeral River Delta on the
Central California Coast. Marine Geology '77, 305-318.
Houston, J.R., 1996. Simplified Dean's Method for Beach-Fill Design. Journal of Waterway,
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Inman, D.L., 1973. Shore processes. p. 317-338 in R.C. Vetter (ed.), Oceanography: The Last
Frontier, Basic Books, Inc., New York, 399 pp. [Reprinted, 1974, p. 350-372 in R.C. Vetter
(ed.), Oceanography: The Last Frontier, Voice of America Forum Series, U.S. Information
Agency, Washington, D.C., 430 pp.
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Inman, D.L., 1985. Damming of rivers in California leads to beach erosion Oceans 85: Ocean
Engineering and the Environment, Marine Technological Society & IEEE, v. I, 22-26.
Inman, D.L., M.H.S. Elwany, and S.A. Jenkins, 1993. Shorerise and Bar-Benn Profiles on
Ocean Beaches. Journal of Geophysical Research, v.98, No. CIO, 18,181-18,199.
Inman, D.L. and J.D. Frautschy, 1965. Littoral Processes and the Development of Shorelines.
Proceedings of the 10th International Conference on Coastal Engineering, American Society
of Civil Engineers, Santa Barbara, CA, 511-536.
Inman, D.L. and S.A. Jenkins, 1983. Oceanographic Report for Oceanside Beach Facilities.
Prepared for the City of Oceanside, CA, unpublished, 206 pp.
Inman, D.L., P.D. Komar, and A.J. Bowen, 1968. Longshore transport of sand. Proceedings of
the J lh International Conference on Coastal Engineering, American Society of Civil
Engineers, 298-306.
Inman, D.L., and P.M. Masters, 1994. Status of research on the nearshore. Shore and Beach,
July, 1994, v. 62, n. 3, 11-20.
Jenkins, S.A. and D.W. Skelly, 1988. An Evaluation of the Coastal Data Base Pertaining to
Seawater Diversion at Encina Power Plant, Carlsbad, California. Prepared for San Diego
Gas & Electric Co., Carlsbad, CA, July 1988, 56 pp.
Jenkins, S.A., D.W. Skelly, and J. Wasyl, 1989. Dispersion and Momentum Flux Study of the
Cooling Water Outfall at Agua Hedionda. Scripps Institute of Oceanography, University Of
California San Diego, Prepared for San Diego Gas & Electric Co., Carlsbad, CA, September
1989, 36 pp. plus 3 appendices.
Jenkins, S.A. and J. Wasyl, 1994. Numerical Modeling of Tidal Hydraulics and Inlet Closures at
Agua Hedionda Lagoon, Part II: Risk Analysis. Submitted to San Diego Gas & Electric Co.,
Carlsbad, CA, 45 pp.
Jenkins, S.A. and J. Wasyl, 1997. Analysis of Tidal Inlet Closure Risks at Agua Hedionda
Lagoon, California and Potential Remedial Measures; Part II. Submitted to San Diego Gas &
Electric Co., Carlsbad, CA, 152 pp. plus appendices.
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Larson, M. and N.C. Kraus, 1989. SBEACH: Numerical Model for Simulating Storm-Induced
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Research Center, Vicksburg, MS, 256 pp., plus 2 appendices.
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CA, 35 sheets
Nordstrom, C.E. and D.L. Inman, 1975. Sand Level Changes on Torrey Pines Beach, California.
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Vicksburg, MS, December, 1975, Miscellaneous Paper No. 11-75.
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Woodward-Clyde Consultants, 1996a. Grain size distribution test results, proposed imported
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
APPENDIX A. SUBTIDAL TOPOGRAPHICAL SURVEYS
Subtidal-topographical surveys were conducted in the vicinity of Agua Hedionda Lagoon to
obtain accurate bathymetry for the area, locate areas of hard-bottom substrate, locate areas with
valuable sub-tidal marine habitat, and determine the sand thickness. The volume of nearshore
sand is needed to assess the relative importance of the disposal of dredged sediment from Agua
Hedionda Lagoon on the nearshore sand transport regime. Computation of the longshore
transport by Equation (3-1) gives the potential sand-transport rates. If there is no sand, then the
longshore sand transport is zero. If there is sand in the beach system, then adding a small
volume of sand ( < 100 yd3 /yd) may not significantly change the actual longshore sand transport.
However, large quantities of sand added to the beach system (> 600 yd3 /yd) may change the
longshore sand transport if the angle between the shore normal and the approaching waves is
changed.
A.1. BATHYMETERY, HARD SUBSTRATE, AND SAND THICKNESS
Bathymetry, sub-bottom profile, and hard-substrate surveys were made in the vicinity of
Agua Hedionda Lagoon for this study on March 12, 1998. These surveys covered the study area
from 4,500 ft north of the intake channel to 1,500 ft south of the discharge channel. The total
length of the survey was about 9,000 ft. The surveys were conducted with a survey vessel
equipped with a Digital Global Positioning System (DGPS) navigation system, a 200-k:Hz
fathometer, a 3.5-kHz sub-bottom profiler, and a 500-kHz Side-Scan Sonar system. Figure A-1
shows the vessel survey tracks. Data from the surveys and DGPS were simultaneously recorded
on a computer. Figure A-2 shows the bathymetry plot for the area between 10-and 60-ft water
depth with the hard-bottom substrate. The contour lines of the bathymetry are, in general,
parallel to the shoreline.
The presence of sub-tidal hard substrate was determined with a Klein 595 Side-Scan Sonar
System and a 500-kHz tow fish. The hard-bottom substrate was divided to three classes
(10-30%, 30-60%, and 60-100%). These percentages give the ratio of hard material to sand
coverage, (i.e., I 00% = no sand). The sub-tidal hard-substrate area extends from the ·intake
channel to about 2,000 ft to the north and to about 500 ft to the south.
The inshore extent of this reef was also investigated. On July 1998, Mr. William Boyd, a
scientific diver, field manager, and electrical engineer, dove on the north sub-tidal hard-substrate
reef to determine its inshore boundary. He concluded that the reef is well exposed from the low
tide terrace out to the offshore extent of his survey (8-10 ft) along the total length from the north
jetty to about 2,000 ft north (Figure A-2). At this point, he noticed an abrupt change to the sand
bottom and the reef rock was less visible (Boyd, 1998).
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
3610007
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Reference Number 98-11
A-2
' 1666000 1667000
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
362000-__ _,_ ___ --'--~
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Figure A·2. Bathymetry and substrate exposure In the vicinity of Agua Hedlonda Lagoon.
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Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Figure A-3 shows the sand-thickness contours between 10 and 60 ft of water. The sand
thickness varied from about 2 ft or less near the 10-ft contour to 8-12 ft at water depths greater
than 60 ft. The sand thickness at Middle and South Beaches was greater than the sand thickness
on North Beach. This is probably resulting from the continuous replenishment of dredged sand
to Middle and South Beaches since 1954.
A sand-thickness survey of the beach was conducted on 19 May 1998 by water-jet probing
into the beach down to bedrock with a 20-ft long probe. The survey extended from the shoreline
to -10 ft. Sand probing was conducted at seven ranges (CB-0805, CB-0810, CB-0820, CB-0825,
CB-0830, CB-0835, and CB-0850). The locations of these ranges are shown in Figure A-4. At
each point, the sand thickness was estimated with standard survey techniques (Total Station) as
explained in Appendix C. Data from this beach survey were combined with the offshore sand-
thickness data. The results of the combined inshore and offshore sand-thickness surveys are
shown in Figures A-5, A-6, and A-7 for North, Middle, and South Beaches, respectively.
The errors in estimating the sand thickness by water-jet probing are minimal and the errors in
estimating sand thickness with a sub-bottom profiler is about ±1 ft. The errors associated with a
sub-bottom profiler were estimated by ground-trothing the data obtained by the sonar sub-bottom
profiler with water-jet probing measurements that were conducted by diver at several offshore
locations during the survey.
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Reference Number 98-11
A-4 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
3610007
I
I
3600oo~
357000-
356000
355000
Pacif i c Ocean
D 10-30% hard substrate
• 30-60% hard substrate
60-100% hard substrate
Scale (ft)
1000 2000
Notes:
3000
Coordinates are California State Plane in ft (NAO 1927).
Substrate was derived from 500 KHz side scan recordings
during 1998.
Contour interval is 2 ft. and represents unconsolidated
sediment thickness.
1660000 1661000 1662000 1663000 1664000 1665000
Encsubsslt1 .srf
rev. 7128/98
r
1666000 1667000
Figure A-3. Sediment thickness and substrate exposure in the vicinity of Agua Hedionda Lagoon.
Coastal Environments
Reference Number 98-11
A-5 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Tamarack Ave.
Pacific Ocean
Scale (ft.)
0 2000 -4000 6000
Figure A..t. Location of profiles where sand probing was conducted.
Coastal Environments
Reference Number 98-11
A-6 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
20
N earaho re: 11-Ju l-11
Ofhhore: 12-M er-18
c::==i Sand th.lckn••• 1re1 0
Eatlm ated profll•
{no dalo ovollol>le) -20
~ " z -40 !£
-10
I i ~
•• 0
.100
4500 4000 3100 3000 2500 2000 1500 1000 500 0
20
Nearahore : Ol·M•y•II
Offehore: 12-M ar-11
0
c:::=J Sand thlckn••• area
. -20
~ i -40 £
C 0
-60 1
•• 0
-1 00 4100 4000 3500 3000 2500 2000 1100 1000 500 0
20
Ne1rehor1 : 01-M 1y-ll
O ffah ore: 12-Mar-U • 0
c==i Sand thlcknaaa area
-20
~ " z • -40 g
C 0 "'
-10 E m
-10
-1 DO
4100 4000 31D0 3000 2500 2000 UOD tOOO 500 0
D la tan ca {ft)
Figure A-5. Sand thickness along North Beach at ranges CB-0850, CB-0835, and CB-0830.
Coastal Environments
Reference Number 98-11
A-7 Final Report
Study of Sediment Transport Conditions, Agua Hedionda lagoon, Carlsbad, CA
20 B .o 121
Ne•r•hor•: 01 •MI y-11
Off•hore: 12-Mar,II 0
c:=:::i S•ncl lhlckn••• u••
Eatlffl •t•cl profll•
fno <1111 1v1ll1ble) .20
~ " z .o E
C 0 ,..
•IO j ..
. ,o
,t 00
4100 4D00 HOO 3000 2100 2000 1500 1D00 SOD D
2D
Near1hor1: 11-Jul-tl
0 ff•h ore : 12-M ar-11 D S•nd ttllckne11 er••
E1tln, 1lecl pro Ille ,,
(no d 111 1v1ll1 bl•) -20
~ " z
·40 E
C 0 =
.1 D
;!
!l IU
•I 0
-1 DD
4500 4000 3100 UDO 2'00 2000 110D 1000 500 0
ZD
Ne1rthor1: 01-M ay-11
0 ff1hor1 : 12-M • r-11 • c:::::J aand thlc:kn••• area
E1llm1t1d prollle
(no date 1v1ll1bl1) -zo
I z .40 ~
l
-10
r di
-10
-100
4100 4001 UOI uoo 21DI JOOI 1100 toeo 500 0
0 1111~•• flt)
Figure A-6. Sand thickness along Mlddle Beach at ranges CB-0825, CB-0820, and CB-0810.
Coastal Environments
Reference Number 98-11
A-8 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
20
Nearshore: 09-May-98
Offshore: 12-Mar-98
0
c::::J Sand thickness area
Estimated profile
(no data available) -20
c > C) z
-40 e:
C 0 =
-60
ta > .!! w
-80
-------------------~-------t------'----+ -100 4500 4000 3500 3000 2500 2000 1500 1000 500
Distance (ft)
Figure A-7. Sand thickness along South Beach at range CB-0805.
Coastal Environments
Reference Number 98-11
A-9
0
Final Report
Study of Sediment Transport Conditions. Agua Hedionda Lagoon. Carlsbad, CA
APPENDIX B. WAVE EXPERIMENT
B.1. WAVE MEASUREMENTS
Wave measurements were obtained in water depth of 33 ft (10 m) just offshore from Agua
Hedionda Lagoon and Oceanside, California. The Carlsbad station was located 1,000 ft (330 m)
south of the intake channel and the Oceanside station was located at the former CDIP wave
gauging station at Oceanside, CA. InterOcean pressure/horizontal velocity sensors (PUV
instruments) were deployed at each location, which simultaneously collected data from 9 July
1998 to 22 September 1998. The PUV instruments record data to estimate both wave energy and
some basic properties of the local directional wave spectrum, such as the mean wave direction
and the longshore component of radiation stress, ,Sry. The instruments are described further by
Trageser and Elwany (1990). The locations of the PUV gauges are shown in Figure 3-7.
The PUV instruments sampled at a frequency of 1 Hz (sampling period of J second) for
approximately 35 minutes (2,048 data points) every 6 hours. Wave parameters for each data run
were estimated by splitting the 2,048 record into two 1,024-point records. The mean and second-
degree polynomial trends were removed from these records (de-tiding). A triangle taper (Parzen
Window) was applied, and each record was Fast-Fourier Transformed (FFT'd). The resulting
Fourier coefficients were "surface corrected" using linear wave-theory relationships. The
corrected coefficients were then combined to form the cross-spectral matrix between the pressure
and two components of velocity, and these matrix values were merged into approximately
0.01-Hz-wide frequency bands.
B. 1. 1. Data Analysis
Spectral Parameters
For each frequency band,/, the cross-spectral matrix defines ao, a1, b1, a2, and b2, which are
the first five terms of the frequency-directional spectrum, S(f,8), expressed as an infinite Fourier
series,
S(/,8) = !1[1 + 2f an(f)cos(nB) + bn(f)sin(n8)]
21l' 11•1
(B-1)
The directional Fourier coefficients are then used to estimate the wave energy as a function
of frequency (the energy spectrum), E(f),
Coastal Environments
Reference Number 98-11
E(f) = ao(f),
8-1
(B-2)
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Car1sbad, CA
and mean wave direction
0(/) = tan-1[h1 (/)/ ] /01(/)
(B-3)
In addition, the peak period, Tp, and mean direction at the peak period, Dp, are defined as
(B-4)
wheref,.a,; is the frequency band of the energy spectrum E(f) with the maximum energy. Energy
spectra and mean wave-direction estimates for one of the four daily samples collected during the
deployment period are shown in Volume II (Appendix C).
Integral (Bulk) Spectral Parameters
Spectral parameters are typically integrated (summed) across all energetic wave frequencies
to obtain more statistically stable estimates of integral (or bulk) properties of the wave field.
Energetic waves on the U.S. West Coast are generally found between 0.05 and 0.26 Hz
(4-20 second periods). Spectral data in this frequency range were integrated to obtain estimates
of the significant wave height, Hs, and the total radiation stress, S;,:y,
0.26Hz
HS = 4,.jE, ; E, = J E(f)df
O.OSHz
l 0.26Hz
Sxy(f) = 2 Jao(f)b2(f)df
O.OSHz
The bulk mean wave direction, 0,, ,
Coastal Environments
Reference Number 98-11
0.26Hz
Jb1(f)df
0.0SHz
0.26Hz
Ja,(f)df
O.OSHz
B-2
(B-5)
(B-6)
(B-7)
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Spectral parameters are shown for the entire deployment period in Figure B-1 and provide a
synopsis of the observed wave conditions. The bulk parameters are also used to establish criteria
for adjusting historical Hs and SXJI data at the Oceanside site to more accurately reflect the
conditions at the Carlsbad location.
Synopsis of Measured Conditions
The instrument deployment period spanned the second half of the summer wave season in
southern California. This portion of the year is characterized by a nearly continuous presence of
long wave-period Southern Hemisphere Swell from the south, and more episodic, short-period
seas from the west generated by local winds. Both of these types of wave events can be seen in
the data summary plots in Figure B-1.
The largest wave events during the study had significant wave heights of 3 to 5 ft (1 to
1.5 m), Figure B-1 (top panel), with higher waves being measured at Oceanside compared to
Carlsbad. The larger events were swells with 12 to 20 second peak periods (second panel,
Figure B_.1) and approached the beaches from the south (third panel, Figure B-1). Local seas
with 5 to 8 second peak periods and directions from the west dominated briefly on three
occasions 1) Mid July, during the first few days of the deployment, 2) the end of July, and 3)
Mid September, during the last few days of the deployment.
The mean wave directions at the peak period at the two sites are offset by approximately 17°
for south swell, and 10° for west seas (third panel, Figure B-1). This is primarily because of the
change in coastline orientation from Carlsbad to Oceanside, with Oceanside being a more
southerly-facing beach. The direction of normal incidence (waves propagating straight into the
beach) is 230° for Oceanside and 242° for Carlsbad. The approach angels for the south swell are
further affected by Carlsbad Submarine Canyon, as discussed in Chapter 6.
The total radiation stresses, SXJI (bottom panel, Figure B-1 ), are calculated relative to the
measurement sites beach normals. The south swell resulted in positive total radiation stresses,
Sxy (bottom panel, Figure B-1 ), and northward sediment transport. The occasional west seas
produce negative total radiation stresses and southward transport. Larger, positive radiation
stresses were measured at Oceanside compared to Carlsbad, but similar negative radiation
stresses were measured at the two sites during the west-sea events.
Coastal Environments
Reference Number 98-11
B-3 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
-1:.
>, )( Cl)
Oceanside vs. Carlsbad : 9 Jul 98 to 22 Sep 98
6
Oceanside' Carlsbad
o.__ _____________ ..__ ______ ....._ ______________ _._ ___ __,
7.5 8 8.5 9 9.5
25,----,.-------,--------r--------.---------,------,
20
10
0 -+ + +
-++-aait ---=>a~ --------1191-0 +-tOOIB 0
~0
0
o o+ • ~ --..... ---~---+.-, -00. _, __ ,
o+ -•
+-tr ' 0 ~ ill?,'o 5L----L----~::._...LU------L-------L----
7.5 8 8.5 9
~ ++
010111
c1i
0
9.5
300..------.-------....... ------..-------,-------..------.
150.__ _ __,_ ______ ...._ ______ .__ _____ ____._ ______ ..._ __ __,
7.5 8 8.5 9 9.5
0.2
0
-0.2
7.5 8 8.5 9 9.5
Month of 1998
Figure B-1. Comparison of wave parameters between Oceanside and Carlsbad.
Coastal Environments
Reference Number 98-11
B-4 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
B. 1.2 The Relationship between Wave Conditions at Oceanside and Carlsbad
From these wave-measurement data, we can conclude that south-swell wave height and
radiation stresses are larger at Oceanside than Carlsbad, but west sea conditions at the two sites
are similar. In this section, wave model simulations were performed for the two sites to better
understand the similarities and differences in south-swell measurements, and to make a
judgement about possible differences for local seas from the south and winter swell arriving from
the west (wave conditions that were not observed during the instrument deployment period). In
addition, the measurements were used to derive empirical formulas to adjust Oceanside's
significant wave heights and total radiation stresses to be more representative of Carlsbad
conditions. These formulas were then applied to modify the long historical record of wave
measurements at Oceanside for use in sediment-transport calculations at Carlsbad.
Wave Modeling
A spectral refraction model (O'Reilly & Guza, 1991) was used to simulate the transformation
of waves over the local continental shelf to the two study sites. It was assumed that the two
locations were close enough together that variations in island sheltering were small compared to
more local bathymetric effects, most notably Carlsbad Submarine Canyon. The spectral-
refraction model calculates a combined wave shoaling and refraction transformation coefficient
between deep water and a shallow site. The transformation coefficient varies as a function of the
deep-water wave frequency, or period and direction and can be expressed as a relative wave
height, H/H0, where Ho is the deep-water wave height. The model was used to estimate
variations in H/H0 as a function of the deep-water wave direction for a typical swell peak period
of 14 second waves (Figure B-2), and a local sea with a peak period of 8 seconds (Figure B-3).
These simulations are not intended to be direct predictors of the observed wave heights. The true
incident deep-water frequency directional spectrum typically span a range of offshore periods
and directions at any given time. This is particularly true oflocally-generated seas.
The model simulations illustrate why Carlsbad has lower wave heights and positive total
radiation stresses compared to Oceanside. South swells typically have deep-water directions
(true compass direction "arriving from") of 170° to 200°. Carlsbad Submarine Canyon refracts
south swell away from the Carlsbad site, producing lower H/Ho values for Carlsbad in the upper
panel of Figure B-2. Carlsbad is essentially blocked from south swell approaching from deep-
water directions less than 180° and heavily sheltered for swell from 180° to 200°. Oceanside is
unaffected by the canyon and is exposed to more southerly deep-water swell directions. This
results in both larger waves and higher approach angles to its beach normal, which further
accentuates the larger positive radiation stresses at this location. The model's estimates of mean
wave directions at the two measurement sites (lower panel, Figure B-2) are consistent with the
Coastal Environments
Reference Number 98-11
B-5 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
range and offset of the measured mean directions for south swell (third panel, Figure B-1) and
provide further support for the inferences drawn from the model results.
Similarly, Carlsbad Submarine Canyon also affects shorter-period seas (Figure B-2). The
differences are not quite as large in the simulations, which is consistent with the fact that shorter-
period waves have shorter wavelength and are less affected by bathymetric features such as
canyons. . Nevertheless, the effect of the canyon on seas appears to be significant, and perhaps
most importantly, Oceanside continues to receive waves from higher southerly angles than
Carlsbad (bottom left comer of bottom panel, Figure B-3). Lacking direct observations of local
sea events from the south, it is assumed that the differences between the two sites for South Seas
is similar to those observed for south swell.
While the wave-height transformation coefficients for the two sites differ significantly for
waves from the south, they are much more similar for west swells and seas. Owing to the
sheltering of the offshore islands, North Pacific winter swell can only approach the
Carlsbad-Oceanside region from approximately 260° to 290°. For this range of deep-water
approach directions, the two locations show very similar transformation characteristics
(Figure B-2). The model simulations also show similar results at both sites for seas from the
west (Figure B-3), which is consistent to what was observed during the study period. (Carlsbad
may have lower wave heights than Oceanside for seas approaching from the northwest (290-
3100, Figure B-3), but the direction is rather extreme, and H/H0 ratios at both sites are drop off
sharply at these angles). In addition, unlike the south seas scenario, seas from the northwest do
not approach the Oceanside site at much larger angles (both sites show a maximum mean wave
direction of approximately 265° to 200° in the bottom panel, Figure B-3). It is unlikely that this
difference in simulated conditions would translate into significant measured differences in the
two sites' wave climates. Therefore, west swell and seas are assumed to be similar enough at
Oceanside and Carlsbad, that Oceanside data can be applied directly for these wave conditions.
In summary, the simultaneous measurements at Oceanside and Carlsbad only covered south
swell and west sea conditions. However, the model simulations suggest that differences between
the sites are primarily a function of the deep-water wave direction rather than the wave period.
Specifically, Carlsbad Submarine Canyon shelters the Carlsbad site from south swell and seas.
Therefore, it is assumed that observed differences between Oceanside and Carlsbad for south
swell can also be applied to local seas from the south, and observed similarities between the sites
for west seas is also true for west swell.
Coastal Environments
Reference Number 98-11
8-6 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
1.6
~o1.4 I
I
'1.2
ai :c
~ 1
~
CD 0.8 ~ 111 I £ 0.6
't:J Cl) t o.4
~
I
I ,,,. I
f .....
I
ID
w 0.2 I
I
I
0 140 160 180 200 220 240 260
Local Deep Water Direction (°'T)
!:260
Cl)
ffi250 ,,,
> ...
:::, ,,,
o.. 240
,,, ,,,
iii ., ,,,
C: / 0 / ~ 230 / ~ / i:5 / I 220
,,
/ ---
C: Sl 210 :::!!:
't:J Cl) 1u200 E ~
w 190
140 160 180 200 220 240 260
Local Deep Water Direction (°'T)
280
... .,
280
Oceansid, Carlsbad
T =14 seconds
300 320
300 320
340
340
Figure B-2. Comparison of Oceanside and Carlsbad waves at 14-second period.
Coastal Environments
Reference Number 98-11
B-7 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
1.6 ,----r---,---""'T""-----r----r-----,r-----r---...----....----.
~o1.4
t 1.2
'ii> ::c I 1
a, 0.8 -~ ]i
~ 0.6
'O Q) I o.4
:.:;
(I)
w 0.2
160 180
200 220 240 260
Local Deep Water Direction (°T)
200 220 240 260
Local Deep Water Direction (°T)
280
280
Oceansidt Carlsbad
T=B seconds
300 320 340
300 320 340
Figure B-3. Comparison of Oceanside and Carlsbad waves at a-second period.
Coastal Environments
Reference Number 98-11
B-8 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Empirical Ad;ustment of Oceanside Data
The simultaneous measurements are used to derive empirical expressions that can be used to
adjust Oceanside measurements of significant wave height, Hs and the total radiation stress, Sxy,
to be more representative of the conditions at Carlsbad.
The field observations and model simulations show that the differences between the sites are
strongly dependent on wave direction. Therefore, the ratio of Hs at Carlsbad and Oceanside was
plotted as a function of the bulk mean wave direction at Oceanside (Figure B-4). There is
considerable scatter in this relationship owing to the fact that multiple events often occur at the
same time (e.g. one or more south swells and west sea) complicating the relationship between the
measured mean-wave direction and differences in wave heights at the two sites. In addition,
although the PlN instruments are very accurate, there is significant statistical uncertainty
associated with estimating wave directions from 34-minute wave records. Attempts were made
to plot these ratios separately for each frequency band, but the statistical scatter in the mean
directions and wave energies was too severe to derive clear relationships. More robust bulk
mean wave directions are used to minimize these errors, but this comes at the expense of
merging sea and swell information, and increasing the scatter during multiple-event conditions
described previously. Nevertheless, the clearest Hs relationships emerged from the data using the
bulk mean direction approach.
The beach normal at Oceanside is at approximately 230°. Directions less than 230° are
approaching from the south and the increasingly smaller Hs ratios for more southerly waves
correspond to the increased sheltering of the Carlsbad site by the submarine canyon. Oceanside
mean directions greater than 230° are the result of west seas and there is no significant trend in
the Hs ratio, which is indicative of the similar wave conditions at these sites for west wave
events.
A linear trend was removed from the Oceanside data with mean directions less than 230°
(dashed line, Figure B-4). This corresponds to an estimate of Hs at Carlsbad, based on Oceanside
Hs, of the form,
Carlsbad Hs = Oceanside Hs [1 -0.02(230-8,, )] for Bi, < 230°
and
Carlsbad H& = Oceanside Hs for "ii,, ~ 230°
where "ii,, is the bulk mean direction at Oceanside. The adjusted values of Hs are shown in
Figure B-5 and are much more representative of Hs measured at Carlsbad.
Coastal Environments
Reference Number 98-11
B-9 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
1.5 .------,-,-----.,------,,.------,..-------,----..... ,----..--------.
--"C as .c
U) -c: as
0
@
Cf)
J:
0 ~
1 ....
a: 0.5 -
190
•
•
• •
• • • . ..
•
• • .. ·. . . . . .,. . • • •• • • ,.. • f ••••• -· . ~ .....
•
•
•• • • /a • ..... .,, . .....,. ,_ .... .
• • •• \ • • -.!. '·.. • • • • • . . .... ...... ., .,,. ... --~-. : . : ,,. ~., .. . .. , ... ,. . " ... .. . .
•• • I I• ' :,• ,_1. .. . . "'11t-...... .c !··· .•. . .. / . .. .
,/ : ":. . .
• •
• •
• . • • . • • • • • • • . • •
• • I • • •
•
,, . .
/
•
. ' . . ' .
200 210 220 230 240 250
Mean Wave Direction (f=0.05-0.26Hz) at Oceanside
-
-
.
260 270
Figure B-4. Ratio of H, at Carlsbad and Oceanside versus mean wave direction at Oceanside.
Coastal Environments
Reference Number 98-11
B-10 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
6
5
l
4 .,~.
g h
4 II) :r 3
2
1
• • • • • •
Oceanside
Adjusted Oceanside
Carlsbad
0'-------''------------'----------__.._ _________ _._ _________ _._ ____ _.
7.5
Coastal Environments
Reference Number 98-11
8 8.5 9
Month of 1998
Figure B-5. Adjusted Oceanside data relative to Carlsbad and original Oceanside data.
8-11
9.5
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
A similar approach was used with measurements of the total radiation stress, Szy at
Oceanside. However, in this case the directionality of the waves is implicit in the Sxy estimates
(positive values resulting from waves from the south and negative values for waves from the
west). Direct comparisons of the measured Sxy at the two sites (top panel, Figure B-6) show
more positive Sxy overall at Oceanside, with increasingly larger differences for large positive
values of Sxy (waves more from south). Two linear trends were used to characterize the
differences in Sxy (dashed lines, upper panel, Figure B-6), resulting in the following adjustment
of the Oceanside data:
Carlsbad Sxy = Oceanside Sxy -20 cm for Oceanside Sxy < 32 cm
Carlsbad Sxy = Oceanside 8;91 / 2.67 for Oceanside Sxy 2: 32 cm
The resulting correction is plotted in the lower panel of Figure B-6 and in time-series format
in Figure B-7. These Hs and Sxy adjustments were applied to the historical Oceanside data
collected by the Coastal Data Information Program at Scripps Institution of Oceanography from
1978 to 1994 to calculate the sediment transport for the study.
Coastal Environments
Reference Number 98-11
8-12 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
0.25 ....... . .........
0.2
0.15
N-e. 0.1 • •
~ Cl) • • •• -0 0.05 a, .0 •• -! a, 0 ........ • ' ... " .. " .. " . 0
-0.05
-0.1
-0.1 0 0.1 0.2
Oceanside Sxy (tt2)
0.25
0.2
0.15 ••••••• ,,, ••••••• ~ .................... 1 •••••• . .
N' 5. 0.1
~
. . . ......................................... . . .. . .
Cl)
1i 0.05
j . :
~ 0
-0.05 · · · · · · · •
-0.1
-0.15 ___ .._ _______________ _._ __ ___.
-0.1 0 0.1 0.2
ADJUSTED Oceanside Sxy (ft2)
Figure B-6. Carlsbad Sxy versus Oceanside and adjusted Oceanside Sxy,
Coastal Environments
Reference Number 98-11
B-13 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
0.3
0.25
0.2
Northward Transport
-0.1 •
-0.15 Southward Transport
-0.2
7.5 8
• • • •
8.5
Month of 1998
• •
9
Oceanside
Adjusted Oceanside
Carlsbad
9.5
Figure B-7. Longshore transport at Carlsbad compared to Oceanside.
Coastal Environments
Reference Number 98-11
B-14
•
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
APPENDIX C. MONITORING THE 1997-1998 SAND DISPOSAL
A beach-monitoring program for the 1997-98 SDG&E sand-disposal project was conducted
by Coastal Environments to determine the response of Middle and South Beaches to the sand
disposal. This program was beyond the contracted scope of work for this study. However,
because of the importance this information a sand-disposal monitoring study was implemented.
Data for this study were collected from twelve beach surveys conducted between May and
September 1998 (Table C-1 ). One survey was conducted at the end of the summer season, on
September 22 to complete the beach-profile survey effort. Measurements were taken along one
or more range lines out of seven range lines that covered the study area. lbree of these ranges
that were surveyed (CB-0820, CB-0830, and GB-0850) are the same ranges surveyed in the past
by the USACE and SANDAG. Figure A-4 shows the location of the ranges. All profile
measurements were restricted to wading depth, extending to about -10 ft below mean sea levC?l.
An electronic Total Station was used in the surveys, providing very accurate survey data. A
rod person carries a prism target at the top of a fixed-length pole along the pre-established
rangeline beginning at the benchmark and stopping every 10 to 15 ft, or at breaks in slope. The
Total Station operator focuses the instrument telescope crosshairs on the prism each time the rod
person stops and sets the rod. The station sends an infrared beam that is reflected by the prism.
The instrument calculates the slant distance and horizontal and vertical angles to the target from
the return signal. A hand-held electronic field-book data logger especially designed for the task
calculates the relative coordinates and elevation, and stores the results. A Sokkia SET-5 Total
Station and a Sokkia SDR-33 Electronic Field Book were the instruments used to conduct the
survey. Figure C-1 illustrates the survey method.
All measurements at a range were made relative to the first reading taken on the benchmark
at that range. Benchmarks have been placed on the back beach as close as practical to the face of
the sea cliff, or at street-ends. This procedure allows the comparison of the new profile data with
previous data gathered at Carlsbad, such as that published by Coastal Frontiers (1996, 1997, and
1998). The survey lines are oriented perpendicular to the mean shoreline using the same fixed
bearings as the historical ranges.
The data from 12 surveys are presented in Volume II. Figures C-2 and C-3 show selected
beach profiles on North and Middle Beaches. Figure C-2 shows that the profile located on North
Beach (CB-0835) eroded during the summer season. And Figure C-3 shows that the profile
located just south of the intake channel accreted during the summer. This is in agreement with
statements in Section 6.1 that during the summer season the longshore sand transport will move
sand to the north.
Coastal Environments
Reference Number 98-11
C-1 Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Table C-1. North, Middle, and South Beaches, Carlsbad, California,
Beach-Profile Program (09-May-98 to 22-Sep-98).
Benchmark
Station CB-0850
Station CB-0835
Station CB-0830
Station CB-0825
Station CB-0820
Station CB-0810
Station CB-0805
date
Coastal Environments
Reference Number 98-11
.,/
.,/
.,/
.,/
.,/
9
.,/
.,/
.,/ .,/
.,/
.r .,/ .r
.,/
.,/
23 1 10
MAY
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.,/
.r .r .r .r
.r
.r .,/ .,/ .,/ .,/ .r
.,/
.,/ .,/
22 30 8 13 19 31
JUN JUL
C-2
.,/
.,/
.,/
.,/ .,/
.,/ .,/
.,/ .,/
.,/ .,/
25 22
AUG SEP
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
Land Survey
.,...-
/ ... ~ .,,,,~ l ---
Figure C-1. Survey method of 1998 beach-profile survey program.
Coastal Environments
Reference Number 98-11
C-3
' Bench
Mark
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
,--------------------------------------------10
CB-0825
-9-May-98
.•••• -23-May-98
--25-Aug-98
.. + . -22-Sep-98
. .
. •
,,. . .,. -..... -.
.•
5
0 g
C, z
£
C 0 .I
-5 !
iii
-10
J---....-------1,.......--..-....---+--------....--+----..----+---------+-------.---+--.---.-----+ -15
700 600 500 400 300 200 100
Distance from Benchmark (ft)
Figure C-2. Beach-profile surveys at CB-0825, located on Middle Beach, just south of the intake channel.
Coastal Environments
Reference Number 98-11
C-4
0
Final Report
Study of Sediment Transport Conditions, Agua Hedionda Lagoon, Carlsbad, CA
-----------------------------------------.--10
CB-0835
-9-May-98
•••••• 23-May-98
----13-Jul-98
.. + •• 22-Sep-98
,/
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'l:' 'll .. .. ~·
/''/ .. : -· : . .
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•
5
0 -~ C> z
E
C 0 :; .-5 i
iii
-10
1---------.---;-------.----+------+-------+-------..--~-----,---1--------+ -15
700
Coastal Environments
Reference Number 98-11
600 500 400 300 200 100
Distance from Benchmark (ft)
Figure C-3. Beach-profile surveys at CB-0835, located on North Beach, north of the intake channel.
C-5
0
Final Report