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An Evaluation of the Coastal Data Base
pertaining to Seawater Diversion at Enoina Power Plant
Carlsbad, CA
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AGUA HEDIONDA
CIRCA 1889
L.
September, 1988
An Evaluation of the Coastal Data Base
Pertaining to Seawater Diversion at Encina Power Plant
Carlsbad, CA
Prepared For
San Diego Gas & Electric Company
by
Scott A. Jenkins, PhD
and
David W. Skelly, M.S.
September, 1988
Table of Contents
1.0 ABSTRACT
2.0 INTRODUCTION
3.0 SEDIMENT BUDGET FOR THE OCEANSIDE LITTORAL CELL
4.0 EFFECTS DUE TO JETTIES AT AGUA HEDIONDA
5.0 EFFECTS OF THE DISCHARGE PLUME AT AGUA HEDIONDA
6.0 SEDIMENT TRAPPING BY AGUA HEDIONDA LAGOON
7.0 CONCLUSIONS
8.0 RECOMMENDATIONS
9.0 REFERENCES
1.0 ABSTRACT
The purpose of this study is to review the existing
body of coastal processes data in the neighborhood of the
Agua Hedionda Lagoon, Carlsbad, CA. This review seeks to
determine if there is any evidence for adverse impact on
coastal processes associated with the sea water diversion
activities of the SDG&E Encina Power Plant. Of particular
concern are the possibilities for: a) obstruction of the
longshore transport of sand by the intake and discharge
jetties; b) diversion of beach sand to offshore areas, by
either the discharge plume or the jetties; c) trapping of
beach sands in the western most lobe of Agua Hedionda
Lagoon; d) long term losses of beach sand as a result of
inadequate maintenance dredging volumes; e) formation of
accretion erosion waves along downcoast beaches due to the
length of the dredging cycle. The data base is reviewed
both locally (the near field) as well as over the entire
length of the Oceanside littoral cell. In this way local
effects are separated from the long term trends of the
region in general.
The conclusions of this study are that: 1) In general
the coastline from Dana Point to Point La Jolla has been
eroding since the 1930's probably because dam building has
constricted the resupply of sediment from stream and river
runoff, 2) There is little evidence to suggest that beach
sediment is being diverted offshore by either the inlet
jetties or discharge plume, 3) The lagoon with its
artificially narrow inlet is indeed__a^.trap^ for beach sands,
4) Dredging records indicate that SDG&E has more than
matched the probable capture rates thru its maintenance
dredging program and therefore does not contribute to long
term downcoast erosion, 5) The dredging cycle is not often
enough to prevent accretion erosion waves in the down coast.
Accretion erosion waves represent local surpluses of sand
when the dredged material is deposited upon the beach during
dredging, followed by a local decline of sand occurring over
the time interval in between dredgings. They cause short
term oscillations in the downcoast sand supply superimposed
upon a steadily declining long-term mean.
2.0 INTRODUCTION
The Encina Power Plant diverts sea water from Agua
Hedionda Lagoon through plant condensers in order to provide
adequate cooling for each of five power generating units.
After passing through the condensers this sea water is
warmed about 10 degrees centigrade and released directly
into the ocean through a discharge channel fortified by a
pair of rubble mound jetties. The sea water taken from the
lagoon in this way is replaced by the tidal circulation
through an intake inlet channel situated at the north end of
the western most lobe of the lagoon system, see figure 1.
To augment this tidal circulation the lagoon system was
artificially deepened through dredging in 1954.
Prior to that dredging effort Agua Hedionda was a
slough with only a few feet of anaerobic hypersaline water
111111
WESTERNSECTION or
LAGOON
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SAN DIEGO GAS & ELECTRIC COMPANY
MAINTENANCE DREDGING
AGUA HEDIONDA LAGOON
ORIGINAL DREDGING LIMITS
AND DEPTHS, AND BEACH
RESTORATION AREA
Figure 1.
which exchanged with the ocean briefly during winter months
when high tides and rain runoff would broach the barrier
berm across the lagoon inlet, (see figures 2 and 3) Southern
Pacific Railroad Surveys (1889) and (1893). The pre-
dredging conditions of Agua Hedionda resembled somewhat the
present day conditions found at nearby Batiquitos Lagoon.
Over a period of 247 days beginning June 1953, a total
of 4,279,000 cubic yards of mostly beach grade sediment was
dredged from the Agua Hedionda Lagoon system. This total
volume broke down between the lobes of the lagoon system as
follows: 1,025,000 cubic yards from the outer or western
most lobe, and 3,254,000 cubic yards from the middle and
inner lobes, see Ellis (1954). This dredged material was
deposited on the neighboring barrier beach forming a large
deltaic shoreline form which had the effect of widening the
beach by an additional 120 meters, see figure 4. In order
to allow the intake and discharge flows to cross this man-
made delta, the intake and discharge channels were armored
with rubble mound jetty structures approximately 270 meters
in length.
The dredge delta caused wave energy to be focused on
this section of shoreline causing it to erode progressively
over time until the original beach width at Agua Hedionda
was re-established by 1956. As the delta eroded, rock was
removed from the intake and discharge jetties until their
lengths were reduced to their present nominal lengths, see
section 4. The 4 million cubic yards of material that had
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made up the dredged delta formation was transported
southward by the longshore current, forming an accretion
wave that was observable in beach profiles measured in
Torrey Pines between 1960 and 1963, see Inman and Jenkins
(1985).
Accretion waves result from the fact that the dredged
spoils migrate basically intact as a unit with a speed of
advance proportional to the longshore drift rate. The speed
of longshore migration of this accretion wave was found to
be 1.7 to 2.5 miles per year. Consequently nourishment to
the beaches at Carlsbad due to dredging of Agua Hedionda
does not remain indefinitely on Carlsbad beaches. Rather
this material migrates down coast, temporarily nourishing
other beaches along the way until eventually this
nourishment is permanently lost down the Scripps and La
Jolla submarine canyons. Following the accretionary part of
the wave is the erosion occurring in between dredgings while
the lagoon intercepts a portion of the longshore drift. The
erosionary cycle also migrates downcoast at the same
longshore migration rate as the accretionary portion of the
cycle. Therefore once dredged nourishment material migrates
past a particular beach site, erosion is generally observed
at that site until the following dredge cycle renourishes
that beach.
The original hydraulic modeling study for the Agua
Hedionda dredging plan, Ellis (1954), ascertained that a
minimum depth of -5 feet NGVD would provide an adequate
tidal prism to maintain inlet velocities sufficient to keep
the intake channel open through current scour action. This
value was consistent with the now widely accepted inlet area
to tidal prism relationship developed by Jarrett in 1976 and
later adopted in the Shore Protection Manual, USAGE (1985),
see figure 5. (The tidal prism is the volume of sea water
exchanged between the lagoon and the ocean in the time
interval between high and low tides.) However a bottom
elevation of -8 feet NGVD was required to provide minimum
working depth for the dredges used to deepen the lagoon
system. Consequently the final dredge configuration of the
lagoon had a mean tidal prism of 55 million cubic feet and a
diurnal tidal prism of 80 million cubic feet. Thus the
tidal prism in the final dredged configuration was somewhat
oversized or conversely the inlet cross section was somewhat
undersized according to the equilibrium inlet criteria in
figure 5. However since the subsequent expansion to five
power generating units and the resulting increased diversion
of tidal prism the ebb flow rates are now near equilibrium
values, see section 6.0.
3.0 SEDIMENT BUDGET FOR THE OCEANSIDE LITTORAL CELL
It is not possible to examine the cause and effect of
disturbances at any particular point along the coastline
with out first considering the trends over the entire
littoral cell which incorporates that point. This is so
because coastal transport does not respect mans political or
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11
geographic boundaries. However coastal transport can be
subdivided into physiographic units or coastal compartments
termed littoral cells. A littoral cell includes all the
sediment sources, transport pathways, and sediment sinks.
Normally a littoral cell includes the coastal mountains with
all the rivers and streams which drain into the sea, the
beaches, harbors, lagoons along the shoreline, and submarine
canyons which serve as the ultimate sediment sink to the
outer reaches of the continental shelf. Littoral cells are
laterally divided by natural barriers such as headlands and
mountains. In the case of the Oceanside littoral cell shown
in figure 6, these barriers are provided by Dana Point to
the north and Point La Jolla to the south.
Prior to mans development of southern California the
Oceanside littoral cell maintained an equilibrium by a
rather simple balance. The mountains provided the primary
source of sediments and the Scripps and La Jolla submarine
canyons provided the ultimate sink for those sediments.
Very little would change in the coastal zone most of the
time due to the benign weather of this area. On occasion
this weather pattern is broken by the occurrence of frontal
extra-tropical cyclones from the Gulf of Alaska in winter
and Mexican tropical hurricanes in late summer or early
fall. These storms typically generate large waves and are
often accompanied by rainfall in the coastal mountains.
Consequently erosion losses which occurred on the beaches
due to these large waves was replaced by the sediment laden
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Figure 6. The dams on the rivers which would otherwise transport sand to
I ittnral
13
runoff from the rainfall, see Brownlie and Brown (1978) and
Stow and Chang (1987). In this way the coastline of the
Oceanside littoral cell more or less maintained a natural
steady state balance between erosion losses to waves and
replacement of these losses by sediment laden rain runoff.
Longshore transport is integral to the function of this
steady state. The sediment load carried to the ocean by the
streams and rivers is initially deposited on the shoreline
as delta formations. These delta deposits are subsequently
spread along the shoreline by wave driven longshore
transport. Although there are periodic reversals, the long
term directional trend of the waves along Oceanside littoral
cell is towards the south, see Inman et al (1986).
Consequently the numerous delta deposits have a net trend of
drifting towards the south over time (termed littoral drift)
see Inman and Brush (1973), and Inman and Chamberlain
(1960). The littoral drift rates are estimated to be
260,000 cubic yards/year, Inman and Jenkins (1983). In
addition there is a seasonal cross shore transport which is
offshore in winter and onshore in summer. This cross shore
transport is estimated to be about 92 cubic yards/yard of
beach length and although it has no net loss or gain effect
on the sediment budget it does act to further smear out the
stream and river deposits along the shoreline. The
effectiveness of this natural nourishment and redistribution
system is evidenced in photographs taken of the Oceanside
14
beaches in 1916 where the beach foreshore was once
approximately 1,000 feet wide, see Inman and Jenkins (1983).
Mans intervention on the coastline began to interrupt
this natural nourishment and redistribution system in the
Oceanside littoral cell beginning with the construction of
the first dams and flood control debris basins in 1922. Dam
building continued within the Oceanside littoral cell until
1970 at which time every major stream and river within the
cell had been obstructed as shown in figure 6. Once this
had happened the sediment load from rain runoff was trapped
behind the dams and thus failed to reach the ocean to
replace the erosion losses of major storms. The total
deficit of sand yield to the beaches as a consequence of
damming of rivers in the littoral cell has been estimated to
be 14.5 million cubic yards, see Inman et al (1986).
Interesting enough this is about equal to the volume flux of
sand down the La Jolla and Scripps submarine canyons
estimated for the same period of time. Thus the steady state
balance between sediment loss and sediment resupply in the
Oceanside littoral cell has been upset in a very large scale
manner.
A portion of the long term sediment deficit of the
Oceanside littoral cell has been made up for by the sediment
yield from cliff erosion. When insufficient volumes of
beach sand cause the beach foreshore to recede landward,
waves are then able to act directly on the base of the sea
cliffs. This action results in the erosion of the sea cliff
15
often producing large scale block failures of the cliff.
The collapse of large blocks of sea cliff release hundreds
of thousands of cubic yards of potentially new beach
sediments into the nearshore zone. Cliff failures in the
Camp Pendleton region during the winters of 1980 and 1983
have released an estimated 1,000,000 cubic yards of beach
grade sediments to the Oceanside littoral cell, see Kuhn and
Shepard (1983), and (1984). However the data on cliff
failures indicate that there has not been nearly enough new
sedimentary material introduced in this way to make up for
the 14.5 million cubic yard deficit incurred since the dam
building and flood control program was initiated.
Furthermore cliff erosion is unacceptable when man has
already built homes, roads, drainage system and other
structures atop the sea cliffs. Additional contributions to
the littoral cell from this source will most likely be
arrested by the construction of shoreline protection usually
in the form of sea walls.
The sediment deficit of the Oceanside littoral cell
has been exacerbated by the way in which the large harbor
breakwater structure at Oceanside has rearranged the limited
existing supplies of sand in the nearshore, see Inman and
Jenkins (1983) and USAGE (1987). The first jetty system for
the Oceanside Harbor was erected in 1942. Since then six
enlargements of the jetty system were made until the final
2,300 foot long configuration was completed in 1957/58.
The enormous length of this breakwater system created a
16
significant obstruction to the longshore transport and
littoral drift, see figure 7. Much of the littoral drift was
impounded on the filet beach north of the jetty while the
configuration of the north jetty diverts the longshore
currents towards the offshore, forming strong rip currents
as shown in figure 7.
These rip currents cause sand to be deposited offshore
of the southern portion of the north jetty out to depths in
excess of 33 feet. One million five hundred eighty thousand
cubic yards of sand was deposited by rip current action
between 1942 and 1950 in this offshore region. An
additional 1,065,000 cubic yards was deposited offshore
between 1950 and 1972, see Inman and Jenkins (1983). The
cumulative effect of this offshore diversion of sand has
caused the depth contours in neighborhood of the Oceanside
Harbor to bulge seaward as shown in figure 7. Furthermore
USAGE (1987) has found that much of this offshore deposition
has formed in echelon bars that remain in deep water and do
not weld onto the foreshore further down drift from the
harbor. Therefore the offshore diversion of sand caused by
the rip current system around Oceanside Harbor has resulted
in a permanent loss of the already depleted reserves of sand
in the Oceanside littoral cell.
In addition to diverting sand offshore the Oceanside
Harbor also impounds beach sands within the harbor itself.
Historically this impoundment has been allowed to continue
for a few years until interfering with navigation in the
OCEANSIOE SHALL CRAFT HARBOR
-10
OCfOSITIOHAL J
OFFSHORE
Figure 7.
18
harbor. Maintenance dredging then placed these impounded
sediments back onto the beach to the south of Oceanside
Harbor. Maintenance dredging has been performed since 1944
in this way, bypassing the following volume to the down
coast beaches:
TABLE 1: MAINTENANCING DREDGING HISTORY AT OCEANSIDE HARBOR
Year Volume (cubic yards)
44 1,501,584
45 218,436
58 800,500
60 40,500
61 482,650
63 3,812,820
65 111,180
66 684,000
67 177,900
68 434,250
69 353,160
71 552,000
73 434,250
75 560,000
76 550,670
78 319,150
81 605,600
82 1,203,360
The average dredge cycle has been about two years and
has resulted in the bypassing of about 600,000 cubic yards
around the harbor each time. Consequently the bypassing
rates due to dredging have keep pace with the longshore
transport rates and therefore the impoundment of sand within
the harbor has not constituted a long term loss of sand to
the Oceanside littoral cell. However erosion has continued
in the interval of time which lapses between dredging while
the harbor is impounding sediment. This erosion then
propagates down coast to the south as an erosion wave at
19
speed of about 1.2 miles per year, see Inman and Jenkins
(1983). This erosion wave is then followed by an accretion
wave when the harbor dredging places 500,000 cubic yards or
more of sand on the beach in just one or two months time.
Dredged volumes of this order are nearly twice the annual
flux of sediment due to the longshore transport. Thus the
accretion/erosionary waves associated with the dredging
activities of Oceanside Harbor constitute large oscillations
in the local sand supply. Hence they are readily observable
in local beach profile measurements. Over the years several
accretion and erosion waves associated with the maintenance
dredging at Oceanside Harbor have been observable in the
beach profiles down coast from Oceanside, see Inman and
Jenkins (1983), Waldorf, et al. (1983), Shaw (1980) Cal Dept
Water Resources (1969), Flick, et al. (1986), and USACOE
(1967 and 1970).
The first erosion wave was observed up to four
kilometers south of the harbor in 1950. This wave was
deduced through comparisons of hydrographic surveys taken in
1934 before harbor construction with those taken in 1950.
The first accretion wave associated with harbor maintenance
dredging was observed beginning in 1963 when 3.8 million
cubic yards of dredge spoils from the harbor were placed on
the beaches south of Oceanside. The accretion wave from
1963 dredging project reached Torrey Pines beach in 1974.
The net effect of these accretion and erosion waves over the
long term has been to cause temporary surpluses of sand on
20
an otherwise steadily declining supply along the beaches of
the Oceanside littoral cell. In an effort to alleviate the
accretion and erosion waves and provide a more continuous
program for the beaches south of Oceanside, the U. S. Army
Corps of Engineers initiated design in 1983 and construction
in 1986 of a sand bypassing system at Oceanside Harbor.
Attempts to find alternative sand supplies for the
increasing sediment deficit of the Oceanside littoral cell
has led to studies assessing the inventory of offshore
sedimentary reserves, see Osborne et al. (1983). Figure 8
gives a map showing the significant deposits of beach grade
sediments that were found between Oceanside and Agua
Hedionda Lagoon. Note that even though there were borings
and vibracores taken in the neighborhood of Agua Hedionda no
significant offshore deposits were found in this area. On
the other hand a very large depositional area was found off
the north breakwater of Oceanside Harbor, consistent with
the offshore diversion of sand by rip currents from the
Oceanside breakwater. There is also another significant
offshore deposit just south of Oceanside Harbor which trails
away, also the bar system of the main offshore accretion
area off the north breakwater. These offshore deposits are
described in USAGE (1987).
In summary railroad surveys and photographic evidence
indicates that the shoreline of the Oceanside littoral cell
was once stable and characterized by sandy low steepness
beaches. This shoreline stability began to breakdown in the
21
I nautical mile «79.6
(UF) (M'F,- Boundary of borrow
area
U. S. Army Corps of
Engineers vibracore
station. Letters in
parentheses denote
sand suitability.2053 '986
(M*,(MF)
Figure 8. Map of area IV showing locations of borrow areas SD-I and SD-II
with associated vibracore numbers and sand suitability symbols.
22
early 1930's following construction of dams and debris
basins on the streams and rivers throughout the Oceanside
littoral cell. Since then an estimated 14.5 to 20 million
cubic yards of beach grade sediment has been prevented from
reaching the ocean by the presence of these dams. In the
same period of time an equivalent volume of sand has been
eroded by waves and carried away by longshore currents to
the south where it has been permanently lost down the
Scripps and La Jolla submarine canyons. Meanwhile the
construction of the Oceanside Harbor has diverted
approximately 5 million cubic yards to offshore bar
formations. Consequently there has been a large scale long
term denuding of sediments from the beaches up and down the
Oceanside littoral cell. The trapping of sand by Oceanside
Harbor has not contributed to this long term deficit because
maintenance dredging volumes have kept pace with longshore
transport rates. However the dredging frequency has not
been often enough to prevent short term erosion accretion
waves which temporarily exacerbate or alleviate the long
term progressive loss of beach sand at any given point along
the littoral cell.
4.0 EFFECTS DUE TO JETTIES AT AGUA HEDIONDA
There are two mechanisms by which harbor and inlet
structures can divert beach sediments into offshore shoals.
These two mechanisms are reflection and rectification as
shown schematically in figure 9. The reflection mechanism
23
MECHANISMS
A) REFLECTION, 0(1)
kb X SURF ZONE X X XBREAKWATERS
I«=FLUX DENSITY
^;::v-;V:: LOW STEEPNESS BEACH^^v^
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Figure 9,
24
results from the fact that rubble mound breakwaters and
jetties are steeper than the native beaches and consequently
reflect a greater percentage of wave energy. In fact the
undisturbed native shoreline reflects no energy whatsoever
since 100% of the incident wave energy is dissipated in the
surf zone. On the other hand the U. S. Army Corps of
Engineers studies due to Madsen and White (1976) have shown
that conventional breakwaters and jetties have a non-
negligible reflectivity (as high as 34%) for typical slopes
of 1/2 and 1/3. As a result there is a non-negligible flux
of reflected energy directed seaward. This seaward energy
flux is acting down the slope of the shelf and therefore a
significant seaward transport of sediment occurs with the
help of gravity.
In addition to the reflected wave carrying sediment
seaward, the longshore current with its suspended load may
also be diverted seaward by the presence of jetties or
breakwaters. This phenomenon was first noted by Wiegle in
1938 after completion of the Santa Barbara Harbor and again
by Inman in 1953 at the Oceanside Harbor as diagramed in
figure 7. There is also a tendency for the weaker mass
transport currents (bottom wind) to converge at the head of
the jetties and breakwaters due to local refraction effects
as detailed by Lamour and Mei (1977). All these mechanisms
can work together in tandem to work sediments from nearby
beaches to shoals that are seaward of the structure. In all
25
cases the magnitude of this seaward flux of sediment is
directly proportional to the length of the structure.
The length of the north jetty at Oceanside Harbor is
4,347 feet, and extends approximately 2,300 feet seaward of
the original mean high water mark on the shoreline. On the
other hand the jetties at Agua Hedionda are only a fraction
of this size. San Diego Gas and Electric surveys of the
intake jetties are shown in figure 10 while the discharge
jetties appear in figure 11. The longest of the two intake
jetties is 368 feet, but only 145 feet of this length is
seaward of the mean high water contour. Only that portion
of the jetty that actually projects into the ocean causes
reflection or diversion of the longshore transport.
Similarly the longest discharge jetty is 376 feet of which
only 110 feet is seaward of the mean high water line . Thus
as far as interference with shore processes is concerned,
the jetties have an effective length of 145 and 110 feet
respectively.
In order to divert littoral sediments seaward by the
mechanisms outlined in figure 9, the length of the jetties
must equal or exceed the width of the surf zone, Xb. On a
plane beach with slope tan ft, the width of the surf zone is
a function of the breaker height Hb and may be estimated by
the following formula,
Hb
b 0.78 tanp
According to the surveys due to Osborne (1983) the mean
beach slope off Agua Hedionda is tan 0 = 0.0278.
26
SAN DIEGO GAS & ELECTRIC
FORM 111-152
PROJECT £*.
SUBJECT
PAGE,of
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liV?TgV?COMPUTED BY ft fi P _ DATg
CHECKED BY.
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SAN DIEGO GAS & ELECTRIC
FORM 111-152 PAGE 2
JOB NO.
COMPUTED BY
CHECKED BY
PROJECT
SUBJECT
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28
Consequently the width of the surf zone will exceed the
effective length of the intake jetty (the portion seaward of
the mean high water contour) when the breaker height is
greater than 1.82 meters or 5.9 feet. Similarly when the
breaker height is greater than 1.49 meters or 4.8 feet the
surf zone will be wider than the effective length of the
discharge jetties.
According to wave refraction studies by Inman and
Jenkins (1983) and direct observations by Seymour et al.
(1982-1988), wave heights in the range of 1.5 to 1.8 meters
are not at all uncommon along the shoreline in the
neighborhood of Agua Hedionda. Figure 12 shows a wave
refraction diagram for the February 16, 1983 storm. This is
a typical wave window for waves generated by the pre-frontal
conditions of Gulf of Alaska storms. We see that the
refraction effects of Carlsbad Canyon cause a convergence of
wave rays or focusing in the neighborhood of Agua Hedionda.
This focusing in turn results in higher wave heights in this
vicinity increasing the likelihood that the surf zone will
be wider than the length of the intake or discharge jetties.
In addition wave observations at Oceanside Harbor by Seymour
et al. (1982-1988) have found that wave heights exceed 1.8
meters 600 hours per year or 6.8% of the time during an
average year such as 1987. In a stormy year such as the El
Nino year of 1983, wave heights were observed in excess of
1.8 meters over 1,000 hours per year or 11.4% of the time.
Estimates of sediment transport by Seymour et al. (1982-
HOVE REFRRCTION DIRGRRM
j 1 OCEANSIOE
•§£PIER
BUEMA ^\
VISTA /I
LAGOON pC
AGUA
HEDIONOA
LAGOON
vo
OCEflNSIDE
'17'. SEC PERTOD 23W DEEP HRTER DTRECTTON-
SCRLE 1CH-0.66KH
CONTOURS IN FATHOMS
Figure 12.
I'*.30
1988) for this same region indicates that 88% of the annual
sediment transport occurred during 1987 when the wave
heights were greater than 1.8 meters. In a stormy year such
as 1983, 94.6% of the longshore transport occurred when the
wave heights were in excess 1.8 meters. Therefore the surf
zone will be wider than the length of the jetties at Agua
Hedionda during the times when most of the longshore
sediment transport is occurring.
When the surf zone is wider than the shoreline
obstructions, such as a pair of jetties, there is a tendency
for the longshore transport to naturally bypass that
obstruction. The mechanism for this natural bypassing has
been outlined in Dean (1980) and elaborated on by Inman
(1987) and involves the formation of a bypassing bar around
the seaward head of the shoreline obstruction as shown in
figure 13. The bypassing bar causes waves to break seaward
of the jetties and allows the surf zone transport to take a
route inside the break point around the jetties as shown in
figure 13. Evidence for a bypassing bar at the head of the
intake jetties at Agua Hedionda was found in hydrographic
surveys taken in 1979 performed in conjunction with the
crater sink fluidization experiments conducted jointly by
San Diego Gas and Electric and Scripps Institution of
Oceanography, see Jenkins et al. (1980). During these
experiments which ran from August through December 1979,
sand tracer measurements determined that approximately 20
cubic yards per hour of beach grade sediments are
AGUA
HEDIONDA
LAGOON
FLOOD
TIDE
LOSSES
SOUTHERLY
LONGSHORE
TRANSPORT
SOUTHERLY
LONGSHORE
TRANSPORT
PREVAILING
NORTHWEST SWELL
BY-PASSING
OVER BAR
Figxore 13.
32
transported around the Agua Hedionda inlet by way of the
bypassing bar. This rate would account for approximately
67% of the annual sediment transport rate. The remaining
33% was found to be carried into the lagoon during the 12
hours of flood tide each day, see section 6.
These local observations suggest a minimal potential
for the diversion of beach sands to the offshore by the
jetties at Agua Hedionda. The available offshore bathymetry
also supports this conclusion. Figure 14 shows a composite
of depth contours out to the 15 fathom contour over the 38
year period from 1934 until 1972 . The original survey,
USCGS (1934) shows the bottom contours as solid lines.
Thirteen years after the construction of the jetties a
second survey was taken by Continental Shelf Data Systems
(1967) shown as dotted contours in figure 14. Five years
after that in 1972 the National Oceanic and Atmospheric
Administration conducted a third survey represented by
dashed lines in figure 14. It is apparent from these
surveys that there are no seaward bulges in the depth
contours to suggest that nearshore sediments are being
diverted offshore by the jetties at Agua Hedionda.
Furthermore the 5 fathom contour is in nearly an
identical location in 1972 as it was in 1934 before the
lagoon was deepened and the jetties constructed. This is
significant because shoreward of the 5 fathom contour is the
zone most influenced by wave driven transport or things
which effect wave driven transport. Seaward of the 5 fathom
USCGS
— •« CSDS
NOAA
34
contour we find a general seaward prograding of depth
contours up and down the coast in the interval between 1934
and 1967. Following 1967 there was a general shoreward
regression of depth contours back to 1934 values. Because
each of these depth contours remained parallel during these
seaward and shoreward progressions and regressions it is
possible that there was some systematic vertical datum error
in the 1967 CSDS survey.
Vibracores taken by Osborne et al. (1983) found no
evidence for recent offshore sedimentary deposits in the
neighborhood of Agua Hedionda. Fischer et al. (1983)
determined that in general the veneer of sediments on the
upper slopes of the continental shelf is uniformly thin from
Dana Point to Scripps Canyon, and noted no special
exceptions to this observation anywhere but in the
neighborhood of Oceanside Harbor. Therefore no particular
significance is assigned to movement in the depth contours
seaward of the 5 fathom line in figure 14 since these
changes are uniform up and down the coastline and would
therefore reflect long term adjustments in the Oceanside
littoral cell if attributed to other factors than
observational error.
Not only does the offshore bathymetry show little
change shoreward of the 5 fathom contour, several beach
profile monitoring programs in this area have given the same
conclusion as well. Waldorf et al. (1983) measured beach
profiles just to the north, in front of, and just to the
35
south bf Agua Hedionda (ranges 5,6 and 7) between December
1981 and February 1983. A typical set of variation in beach
profiles over this period is shown in figure 15 for range 6
located off the barrier beach in front of Agua Hedionda.
Note that all the beach profiles, aside from whatever
seasonal variations they display, merge to the same depths
offshore beyond 350 to 450 meters from the berm crest. This
condition is called closure.
If there was a progressive accumulation of sand
offshore then closure would not have been observed, but
rather each successive profile would end at a slightly
shallower depth. Similarly an independent set of beach
profiles was conducted by the Nearshore Research Group of
Scripps Institution of Oceanography, CCSTWS (1984), over the
period from Oct. 1983 to Oct. 1984. The range line numbers
CB0830, CB0820, and CB0810 corresponded to ranges 5, 6, and
7 of the survey program of Waldorf et al. (1983). This
later set of surveys not only achieved closure in the
offshore over the period from 1983 to 1984 but also achieved
closure with the earlier surveys of Waldorf et al. (1983)
for the period from 1981 to 1983. Therefore direct and
highly accurate observations with good vertical datum
control have found no new accumulations of sediment offshore
of Agua Hedionda during the three year period from 1981 to
1984.
Carlsbad
20 5 82 9 :20, RflNGE 6 C
17 9 82 10:3M, RflNGE 6 (
16 12 82 10:M5, RflNGE 6 L
I I I
100 300 500
SAND VOLUME (m3/m of BEACH)
Figure 15.
DISTflNCE OFFSHORE (M)
- 15
-10
..0
CE
LU
•'-10
00en
-15
H50 400 350 300 250 200 150 100 50 0
37
5.0 EFFECTS OF THE DISCHARGE PLUME AT A6UA HEDIONDA
The same offshore and nearshore bathymetry evidence
provided in the previous section also indicates that the
discharge plume diverts a negligible and unobservable
quantity of beach and littoral sediments into the offshore.
Beach profiles on range lines 6 and 7 from Waldorf et al.
(1983) and range lines CB0820 and CB0810 from CCSTWS (1984)
flank either side of the area effected by the discharge
plume. Even so both these profiles exhibit closure in the
offshore over the period from 1981 to 1984. Furthermore the
CSDS survey in 1967 and the NOAA survey in 1972 show no
seaward bulges or deltaic formations in this region.
There is also indirect biological evidence to indicate
that the discharge plume does not reach a significant
distance into the offshore and does not divert sediment
offshore. The upper lethal temperature limit for kelps
native to this region is between 21° and 22° C, see Kinnetic
Laboratories Inc. (1987). There is very little difference
in the present distributions of kelp compared with that
observed in 1951 prior to the construction and seawater
diversion at the Encina Power Plant, see Jackson and Winant
(1983), (and Wheeler North personal communication). The
local kelp populations have been a highly visible
environmental indicator which SDG&E has been required to
monitor semi-annually as part of the operating permit issued
by the California Coastal Commission. Any large scale
38
deleterious effects which the discharge plume might have
caused in consequence of offshore intrusion would surely
have attracted further scientific scrutiny long before the
writing of this report. Furthermore these populations are
highly sensitive to the amounts of suspended silts and
sediments in the water column and to the temperature of the
water. If the discharge plume, which is heated to as high
as 10.5° C above ambient, were reaching far seaward without
mixing then the warmer waters would undoubtedly have a
deleterious effect on the kelp populations to the south of
the discharge. In a similar way if the discharge plume were
transporting large volumes of sediments offshore then the
holdfasts on which kelp is dependent for rootage would
become buried with the subsequent result of exterminating
these populations.
The intensity of seaward transport by the discharge
plume under varying discharge rates and oceanographic
conditions has never been directly measured. The question
is how long the plume remains intact before mixing dilutes
its transport capacity causing it to drop the sediments it
might be carrying. On the one hand the thermal
stratification provided by the artificially warm water will
inhibit mixing and allow the plume to project further
seaward. On the other hand the plume must penetrate the
surf zone which has been shown in the previous section to
often be wider than the length of the jetties. The surf
zone is a very efficient mixing machine and will act to
39
diffuse the transport momentum of the discharge plume.
Observations by Woodward-ENVlCON (1974) indicates that the
thermal discharge plume is reduced to within 0.6° C of
ambient within a distance of 1,000 feet from the discharge
jetties. This indicates that mixing is indeed vigorous,
corresponding to a diffusivity of the order e = 10 cm2/sec,
i.e. four orders of magnitude greater than molecular
diffusion. Diffusivities of this order of magnitude are
quite typical in the surf zone, see Sleath (1987).
The axial velocities of the discharge plume can be then
estimated to decay seaward of the discharge jetties at a
rate proportional to the -1/2 power of the seaward distance
in accordance with the behavior of a two dimensional
turbulent free jet, see Schlichting (1968). The mean
velocity in this two-dimensional jet will decay to less than
the threshold velocity of the sediment, ut, once the plume
has traveled a distance x seaward:
x=(u/ut)2w (2)
where u is the initial discharge velocity and w is the width
of the channel. Measurement of the discharge velocities at
maximum flow rate of 450,000 gpm by SDG&E (1987) found
values as high as 3.5 knots (1.8 m/sec) at the discharge
culvert. However by the time this flow reaches the end of
the outfall jetties the velocity is about 1 knot (0.5
m/sec). The discharge channel width at mean water is 17
meters according to a recent survey. Therefore the
discharge plume drops below the minimum velocity required to
40
I «!• transport 176-210 micron sized sand (u^. = 20 cm/sec) after
having traveled seaward a distance of 106.2 meters or 348
feet. Thus it would appear that transport of beach sand by
I the discharge plume is not possible more than 106 meters
seaward of the jetties even in the complete absence of
waves.
To estimate the seaward influence of the discharge
plume in the presence of waves we examine the conditions
under which the seaward flux of energy of the discharge
plume is balanced by the shoreward flux of energy due to the
incoming waves. The wave height for which the outgoing and
incoming energy flux is just balanced is given by :
(3)
where g is the acceleration of gravity. Again taking
maximum discharge velocities to be of the order 1.8 m/s
(SDG&E 1987) we find that the seaward flux of energy by the
discharge plume will be arrested by the shoreward flux of
energy due to the waves when the breaker height exceeds 0.38
meters or 1.26 feet. Note that this is significantly less
than the minimum wave height which make surf zones wider
than the lengths of the jetties and transports the
preponderance of littoral drift (see section 4.0). Thus the
discharge plume appears to be able to exert little influence
on seaward directed sediment transport under the conditions
when most of the sediment transport occurs.
41
6.0 SEDIMENT TRAPPING BY AGUA HEDIONDA LAGOON
There are two dominant mechanisms which cause the Agua
Hedidnda Lagoon to trap beach and littoral sediments: 1)
the intake velocities which oscillate at tidal frequencies
are rectified by diversion of a substantial fraction of the
tidal prism through power plant condensers and, 2) the
intake jetties leak and are too short to prevent the
ingestion of surf zone suspension during flooding tides. In
natural lagoon systems the same volume of water which enters
the lagoon inlet on flooding tide leaves the lagoon through
that same inlet during ebbing tide. This is not the case at
Agua Hedionda where some of the water leaves the lagoon by a
separate route via the cooling plant condensers and the
discharge channel. Consequently the ebbing currents through
the lagoon inlet are weaker than the flooding currents.
This asymmetry in the inlet flow velocities is called
rectification. This feature alone would preferentially
favor sediment transport into the lagoon. In addition there
is no agitation mechanism at work inside the lagoon
analogous to that provided by the waves in the surf zone
outside the lagoon. Consequently beach size sediment is not
readily mobilized into suspension within the lagoon to
induce seaward transport on the ebbing currents.
According to direct measurements with all five power
generating units in operation, the maximum flow rate thru
the condenser systems of the Encina Power Plant is
648,000,000 gallons/day, see SDG&E (1987). During any given
42
6 hour period 21,657,754 cubic feet of seawater, or 27% of
the diurnal tidal prism, is diverted through the plant
condensers. Consequently the maximum intake flow rates will
be 3,704 cubic feet/sec through the inlet during flooding
tide while only 2,701 cubic feet/sec will return seaward
through the inlet during ebbing tide. A nominal discharge
rate through plant condensers more typical of average
operating conditions would be 550,000,000 gallons/day. This
accounts for 33% of the mean tidal prism, and will result in
flood tide flow rates through the inlet of 2,546 cubic
feet/sec as compared to ebb tide flow rates of only 1,695
cubic feet/sec. Thus the tidal circulation through the
inlet is highly rectified providing about three times more
energy flux to transport sediment into the lagoon on
flooding currents as compared to the energy flux available
in the residual tidal prism on ebbing flow. Energy flux of
the tidal flow is proportional to the cube of the flow
velocity. This fact tends to make the lagoon tidal
circulation a one-way transport engine favoring transport
into the lagoon.
The short length feature of the intake jetties was
found to be critical in minimizing their impact in the
offshore region, see section 4.0. However this same feature
makes them ineffective in blocking the ingestion of surf
zone suspension on flooding tide. This ingestion is
particularly active during storm periods when the surf zone
is wider than the length of the jetties (when the breaker
43
heights are greater than 1.8 meters, see section 4.0).
These are the same conditions when transport in the surf
zone is most active accounting for 88-94% of the annual
littoral drift (see section 4.0). The proximity of the
bypassing bar to the inlet (see figure 13), allows the
littoral drift to be freely entrained by the flood tide
currents entering the lagoon. In addition the intake
jetties are of typical rubble mound construction and are
therefore quite porous. Consequently there are steady
ventilation flows which leak laterally through the jetties
carrying a portion of the surf zone suspension directly into
the inlet channel.
Flood flow currents through the inlet are typically on
the order of 1.75 knots or 0.79 m/s which is more than
adequate to exceed the threshold scour stress for the 176
micron size sediment that typifies the accumulations within
the lagoon, see Table 2 and Leighton and Associates, (1988).
Normal threshold current speeds for sand of this size is on
the order of 0.4 knots or 0.2 m/s. Once the flood flow
passes under the Highway 101 bridge the lagoon rapidly
widens causing the flood flow to diverge and the currents to
rapidly decelerate. Consequently flooding flows drop below
the threshold speeds once entering the lagoon causing the
suspended load to be dropped to form a sand bar at this
point as shown in figure 13. Since ebbing flows are weaker
than flooding flows, threshold current speeds will not be
44
reached at the point of deposition during the ensuing ebb
tide.
TABLE 2: Grain Size Distribution Beach Sands at Agua
Hedionda
MICRONS PHI PERCENT CUM PERCENT
2000.000
1414.213
1000.000
707.107
500.000
353.553
250.000
176.777
123.000
83.388
62.500
44.194
31.250
22.097
-1.000
-0.500
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
4.500
5.000
5.500
0.000
0.016
0.000
0.063
0.973
9.616
37.835
39.953
10.212
1.302
0.031
0.000
0.000
0.000
0.000
0.016
0.016
0.078
1.051
10.667
48.502
88.455
98.667
99.969
100.000
100.000
100.000
100.000
Another feature of the lagoon which exacerbates the
entrainment of suspended sediment during flooding flow is
the fact that the lagoon inlet is artificially too narrow
for the flood tide flow rates and tidal prisms. As
discussed in section 2.0 this condition resulted from the
requirement to overdredge the lagoon during construction to
accommodate the draft of the dredges used for the project.
In view of the subsequent expansion to five power generating
units this overdredging was a fortuitous event. Even though
flooding currents through the inlet are approximately 37.5%
larger than those required for equilibrium, see figure 5,
the ebb flow currents are now close to equilibrium values in
consequence of the increased diversion of tidal prism
through the additional condenser units. It is certainly
45
more important to have near equilibrium ebb flow rates in
order to minimize the possibility of diverting excessive
amounts of sands into offshore bars.
These inlet dynamics coupled with the deceleration in
tidal flow below threshold velocities upon entering the
lagoon result in nearly 100% of the trapped littoral sands
being deposited in the bar formations of the outer lobe of
the lagoon (see figure 13). The first maintenance dredging
activities to return these sands to the beach began in 1955.
Such maintenance dredging is actually an episodic version of
sand bypassing. The bypassing rates can by derived from the
dredging history at Agua Hedionda Lagoon which were compiled
by Shaw (1980) and SDG&E as follows:
Table 3
DATE QUANTITY (cubic yards)
1955 111,000
1957 231,000
1960 370,000
1961 225,000
1963 307,000
1965 220,000
1967 159,000
1969 96,700
1972 259,000
1974 341,110
1976 331,090
1979 397,555
1981 292,380
1983 278,506
1985 447,464
1988 333,930
It can be seen from this dredging history that
maintenance dredging at Agua Hedionda Lagoon bypasses
effectively 110-120,000 cubic yards/year around the lagoon
system. Since these dredging activities have maintained a
46
steady state configuration in the outer lobe of the lagoon
for over 30 years, it can be concluded that the lagoon traps
approximately 40% of the annual littoral drift. This agrees
roughly with the results of the sand tracers study conducted
in the neighborhood of the bypassing bar, see section 4.0
and Jenkins, et al. (1980), which concluded the lagoon
captured about 1/3 of the annual drift. Because this
material is always returned to the beaches at the end of
each dredging cycle, this trapping rate does not present a
long term loss to the Oceanside littoral cell. However
between the dredgings there is sufficient time for short
term reduction in sand supply to occur along beaches
immediately down drift (south) of Agua Hedionda Lagoon while
sand remains impounded in the lagoon system.
Typically, erosion waves can develop in-between
dredging cycles followed by accretion waves once the dredged
material has been returned to the beaches, Inman and Jenkins
(1983). The evolution of an erosion wave between the 1981
and 1985 dredge projects is clearly evident in the beach
profiles taken at down coast beaches by Waldorf, et al.
(1983) and Seymour, et al. (1984). These profile programs
were begun immediately following the 1981 dredging when
292,380 cubic yards were placed on the beaches. Initially
range 7 of Waldorf, et al. (1983) showed wide berm crests
and a broad foreshore. Subsequent surveys in 1983 and 1984
by Seymour, et al. (1984) at ranges CB0810, CB0800, and
CB0790 show a retreat of 35 meters in the position of the
47
berm crest, and a corresponding reduction in the foreshore
width. These changes persisted even after the seasonal
cross shore transport of the summer waves had returned bar
deposits from the offshore to the foreshore.
The likelihood of accretion erosion waves caused by
episodic dredging of Agua Hedionda Lagoon was brought to the
attention of the power plant manager in 1978. In response
to a memo from the Scripps Institution of Oceanography dated
May 1978, SDG&E contributed both personnel and equipment to
two joint sand bypassing experiments in the summers of 1978
and 1979. The purpose of these experiments was to test a
bypassing concept employing crater sink fluidization,
Jenkins, et al. (1980). The two experiments succeeded in
bypassing only 2,680 cubic yards from the flood tide bar in
the outer lagoon to the barrier beach. Although the power
and transport relation for the crater sink fluidization
concept were validated (Jenkins et al., 1980), long term
operation of the system was frustrated by the fouling
problem encountered with kelp which was carried into the
lagoon on a flooding tide and subsequently entangled itself
about the fluidizer pipes. As a result of these experiences
a modification to the fluidization system was developed
utilizing flexible hoses which were deployed and retrieved
on a daily basis to form a centrally located crater fed by a
radial array of fluidized trenches. This concept was
subsequently incorporated in the U.S. Army Corps of
Engineers Phase 1A bypassing plant at Oceanside.
48
The middle and inner lobes of Agua Hedionda Lagoon system
also trap sediments. Recent surveys conducted by SDG&E,
indicate that 290,000 cubic yards of sediments have
accumulated in the middle and inner lagoons since the
initial construction dredging in 1954. Grain size analysis
of these sediments by Leighton and Associates (1988)
indicate a high percentage of silt size material stratified
in layers between fine sand material. These grain sizes and
deposition patterns suggest that the source of these
sediments was likely to be runoff from surrounding flower
fields and urban developments. In fact most of the 290,000
cubic yards of infilling has occurred on the northeast side
of the inner lobes where most of the new develop and housing
construction has occurred. Runoff from the record rainfalls
of 1978, 1980 and 1983 produced debris flows which deposited
these sediments in the observed layers. It is highly
unlikely that any of this material entered the lagoon
through the ocean inlets since the coarsest fractions of
these deposits in the inner lagoons were finer than the
median grain sizes found in the outer lagoon and on the
adjacent beaches (176-210 microns). The deposition of this
terrigenous runoff material has had the effect of reducing
the initially oversized tidal prism of the lagoon system by
approximately 9.8%.
49
7.0 CONCLUSION
I). The coastline of the Oceanside littoral cell has been
suffering from chronic large scale erosion since the 1930's.
The root cause of this erosion^ is a diminishing supply of
sand to the shorelinefal; a consequence} of the constructionv^ ~ -"
of dams and debris basins on the streams and rivers on the
Oceanside littoral cell. Estimates of additional sand
volumes provided by cliff erosion and beach nourishment
programs have compensated for only a small fraction (22%) of
the estimated 20 million cubic yards sand deficit incurred
in the Oceanside littoral cell since construction of the
first dams in 1922.
II). There are no effects attributable to the Agua Hedionda
jetties and discharge plume apparent in the offshore
bathymetry or the beach profile monitoring programs
conducted throughout the region since the 1960's. Agua
Hedionda jetties are shorter than the width of the surf zone
during the times when the majority of the littoral drift is
occurring. Under these same conditions the shoreward energy
flux of the waves will exceed the seaward energy flux of the
discharge plume.
Ill). Agua Hedionda Lagoon is a trap for littoral
sediments. This trapping is unavoidable due to short
jetties and the diversion of 27-33% of the tidal prism
through plant condensers.
50
IV). Sand tracer experiments and dredging records at Agua
Hedionda indicate that maintenance dredging has at least
kept pace with the probable capturejrates^ of_the laigoon.
C 1 *~
V). Dredging frequencies as long as once every 30 months
are probably too long to avoid down coast accretion erosion
waves. Evidence for these erosion accretion waves are found
in beach profile monitoring data dating back as far as 1960.
51
8.0 RECOMMENDATIONS
I) Although SDG&E has taken a wide variety of physical
measurements of the discharge plume, most of these are not
suitable for measuring the mixing or seaward penetration of
discharge momentum. This could be determined by measuring
the offshore distribution of sea surface and near bottom
temperatures over a high density survey grid. The
diffusivity of momentum can then be inferred from the
diffusivity of heat by way of the Prandtl number. The use
of mini-range positioning equipment would be essential to
achieving sufficient grid resolution. A string of current
meters could be deployed seaward of the axis of the
discharge channel concurrently with the temperature
measurements in order to correlate in extent of seaward
intrusion of discharge momentum with the observed mixing.
Once this instrumentation is in place the experiment could
be conducted over the course of one or two tide cycles. It
would be desirable to measure both nominal and maximum plant
discharge rates during this interval of observation.
II) The long term bathymetry and beach profile data is
spotty. Therefore it has been difficult to separate effects
due to Agua Hedionda from the effects attributable to nearby
Oceanside Harbor and the long term depletion of sand along
the coastline of the Oceanside Littoral Cell in general.
Because such data is expensive and will not improve the
existing knowledge base for many years yet to come, a long
J< 52
f term program of beach profile and offshore bathymetry
monitoring would have only marginal value in present
planning.
III) The intake jetties should not be lengthened in order
i
to reduce trapping of sand by Agua Hedionda Lagoon. The
I '
present jetty length appears to have minimal adverse impact
on littoral processes. However sand ingestion by Agua
Hedionda could be reduced somewhat by grouting the existing
jetties. By reducing the porosity of the jetties it is
likely that a fillet beach will form along the adjacent
Carlsbad shores on the North side of the inlet jetty. This
fillet beach will also aid in natural bypassing of sand
around the inlet by forming a bridge between the surf zone
and the offshore bypassing bar.
IV) Coordination of dredging activities by SDG&E at Agua
Hedionda with bypassing and dredging activities at Oceanside
Harbor could result in a more uniform sand supply to the
beaches further to the south.
53
9.0 REFERENCES
Armstrong, G.A., 1977, "Assessment and atlas of shoreline
erosion along the California coast," State of California,
Department of Navigation and Ocean Development, 70 pp. and
277 pp. atlas.
Brownlie, W.R. and W.R. Brown, 1978, "Effects of dams on
beach and sand supply", p. 2273-2287 in Coastal Zone 78.
v.l, ASCE, NY.
Brownlie, W.R. and B.D.Taylor, 1981, "Sediment management of
southern California mountains, coastal plains and shoreline
Part C, Coastal sediment delivery by major rivers in
southern California" California Inst. of Technology.
Environmental Quality Lab Report 17-C, Pasadena, CA 314pp.
California Department of Water Resources, Interim Report on
Study of Beach Nourishment Along the Southern California
Coastline. July 1969.
California State Lands Commission, "An Index to Historical
Hydrographic and Topographic Charts of the California
Coast", Located in the Files of the State Lands Commission,
October 1979, 84pp.
Continental Shelf Data Systems, 1967 Bathymetric Map
Southern California, Oceanside 120:37:1, Zone 6
Ellis, J.D., 1954, "Dredging Final Report, Agua Hedionda
Slough Encina Generating Station" , San Diego Gas and
Electric, 44 pp.
Emery, K.O., "Beach Nourishment Along the Southern
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