HomeMy WebLinkAboutSDP 05-17; DONALD CONDOMINIUMS; WAVE ACTION & COASTAL HAZARD STUDY; 2006-06-13COASTAL HAZARD
&
WAVE RUNUP STUDY
2497 OCEAN STREET
CARLSBAD,CA
June 2006
Prepared For
R & M Enterprises
June 13, 2006
Mr. Peter Donald
C/O R & M Enterprises
24423 Whitaker Way
Murrieta, CA 92562
GeoSoils, Inc.
SUBJECT: Wave Action & Coastal Hazard Study 2497 Ocean Street, Carlsbad.
Dear Mr. Donald:
At your request, we are pleased to present the following report concerning the
vulnerability of the subject property to wave action and coastal hazards. This property,
and neighboring Carlsbad beaches, are situated along a moderately high wave energy
portion of the Southern California coast. This report constitutes an investigation of the
wave and water level conditions expected at the property in consequence of extreme storm
and wave action. It also provides conclusions and recommendations regarding the
stability of the beach, low bluff, and pre-Coastal Act seawall, garden walls and steps, and
the proposed new condominium home development envelope, and their respective
vulnerability to coastal hazards.
INTRODUCTION
The subject property, located at 2497 Ocean Street, Carlsbad, California, extends
between Ocean Street right-of-way on the east to the Mean High Tide Line (MHTL) of the
Pacific Ocean on the west (Lindvedt, McColl, 2006). The property has about 75 feet of
beach frontage. At present the topography of the site consists of a relatively flat beach,
a low wave cut bluff at about elevation +10 feet NGVD (GeoSoils, Inc 2006), and a
previously farmed, graded and residentially developed aeolian dune ridge (Shlemon,
2003). Figure 1 is an aerial photograph showing the site and adjacent shoreline in
October 2004. Figure 1 was down loaded, with permission, from the California Coastal
Records Project web site (http://www.californiacoastline.org/). A low height seawall,
constructed prior to 1950, is located about 140 west of the Ocean Street property line.
Presently vegetation and some wind blown sand cover the seawall . The beach between
the seawall and the MHTL consists of beach nourishment sand, covering native sand,
cobbles, and the formational sandstone (Santiago Formation). The beach in this area was
nourished by the SAN DAG regional beach replenishment project in the Fall of 2002. Much
of that nourishment sand is still in the beach profile above low water. The nourishment
sand thickness on the beach varies from 1 foot near the shoreline (0.0 feet MSL) to over
5 feet at the seaward side of the seawall.
5741 Palmer Way, Suite D, Carlsbad CA 92008 W.O. 5181 Phone 760-438-3155
GeoSoils, Inc.
Figure 1. Subject property and adjacent properties and shoreline in October 2004.
DATUM
2
The datum used in this report is the National Geodetic Vertical Datum (NGVD). In the
open ocean off the San Diego County coast, Mean High Water (MHW (Mean High Tide
Line]) is 1. 87 feet above MSL. The units of measurement in this report are feet (ft), pounds
force (lbs), and second (sec). A initial topographic survey was performed by Lindvedt,
McColl Associates on January 12, 2004, and additional site elevations where taken in May
2006. In May 2006 the MHT line was surveyed and located about 120 feet seaward of the
seawall.
5741 Palmer Way, Suite D, Carlsbad CA 92008 W.O. 5181 Phone 760-438-3155
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PROPERTY AND SITE INSPECTION
A visual inspection of the property and the adjacent properties was performed on
January 22, 2004, and again on June 9, 2006. The property is currently developed with
an older large single family home, various fieldstone garden walls and paths, private steps
to the beach and a low height, concrete and fieldstone, seawall. A review of historical
aerial photographs show that all of the improvements seaward of the residence have been
in place prior to 1950. The fieldstone garden wall (with a top elevation +17 .5 feet to + 19.5
feet MSL, some 15 feet to the rear and 7 feet above the top of the seawall) serves no
significant shoreline or bluff protective functions against combined high tide and storm
wave erosion, but may episodically (occasionally) be subject to wave splash. At present,
the visible top of the fieldstone garden wall tilts forward and should be maintained and
repaired in the next several years. The vegetation on this slope, between the seawall and
the fieldstone garden wall, shows now signs of erosion during the site inspections and in
the aerial photographs reviewed as part of this report. Hand excavation in January, 2004
at two test pits located near the northwesterly and southwesterly seaward ends of the
seawall to ~ +5.5 feet MSL identified the concrete and fieldstone construction of the wall,
and that it remains in fair condition. No structural development occurs on the beach part
of the property to the west of the seawall. The beach, on both days of inspection, was
relatively wide ( over 100 feet). The adjacent properties to the south of the subject property
are fronted by a quarry stone revetment and the properties to the north have vertical
concrete seawalls (see Figure 1 ).
OCEANOGRAPHIC PARAMETERS
The wave, wind and water level data used in the hazard analysis are taken from the
historical data reported in USACOE Coast of California Storm and Tidal Wave Study and
updated as necessary. The nearshore beach profile information was taken from nautical
charts. The shoreline erosion and beach change profile data was taken from the SAN DAG
regional beach profile monitoring program and the referenced US Army Corps of
Engineers Reports. Other oceanographic information is based upon our experience and
knowledge of the area.
Retreat Rate
There are two retreat rates of interest at this property: the retreat of the beach and
the retreat of the bluff. Over long enough periods of time, usually centuries, these rates
are essentially equal. However over shorter periods of time, usually days to months, a few
years and, as here, almost eight decades, they are seldom equal. The short term beach
retreat rate can be high with little or no bluff retreat over a longer time period. On the other
hand, the beach can significantly and temporarily erode such that the bluff may be
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exposed to wave action resulting in rapid bluff retreat. The aerial photographic record
since 1928 indicates that at the subject property a sand beach exists much of the time,
which reduces the likelihood of wave attack on the low seawall. However, as in 1989-
2001, erosion of that sand resulted in exposure of the cobble layer that underlies the
beach sand. A review of historical aerial photographs over the last eight decades shows
no measurable change in the location of the low coastal bluff. The US Army Corps of
Engineers (USACOE) as part of the Coast of California Storm and Tidal Wave Study
performed extensive analysis of the long term shoreline and bluff retreat rates along this
section of shoreline. USACOE 1987 concluded that comparison of 1 :24,00 scale maps
indicate from 1947 through 1983 there "was little or no change in the position of the base
of cliff or toe of slope" in the area. In addition, Figure 3-6a from USACOE 1991 also shows
no movement in the shoreline over the period from 1940 to 1989.
The referenced SANDAG report shows that for beach profile location CB-0880,
which is about 400 feet north of the subject property, the retreat rate ( changes in mean sea
level shoreline position) from Fall 1989 to Fall 2000 was 0.5 fUyr. This measured short
term erosion rate coupled with the fact that the beach was likely eroded back to the
existing shore protection during the 1978, 1982-83, 1988 and 1997-98 winters suggests
that the beach at the subject property may again be subject to extreme wave attack during
super elevated water conditions in the future. Due to the~ +11 feet NGVD elevation of the
seawall, the area between the top of the seawall and the fieldstone wall may have been
subject to rare episodic wave overtopping or spray. However, this filled slope seaward or
landward of the fieldstone wall shows no signs of erosion as a result of waves overtopping
and splash. The fieldstone wall, located at the top of a low bluff rather than in front of the
bluff face, does not function as a seawall.
Beach Slopes and Maximum Scour Depth
The slope of the beach face, the nearshore slope (below low tide) and the maximum
scour depth are used in the coastal processes analysis for this report. The beach slope
was measured in May 2006 and previously in January 2004 during the site inspections.
Due to the presence of the sand nourishment the actual beach slope was about 1 /1 O and
the nearshore slope to water depths of about 20 feet is 1 /40. The maximum scour depth
is not determined by the lowest water but rather by the materials that make up the beach.
There is a significant cobble layer beneath the existing beach sand. Once the sand is
eroded and transported offshore the cobble layer is exposed. The cobble layer is very
resistant to wave transport and remains intact even under extreme wave conditions.
Based upon direct observation of nearby beaches during the 1982-83 El Nino winter a
conservative maximum scour elevation is about -1.0' NGVD. It should be noted that this
scour elevation is used only for extreme wave runup and wave force analysis and not the
maximum scour at the seawall. Based upon a review of photographs taken when the
5741 Palmer Way, Suite D, Carlsbad CA 92008 W.O. 5181 Phone 760-438-3155
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beach was entirely eroded, and subsurface analysis on nearby properties where the
bedrock has been exposed, the bedrock Santiago formation at the subject property is at
about elevation +5 feet NGVD at the seawall. The onshore wind speed was chosen to be
40 knots.
Water Level
The following table is from the NOAA website for tidal datums and elevations for the
La Jolla Scripps Pier. This is valid for the Carlsbad shoreline.
HIGHEST OBSERVED WATER LEVEL (08/08/1983) = 7.81
MEAN HIGHER HIGH WATER (MHHW) = 5.37
MEAN HIGH WATER (MHW) = 4.62
MEAN TIDE LEVEL (MTL) = 2. 77
* NATIONAL GEODETIC VERTICAL DATUM-1929 (NGVD) = 2.56
MEAN LOW WATER (MLW) = 0.93
MEAN LOWER LOW WATER (MLLW) = 0.00
LOWEST OBSERVED WATER LEVEL (12/17/1933) = -2.60
Relative to mean sea level (MSL=NGVD29) and based upon the available data, the
highest observed water level is +5.25 feet MSL ( 7.81 feet MLLW). The lowest observed
water level is -5.21 feet MSL. During storm conditions the sea surface rises along the
shoreline (super-elevation) and allows waves to break closer to the shoreline and runup
on the beach to the seawall and potentially to the low bluff and fieldstone garden wall.
Superelevation of the sea surface can be accounted for by: wave set-up (1 to 2.5 feet),
wind set-up and inverse barometer (0.5 to 1.5 feet), wave group effects ( 1 to 2.5 feet) and.
El Nino effects (0.5 to 1.0 feet). These conditions do not occur simultaneously. The EPA
estimates an 8 -10 inch rise in sea level over the next 75 years (Titus and Narayanan,
1995). The extreme water elevation used in this analysis is +6.0 feet NGVD ( an
estimated100 year recurrence water level). This is basically the highest observed water
elevation of +5.25 feet MSL plus the predicted sea level rise of 0. 75 feet.
Waves
The San Diego North County shoreline, and the subject property, have experienced
a series of major storms over the years. These storm wave events have impacted coastal
property and beaches depending upon the severity of the storm, the direction of wave
approach and the local shoreline orientation and condition. During the 1982-83 El Nino
winter the beach along the Carlsbad shoreline was severely eroded and large waves
occurred during very high water levels. The existing seawall was likely overtopped but
there is no indication or record of significant damage to the fill slope or improvements
5741 Palmer Way, Suite D, Carlsbad CA 92008 W.O. 5181 Phone 760-438-3155
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behind the seawall. The design wave is not the largest wave to come into the area. The
larger waves break offshore of the beach and lose most of their energy before reaching
the shoreline. The design wave for the analysis uses the maximum still water level and
maximum scour depth to determine the maximum water depth at the toe of the seawall.
This water depth is used to compute the maximum breaking wave height at the seawall.
This wave will provide the maximum runup and wave force on the seawall. Again,
historical storms account for much higher wave heights but those waves break offshore
and do not impact the site as much as the wave breaking at the toe of the seawall. If the
total water depth is 7 feet, based upon a maximum scour depth at the toe of the seawall
of-1.0 feet NGVD and a water elevation of +6.0 feet NGVD, then the design wave height
would be about 6 feet.
WAVE RUNUP AND OVERTOPPING ANALYSIS
As waves encounter the beach in front of this section of shoreline the water rushes
up the beach and may runup the seawall. Often, wave runup strongly influences the
design and the cost of coastal projects. Wave runup is defined as the vertical height
above the still water level to which a wave will rise on a structure of infinite height.
Overtopping is the rate at which the runup water flows over the top of a finite height
structure. The seawall height for the analysis is ~+11.0 feet NGVD . Wave runup and
overtopping for the existing seawall is calculated using the United States Army Corps of
Engineers Automated Coastal Engineering System (ACES). ACES is an interactive
computer based design and analysis system in the field of coastal engineering. The
methods to calculate runup and overtopping implemented within this ACES application are
discussed in greater detail in Chapter 7 of the Shore Protection Manual ( 1984 ). The ACES
analysis was performed on oceanographic conditions that represent a typical 100+ year
recurrence interval. Table I is the ACES output for these design conditions.
AUTOMATED COASTAL ENGINEERING SYSTEM ... Version 1.02 2/11/2004 16:30
Project: WAVE HAZARD STUDY 2497 OCEAN STREET CARLSBAD
WAVE RUNUP AND OVERTOPPING ON IMPERMEABLE STRUCTURES Item Unit Value
Wave Height at Toe Hi: ft 6.000
Wave Period T: sec 18. 000 COTAN of Nearshore Slope 60.000
Water Depth at Toe ds: ft 7.000
COTAN of Structure Slope 0.000
Structure Height Above Toe hs: ft 12.000
Deepwater Wave Height HO: ft 3. 4 01
Relative Height {ds/H0): 2.058
Wave Steepness {H0/gT-2): 0.326E-03
Wave Runup R: ft 30. 876
Onshore Wind Velocity U: ft/sec 67.512
Overtopping Coefficient Alpha: 0.300E-01
Overtopping Coefficient Qstar0: 0.500E-01
Overtopping Rate Q: ft-3/s-ft 3 .193
Smooth Slope
Runup and
Overtopping
5741 Palmer Way, Suite D, Carlsbad CA 92008 W.O. 5181 Phone 760-438-3155
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The calculated overtopping rate is about 3 ft3/sec-ft of wall. Once this rather small
amount of wave runup water comes over the top of the sea wall it travels across and
upslope of the vegetation, which initially present substantial friction to the movement of
the water. Initially, the likely maximum distance the water will travel is on the order of a few
feet, well short of the fieldstone garden wall some 15 feet landward of the seawall and
about 7 feet above the seawal I. Wave run up to the fieldstone garden wall above elevation
+16 feet MSL is unlikely, since the design wave does not have enough energy to run up
beyond this elevation. As in the past this overtopping water is manageable with preferably
deep-rooted native vegetation on the slope below the fieldstone garden wall.
TSUNAMI HAZARD
Tsunami are waves generated by submarine earthquakes, landslides, or volcanic
action. Lander et. al. (1993) discusses the frequency and magnitude of recorded or
observed tsunami in the southern California area. James Houston ( 1980) predicts a
tsunami of less than 5 feet for a 500 year recurrence interval for this area. Any wave,
including a tsunami ,that approaches the Carlsbad Beach will be depth limited, that is to
say it will break in water depth that is about 1.3 times the wave height. The wave runup
and overtopping analysis herein considers the maximum possible unbroken wave at the
structure. This wave is about 6.5 feet high. The runup and overtopping analysis can also
serve to estimate the amount of wave overtopping as a result of a tsunami occurring at the
peak high tide. A 5 foot high tsunami, during a very high tide, will impact the site much
like the 100-year recurrence interval wave height overtopping. The tsunami, much like
the design extreme wave, will break on or before the structure, losing much of its energy.
Due to the infrequent nature and the relatively low 500 year recurrence interval tsunami
wave height, the site is reasonably safe from tsunami hazards.
CONCLUSIONS
■ The existing low seawall on the property is a pre-Coastal Act structure in fair
condition and whose continued stability will substantially protect the improvements
on the property behind it against most infrequent extreme wave run up or
overtopping.
■ The existing ice plant vegetation between the seawall and the low coastal
bluff/fieldstone garden wall contributes to protection of this older fill slope against
surficial erosion from infrequent wave runup or overtopping of the seawall.
■ The shoreline and bluff along this section of beach have been relatively stable
during the past almost 80 years. However, extreme events in the past have
resulted in significant erosion of the sandy beach, which most recently has been
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restored through a beach nourishment program. Future beach nourishment will
likely be required due to the negative sand budget in the littoral cell and sub-cell.
■ The site has been subject to wave attack from extreme oceanographic events in the
past. The existing seawall was constructed well prior to 1972 and remains in fair
condition. In conjunction with the vegetation behind it, it provides adequate
protection of existing improvements against wave overtopping and runup. The
fieldstone garden wall provides minor additional protection against any incidnetal
spray above the top of the low bluff.
■ The parcels (lots) to either side of the subject property have shore protection either
in the form of seawalls or quarry stone revetments.
■ Removal of the low seawall is not recommended, however, because it would likely
result in accelerated scouring of the back beach, especially in the areas adjacent
to the upcoast seawall or downcoast revetment, as well as instability in the slope
to the east and the improvements on it.
RECOMMENDATIONS
■ The existing, pre-Coastal Act seawall should be regularly inspected at 5 year
intervals, or following a 100-year recurrence high tide-storm wave event, to
determine whether maintenance or repair of the seawall is indicated.
■ No new seawall is needed to protect the proposed new two condominium homes
against projected shoreline erosion or bluff retreat, based on currently available
data.
■ The existing pre-Coastal Act fieldstone garden wall, located approximately 15 feet
to the east of the seawall, is in need of repair and maintenance, within the next
several years, to insure its continued proper function as a low retaining wall.
■ Deep-rooted native vegetation should be planted on the west facing slope of the
property and irrigation should be limited to its establishment or during extended
periods of drought, to minimize the application of water onto the west-facing slope.
All drainage of storm runoff from the proposed new condominium homes, decks,
driveway, patios/balconies, and walkways should be directed toward the City storm
drain system in Ocean Street and away from the beach, low bluff, and west-facing
slope on the property.
5741 Palmer Way, Suite D, Carlsbad CA 92008 W.O. 5181 Phone 760-438-3155
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■ The coastal hazards associated with the site are mitigated by the existing shore
protection provided by the seawall and vegetation on the slope behind the seawall.
In addition, the existing fieldstone garden wall provides minor protection against
sea spray. The coastal hazards do not create a danger for the proposed
development project. The existing seawall and garden wall, or the proposed site
development, do not create a hazard for, or adversely affect, the adjacent
properties or structures.
LIMITATIONS
Coastal engineering is characterized by uncertainty. Professional judgements
presented herein are based partly on our evaluation of the technical information gathered,
partly on our understanding of the proposed construction, and partly on our general
experience. Our engineering work and judgements have been prepared in accordance
with current accepted standards of engineering practice; we do not guarantee the
performance of the project in any respect. This warranty is in lieu of all other warranties
expressed or implied.
If you have any question regarding this letter report please contact us at the number
below.
Respectfully Submitted,
David W. Skelly MS,
RCE#47857
Coastal Engineer
No. C 47857 ,,
Exp. 12:{~, IO * .. if
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5741 Palmer Way, Suite D, Carlsbad CA 92008 W.O. 5181 Phone 760-438-3155
GeoSoils, Inc. 10
REFERENCES
Buck Thompson, Donald Condominiums Site Plan, 2497 Ocean Street, Carlsbad,
California, June, 2006.
Beck Architect, 1956 Photograph of Seawall, 2497 Ocean Street,
Carlsbad, California and Declaration, June, 2006. [Young lady on
seawall; declaration by same person.]
California Department of Boating and Waterways, 1994 "Shoreline Erosion
Assessment and Atlas of the San Diego Region" Volume II.
GeoSoils, Inc., Limited Geotechnical Investigation, 2497 Ocean Street, Carlsbad,
California, June, 2006.
Johnsson, Mark, Calif. Coastal Commission staff geologist. Email to Dave Skelly,
2003, re concurrence in Shlemon peer review document, 2004.
James R. Houston, 1980, "Type 19 Flood Insurance Study: Tsunami
Predictions For Southern California," USACOE Technical Report HL-80-18
Lander, James F., P. Lockridge, and M. Kozuch, 1993, "Tsunamis Affecting the
West Coast of the US, 1806-1992," NOAA National Geophysical Data Center
publication.
Lintveldt, McColl & Associates, Topographical Survey Map, 2497 Ocean Street,
Carlsbad, June, 2006.
SANDAG 2002, "State of the Coast Report Spring 2002, Beach and LAGOON
Mouth Monitoring Program" 44 pgs + Appendices
SANDAG, Beach Profile Monitoring Reports, 2002-200_.
Shlemon, Roy, Peer Review, 2649 Ocean Street, Carlsbad GEi Geotechnical
Investigation, K&AES Soils Analyses, and Skelly Engineering Wave Runup and
Coastal Hazards Study. 2003
Titus and Narayanan, 1995, "The Probability of Sea Level Rise" (EPA 230-R-95-
008).
USACOE 1984 Shore Protection Manual.
5741 Palmer Way, Suite D, Carlsbad CA 92008 W.O. 5181 Phone 760-438-3155
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USACOE 1987 CCSTWS report#87-2 "Coastal Cliff Sediments San Diego Region."
USACOE 1988 CCSTWS report #88-6 "Historic Wave and Water Level Data
Report San Diego Region."
USACOE 1991 CCSTWS -Main Report, "State of the Coast Report San Diego
Region."
5741 Palmer Way, Suite D, Carlsbad CA 92008 W.O. 5181 Phone 760-438-3155