HomeMy WebLinkAboutSDP 05-17; DONALD CONDOMINIUMS; WAVE ACTION & COASTAL HAZARD STUDY; 2004-02-23",,'
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Prepaf~d.' ~or :. ..•••
•• 1111,i: .. Peter /JonaJrJ.
~SKELLY ENGINEERING (GeoSoils, ·Inc.)
February 23, 2004
Mr. Peter Donald
C/O R & M Enterprises
_ 24435 Adams Ave
Murrieta, CA 92562
SUBJECT: Wave Action & Coastal Hazard Study 2497 Ocean Street, Carlsbad.
Dear Mr. Donald:
At your request, we are pleased to present the following letter report concerning
wave action and the vulnerability to coastal hazards at the subject property.
INTRODUCTION
The subject site, located at 2497 Ocean Street, Carlsbad, California, lies on top of
a wave cut sea cliff which backs a sand beach (see Figure 1 ). Figure 1 was down loaded
from the California Coastal Records Project web site ( http://www.californiacoastline.org/).
The lot is about 75 feet in width along the ocean parallel property line. The seaward
portion of the property is protected from wave attack by a low height seawall. The beach
in front of the site currently consists of sand, covering ·cobbles that overly a formational
sandstone. The beach in this area was nourished by the 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 to over 8 feet at the seaward property line. Below the sand is a cobble layer
which is underlain by the Santiago Formation, an Eocene bedrock material. This site, 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 site in consequence of extreme storm and wave
action. It also provides conclusions and recommendations regarding the stability of the site
and vulnerability to coastal hazards.
5741 Palmer Way, Suite D, Carlsbad CA 92008 Phone 760-438-3155
9:: SKELLY ENGINEERING (GeoSoils, Inc.)
Figure 1. Subject site in Fall 2002. Note the relatively wide sand beach and shore
protection fronting the adjacent properties.
DATUM
2
The datum used in this report is Mean Sea Level (MSL), which is +0.19 feet National
Geodetic Vertical Datum (NGVD). In the open ocean off the San Diego County coast,
Mean High Water (MHW) is 1.87 feet above MSL. The units of measurement in this report
are feet (ft), pounds force (lbs), and second (sec). A survey performed by Lindvedt, McColl
Associates on January 12, 2004 was used for site elevation.
SITE INSPECTION
A visual inspection of the site and the adjacent properties was performed on
January 22, 2004. There currently exists a single family residence on the site. The beach,
on the day of the inspection, was relatively wide (over 100 feet) as a result of the
nourishment project in the Fall of 2001. The site is currently protected from extreme wave
5741 Palmer Way, Suite D, Carlsbad CA 92008 Phone 760-438-3155
~SKELLY ENGINEERING (GeoSoils, inc.)
3
attack by a fieldstone type seawall. Because the seawall is almost fully covered by the
beach sands placed during the recent nourishment, a complete inspection of the entire wall
was not possible. However, two hand dug excavations at the either end of the wall were
performed during the site inspection. The excavations extended down to the water table,
which was about 5 feet below grade. The elevation of the top of the wall was surveyed
during the inspection and is~ +11 feet MSL. The bottom of the wall was estimated to be
. at or below +6 feet MSL. The wall was built prior to 1960 and appears to be in fair condition
based upon our limited visual inspection. About 15 feet landward of the seawall is another
field stone wall. This wall is referred to herein as the secondary fieldstone wall. This
secondary wall is in poor condition. This wall may have been constructed to stop wave
run up that overtops the seawall. The top of this secondary wall is at about +18 feet MSL.
The area between the two walls is covered with vegetation which does not show signs of
any recent exposure to wave overtopping. The properties to the south of the site are
fronted by quarry stone revetments for shore protection and the properties to the north
have vertical concrete seawalls for shore protection.
OCEANOGRAPHIC PARAMETERS
The wave, wind and water level data used in the ha'zard analysis are taken from the
historical data reported in USACOE Coast of California Storm and Tidal Wave Study and
_updated as necessary. The beach profile information was taken from nautical charts. The
'and shoreline erosion and beach change profile data was taken from the SANDAG
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 site: 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 a few years, 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 is exposed to wave action resulting in rapid bluff
retreat. There is a beach fronting the subject property most of the time which reduces the
likelihood of wave attack on the seawall. A review of historical aerial photographs over the
last four decades shows no change in the location of the bluff and little if any shoreline
retreat. 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 concludes that
5741 Palmer Way, Suite D, Carlsbad CA 92008 Phone 760-438-3155
~SKELLY ENGINEERING (GeoSoils, Inc.)
4
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 shows no movement in the shoreline overthe period from 1940
to 1989.
The referenced SANDAG report shows that for beach profile location CB-0850,
_ which is close to the site, the retreat rate (changes in mean sea level shoreline position)
from Fall 1989 to Fall 2000 was 0. 7 ft/yr. This relatively high 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 winters demonstrates that subject property may again be subject to
extreme wave attack during super elevated water conditions in the future. The area
between the seawall and the fieldstone wall has likely been subject to wave action
overtopping the seawall. However, this area does not show any signs of significant
erosion due to waves overtopping the seawall. The lack of significant erosion in this area
is likely due to the presence of the vegetation and the secondary fieldstone wall.
Beach Slopes and Maximum Scour Depth
The slope of the beach face, the nearshore slope and the maximum scour depth are
used in the coastal processes analysis for the project. The beach slope was measured
during the time of the site inspection. Due to the presence of the sand nourishment the
'actual beach slope was about 1/10 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' MSL.
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 beach was entirely eroded, it is believed that the bedrock
Santiago formation is at about elevation +5 feet MSL 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
5741 Palmer Way, Suite D, Carlsbad CA 92008 Phone 760-438-3155
~ SKELLY ENGINEERING {GeoSoils, Inc.)
MEAN HIGHER HIGH WATER (MHHW) =
MEAN HIGH WATER (MHW) =
MEAN TIDE LEVEL (MTL) =
* NATIONAL GEODETIC VERTICAL DATUM-1929 (NGVD) =
MEAN LOW WATER (MLW) =
MEAN LOWER LOW WATER (MLLW) =
LOWEST OBSERVED WATER LEVEL (12/17/1933) =
5.37
4.62
2.77
2.56
0.93
0.00
-2.60
5
Relative to mean sea level .MSL=NGVD29 and based upon the available data, the
highest observed water level is +5.25' MSL( 7 .81' MLLW). The lowest observed water level
is -5.21' 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 sometimes to the secondary fieldstone 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 rarely 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 MSL (100 year recurrence water level). This
is basically the highest observed water elevation of +5.25' MSL plus the predicted sea level
rise of 0.75 feet.
Waves
The San Diego North County shoreline, and this site, has experienced a series of
storms over the years. These events have impacted coastal property and beaches
depending upon the severity of the storm, the direction of wave approach and the local
shoreline orientation. During the 1982-83 El Nino winter the beach fronting this site was
severely eroded and large waves occurred during very high water levels. The existing
seawall was likely overtopped but there is no record of significant damage to the
improvements 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' MSL and a water elevation of +6.0' MSL, then the design wave height
would be about 6 feet.
5741 fa/mer Way, Suite D, Carlsbad CA 92008 Phone 760-438-3155
~SKELLY ENGINEERING {GeoSQils, Inc.)
6
WAVE RUNUP AND OVERTOPPING ANAL vs·,s
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 flow over the top of a finite height structure. The
seawall height for the analysis is +11.0' MSL. . 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.
Table I
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
Wave Height at Toe
Wave Period
COTAN of Nearshore Slope
Water Depth at To~
COTAN of Structure Slope
Structure Height Above Toe
Hi:
T:
ds:
hs:
Deepwater Wave Height
Relative Height
Wave Steepness
HO:
(ds/H0) :
(HO/gTA2):
Wave Runup
Onshore Wind Velocity
Overtopping Coefficient
Overtopping Coefficient
Overtopping Rate
R:
U:
Alpha:
Qstar0:
Q:
Unit
ft
sec
ft
ft
ft
ft
ft/sec
ftA3/s-ft
Value
6.000
18.000
60.000
7.000
0.000
12.000
3.401
2.058
0.326E-03
30.876
67.512
0.300E-0l
0.500E-01
3.193
Smooth Slope
Runup and
Overtopping
The calculated overtopping rate is about 3 ft3/sec-ft of wall. Once the wave run up
water comes over the top of the wall it travels across the vegetation ar,d possibly to the
secondary fieldstone wall. The desigri' wave does not have eriough energy to runup to
elevations greater than about + 16 feet MSL. As in the past this overtopping water is
manageable with~the vegetation and secondary fieldstone wall.
5741 Palmer Way, Suite D, Carlsbad CA 92008 Phone 760-438-3155
~SKELLY ENGINEERING (GeoSoils, Inc.)
7
CONCLUSIONS
• The shoreline and bluff along this section of beach have been relatively stable over
the last several decades. However, extreme events in the past have resulted in
significant but temporary erosion.
• The site has been subject to wave attack from extreme oceanographic events in the
past. The existing seawall is in fair condition and in conjunction with the vegetation
and secondary fieldstone wall provides adequate protection of the site
improvements from wave runup and overtopping. Wave runup can reach as high
as approximately +16' MSL under the existing conditions.
• The properties to either side of the site have shore protection either in the form of
seawalls or quarry stone revetments.
RECOMMENDATIONS
■ Long term stability of the site and the adjacent properties depends on the presence
of the seawall, vegetation, and secondary fieldstone wall. Maintenance of the
seawall, vegetation, and secondary wall will insure the continued protection of the
site from infrequent extreme wave runup and overtopping.
■ The existing secondary fieldstone wall is in need of repair/replacement to insure its
continued proper function and protection of site improvements.
The coastal hazards associated with the site are mitigated by the existing shore
protection (seawall, vegetation, and secondary fieldstone wall). The coastal hazards do
not create a hazard for the site improvements provided that the existing walls are
maintained in proper condition. The seawall and site development does not create a
hazard for the adjacent properties or structures and does not adversely impact 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
5741 Palmer Way, Suite D, Carlsbad CA 92008 Phone 760-438-3155
~ SKELLY ENGINEERING {GeoSoils, Inc.)
8
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, P
RCE#47857
Coastal Engineer
REFERENCES
California Department of Boating and Waterways, 1994 "Shoreline Erosion
Assessment and Atlas of the San Diego Region" Volume II.
SANDAG 2002, "State of the Coast Report Spring 2002, Beach and LAGOON
Mouth Monitoring Program" 44 pgs + Appendices
Titus and Narayanan, 1995, "The Probability of Sea Level Rise" (EPA 230-R-95-
008).
USACOE 1984 Shore Protection Manual.
USA COE 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 Phone 760-438-3155