HomeMy WebLinkAbout20201223_Coastal Hazards_12-17-2020Geotechnical C Geologic C Coastal C Environmental
5741 Palmer Way C Carlsbad, California 92010 C (760) 438-3155 C FAX (760) 931-0915 C www.geosoilsinc.com
December 17, 2020 WO S7677
Mr. Jeff Bethel & Mr. Steve Ridge
365 Via Lido Soud
Newport Beach CA 92663
SUBJECT: Supplemental Coastal Hazard and Sea Level Rise Discussion for Two New
Residences, 361 & 365 Via Lido Soud, Newport Beach, Orange County,
California.
Dear Mr. Bethel & Mr. Ridge:
In accordance with your request and authorization, GeoSoils, Inc. (GSI) is pleased to
provide this supplemental discussion regarding the potential coastal hazards, including the
impact of future sea level rise (SLR), on the proposed construction of two new residences
in Newport Beach, California. The purpose of this report is to provide the hazard
information for your permit amendment application requested by the City of Newport Beach
and the California Coastal Commission (CCC). Our scope of work includes a review of the
State of California Sea-Level Rise (SLR) Policy Guidance document (March 2018), CCC
SLR Guidance (November 2018), a discussion of the proposed project, a site inspection,
and preparation of this letter report.
INTRODUCTION
The proposed project is construction of two new residential structures, in the City of
Newport Beach. Each structure will include a traditional housing unit and a junior
accessory dwelling unit. Figure 1, downloaded from Google Maps (Bird’s Eye View),
shows the site in relation to the adjacent properties, the public boardwalk, an intertidal
beach, and the navigation channel within Newport Bay. The two parcels are rectangular
shaped, about 35 feet by 90 feet each. The proposed project will create two new
residences on the two new parcels. The finished grade at the site is ~+12.1 feet NAVD88
with a lowest finished floor (FF) at about +12.8 feet NAVD88 for both residences. The top
of the City of Newport Beach owned boardwalk is at about elevation 8.5 NAVD88. The
project includes a privacy wall on the landward side of the public boardwalk and exterior
structure stem walls that extend to elevation +14.40 feet NAVD88. The site is currently
mapped by FEMA to be in the unshaded X Zone, with the adjacent bay mapped in the AE
Zone, with a base flood elevation of +8 feet NAVD88. The elevation of the Via Lido Soud
street flow line elevation is about +11.2 feet NAVD88.
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Figure 1. Subject site, 365 Via Lido Soud, adjacent properties, public boardwalk, and
Newport Bay channel.
DATA & DATUM
The datum used in this report is NAVD88, which is about 2.62 feet below the mean tide
level (MTL). The units of measurement in this report are feet (ft), pounds force (lbs), and
seconds (sec). Site elevations were taken from a topographic map prepared by Forkert
Engineering and Surveying, Inc., dated 10/26/18. Plans by Craig S. Hampton Architect,
showing future coastal hazard adaptability measures, dated 12/16/20, were reviewed. A
site reconnaissance was performed in July 2019. The offsite walkway/boardwalk was
observed to be in good condition. There is a small (about 5 feet wide) supra-tidal beach
fronting an intertidal beach bayward of the boardwalk.
HAZARD ANALYSIS
There are three different potential shoreline hazards identified at this site: shoreline
movement/erosion, waves and wave runup, and flooding. For ease of review, each of
these hazards will be analyzed and discussed separately, followed by a summary of the
analysis including conclusions and recommendations, as necessary
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Shoreline Erosion Hazard
There is no actual shoreline at the site proper. There is an intertidal beach bayward of the
public boardwalk/walkway. The beach is a receiver beach from dredging within the Newport
Bay navigation channels. The beach is maintained at a width that is adequate to protect
the public boardwalk. The shoreline is essentially located just bayward of the City
boardwalk. The public boardwalk will prevent shoreline erosion from impacting the
proposed development over the life of the development, provided the boardwalk is
maintained and protected by the City.
Current Flooding Hazard
The National Oceanographic and Atmospheric (NOAA) National Ocean Survey tidal data
station closest to the site with a long tidal record (Everest International Consultants Inc.
(EICI), 2011) is located at Los Angeles Harbor (Station 94106600). The tidal datum
elevations are as follows:
Mean High Water 4.55 feet
Mean Tide Level (MSL) 2.62 feet
Mean Low Water 0.74 feet
NAVD88 0.0 feet
Mean Lower Low Water -0.2 feet
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. Super-elevation of
the sea surface can be accounted for by: wave set-up, wind set-up and inverse barometer,
wave group effects and El Niño sea level effects. The historical highest ocean water
elevation at the Los Angeles Harbor Tide station is +7.72 feet NAVD88 on January 10,
2005. In addition, EICI reported that the elevation of 7.71 feet NAVD88 is the 1% water
elevation. For this analysis the historical highest water elevation will be +7.7 feet
NAVD88.
Future Tide Levels Due to Sea Level Rise
The California Coastal Commission (CCC) SLR Guidance document recommends that a
project designer determine the range of SLR using the “best available science.” When the
SLR Guidance document was adopted by the CCC in 2015, it stated that the best available
science for quantifying future SLR was the 2012 National Research Council (NRC) report
(NRC, 2012). The NRC (2012) is no longer considered the state of the art for assessing
the magnitude of SLR in the marine science communities. The California Ocean
Protection Council (COPC) adopted an update to the State’s Sea-Level Rise Guidance in
March 2018. These new estimates are based upon a 2014 report entitled “Probabilistic
21st and 22nd century sea-level projections at a global network of tide-gauge sites” (Kopp
et al., 2014). This update included SLR estimates and probabilities for Los Angeles Harbor
the closest SLR estimates to Newport Beach. These SLR likelihood estimates are
provided below in Figure 2 taken from the Kopp et al., 2014 report. The report provides
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SLR estimates based upon various carbon emission scenarios known as a “representative
concentration pathway” or RCP. Figure 2 provides the March 2018 COPC data (from the
Kopp et al., 2014 report) with the latest SLR adopted estimates (in feet) and the
probabilities of those estimate to meet or exceed the 1991-2009 mean, based upon the
best available science.
Figure 2. Table from Kopp et al., (2014) and COPC 2018, providing current SLR estimates
and probabilities for the Los Angeles Harbor tide station.
This table illustrates that SLR in the year 2100 for the likely range, and considering the
most onerous RCP (8.5), is 1.3 feet to 3.2 feet above the 1991-2009 mean. Interpolating
the above table, the very low probability SLR (0.5%) for the year 2096, high emissions,
SLR is ~6.0 feet and for the year 2100 it is 6.7 feet. Based upon this table there is a much
lower probability (0.5%) of SLR of about ~6 feet to 6.7 feet. This SLR would account for
future extreme bay water level as high as of 14.4 feet NAVD88 (7.7 feet NAVD88 + 6.7 feet
SLR). As stated before, the present maximum (1%) historical water elevation at the site,
including El Niño effects, is +7.7 feet NAVD88. Based upon the elevation of the access
street flow line (11.5 feet NAVD88), the extreme Newport Bay water level will exceed the
height of the existing street flow line when SLR is 4.2 feet or greater. For the likely COPC
SLR estimate range (high emissions) the access street is safe from flooding until beyond
the year 2100. For the 0.5% SLR case this may occur in after the year 2080. It should be
noted that, if SLR is higher, flooding the street will not occur constantly but rather only a
few times a month, at the full moon and new moon, for a period of about 1 hour. With the
FF at elevation +12.8 feet NAVD88 the structure will be safe from flooding for SLR less
than 5.1 feet. For the 5% COPC SLR estimate range (high emissions) the structure is
safe from flooding until beyond the year 2100.
The Kopp et al. paper used 2009 to 2012 SLR modeling for the probability analysis, which
Low Medium - High Extreme Risk
Ave rsion Risk Aversion Risk A ve rsion
High emissions 2030 0 .3 0 .2 0 .5 0 .6 0 .7 1.0
2040 0 .5 0.4 0.7 0 .9 1.2 1.7
2050 0 .7 0 .5 1.0 1.2 1.8 2 .6
Low emissions 2060 0 .8 0 .5 1.1 1.4 2 .2
High emissions 2060 1.0 0.7 1.3 1.7 2.5 3 .7
Low emissions 2070 0.9 0 .6 1.3 1.8 2.9
High emissions 2070 1.2 0 .8 1.7 2.2 3 .3 5 .0
Low emissions 2080 1.0 0 .6 1.6 2.1 3.6
High emissions 2080 1.5 1.0 2.2 2.8 4.3 6.4
Low emissions 2090 1.2 0.7 1.8 2.5 4.5
High emissions 2090 1.8 1.2 2.7 3.4 5.3 8 .0
Low emissions 2100 1.3 0 .7 2.1 3.0 5 .4
High emissions 2100 2 .2 1.3 3 .2 4 .1 6 .7 9.9
Low emissions 2110' 1.4 0 .9 2.2 3.1 6.0
High emissions 2110' 2 .3 1.6 3.3 4 .3 7.1 11.5
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means the “best available science” as determined by the CCC is almost 10 years old. The
CCC SLR Guidance requires the use of the “best available science.” Dr. Reinhard Flick
from the Scripps Institution of Oceanography has provided information that global sea level
from 1992 to 2018 has resulted in 8.32 centimeters of relatively uniform SLR in the past
26 years. This information is shown on Figure 3 taken from the CCC SLR Guidance
(2015). This current measurement shows that SLR is tracking more on the intermediate
SLR prediction curves, which is more like a 50% (median) probability SLR in the year 2100,
as shown in Figure 2.
Figure 3. Current global SLR plotted on SLR prediction curves (graphic taken from
TerraCosta Consulting).
The recent global SLR measurement provided in Figure 3 shows that the current SLR
trend, is not following the higher SLR estimate curves. It is GSI’s professional opinion that
the methodology and SLR estimates suggested by the CCC SLR Guidance, based upon
current SLR trends, are an overestimate of SLR over the project design life of 75 years.
The City of Newport Beach has recognized that in the future there will be a need to raise
the elevation of the boardwalks and bulkheads around the Newport Bay. The City of
Newport Beach Building Department (CNBBD) has a standard drawing, and a regional plan
for rehabilitating and raising the bulkheads. With SLR greater than 3 feet the entire Lido
Island can be flooded from the several sections of the island shoreline that do not currently
have a shore protection device in place. Flooding through these areas will result in
flooding of the access roads to hundreds of homes which are not on the shoreline. The
island will require some form of shore protection before there is flooding of the residences.
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Modified from Figure 5 of the California Coastal Commission Sea Level Rise Policy Guidance
document adopted August 12, 2015.
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Waves and Wave Runup
The potential surface gravity waves (ocean swell) to arrive at this site is nil. Boat wakes
and wind waves are the only possible waves that can reach the bulkhead fronting the site.
Boast wakes are very small due to speed restriction in the channel and there is insufficient
fetch to produce wind waves or more than a couple inches.
Tsunami
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. Legg, et al. (2002) examined
the potential tsunami wave runup in southern California. While this study is not specific to
the site, it provides a first order analysis for the area. The Legg, et al. (2002) report
determined a maximum open ocean tsunami height of less than 2 meters. The maximum
tsunami runup in the Newport Beach open coast area is less than 1 meters in height. Any
wave, including a tsunami, that approaches the site in will be refracted, modified, and
reduced in height by the Newport jetties, and as it travels into the bay. Due to the
infrequent nature and the relatively low 500-year recurrence interval tsunami wave height,
and the elevation of the proposed improvements, the site is reasonably safe from tsunami
hazards.
It should be noted that the site is mapped within the limits of the California Office of
Emergency Services tsunami innundation map, Newport Beach Quadrangle (State of
California, 2009). The tsunami inundation maps are very specific as to their use. Their use
is for evacuation planning only. The limitation on the use of the maps is clearly stated in
the PURPOSE OF THIS MAP on every quadrangle of California coastline. In addition, the
following paragraph is taken from the CalOES Local Planning Guidance on Tsunami
Response concerning the use of the tsunami inundation maps.
Inundation projections and resulting planning maps are to be used for emergency
planning purposes only. They are not based on a specific earthquake and tsunami.
Areas actually inundated by a specific tsunami can vary from those predicted. The
inundation maps are not a prediction of the performance, in an earthquake or
tsunami, of any structure within or outside of the projected inundation area.
The City of Newport Beach and County of Orange have clearly marked tsunami evacuation
routes for the entire Newport Beach/Bay area.
CONCLUSIONS
• The proposed residential structures with the implementation of the SLR adaptation
strategies (design for future water proofing to above elevation +14.4 feet NAVD88
and the future protection of all the island residences by the City) will not be
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adversely impacted by potential coastal hazards including more than 6 feet to 6.7
feet sea level rise. The site will be part of a community wide response to mitigate
SLR hazards.
• Provided the recommendations of this report are implemented during the project
construction, no site specific protective devices will be necessary to protect the
proposed development from any existing or anticipated future coastal hazards for
the life of the development.
RECOMMENDATIONS
Based upon the analysis and discussion herein, the proposed development is reasonably
safe from coastal hazards for the life of the development including shoreline movement,
waves and wave runup, and flooding with future SLR. It should be noted that future
flooding hazards due to SLR are shared by all development around Newport Bay. The
public roads for access to the site will be impassable due to ocean flooding long before the
flood water level approaches the FF elevation of the development. SLR impacts will be a
regional problem and only solved by a regional management plan. The proposed
development will neither create nor contribute significantly to erosion, geologic instability,
or destruction of the site or adjacent area.
The opportunity to be of service is sincerely appreciated. If you should have any
questions, please do not hesitate to contact me.
Respectfully submitted,
GeoSoils, Inc.
David W. Skelly MS, PE
RCE#47857
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REFERENCES
Everest International Consultants, Inc., 2011, Assessment of seawall structure integrity and
potential for seawall over-topping for Balboa Island and Little Balboa Island, main report,
No Project No., dated April 21.
Kopp, Robert E., Radley M. Horton Christopher M. Little Jerry X. Mitrovica Michael
Oppenheimer D. J. Rasmussen Benjamin H. Strauss Claudia Tebaldi Radley M. Horton
Christopher M. Little Jerry X. Mitrovica Michael Oppenheimer D. J. Rasmussen Benjamin
H. Strauss Claudia Tebaldi “Probabilistic 21st and 22nd century sea-level projections at
a global network of tide-gauge sites” First published: 13 June 2014
Newport Beach, “Waterfront Project Guidelines and Standards, Harbor Design Criteria
Commercial & Residential Facilities,” 2017 Edition
NOAA, 2018, Web Site, Maps http://anchor.ncd.noaa.gov/states/ca.htm Tidal Datums
http://www.opsd.nos.noaa.gov/cgi-bin/websql/ftp/query_new.pl
State of California, County of Orange, 2009, “Tsunami Inundation Map for Emergency
Planning, Newport Beach Quadrangle,” 1:24,000 scale, dated June 1.
State of California Sea Level Rise Guidance 2018 Update, by Ocean Protection Council,
dated in March 2018.
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