HomeMy WebLinkAboutPA2021-094_20210525_Coastal Hazards Report_4-19-21•
Geotechnical • Geologic • Coastal • Environmental
5741 Palmer Way • Carlsbad, California 92010 • (760) 438-3155 • FAX (760) 931-0915 • www.geosoilsinc.com
April 19, 2021
Mr. & Mrs. Voss
3909 Marcus Avenue
Newport Beach,· CA 92663
SUBJECT: Updated Coastal Hazard and Sea Level Rise Discussion for New Duplex,
3909 Marcus Avenue, Newport Beach, Orange County, California.
Dear Mr. & Mrs. Voss:
In accordance with your request and authorization, GeoSoils, Inc. (GSI) is pleased to
provide this updated discussion regarding the potential coastal hazards, including the
impact of future sea level rise (SLR), on the proposed new duplex at 3909 Marcus Avenue
in Newport Beach, California. This update is in response to recent changes in City of
Newport Beach bulkhead elevation requirements. The purpose of this report is to provide
the hazard information for your permit application typically 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 review of City of Newport Beach Municipal
Code (NBMC) 21.30.15.E.2, a discussion of the proposed new duplex plans, a site
inspection, and preparation of this letter report. It is our understanding that a bulkhead
condition report, along with plans to raise the bulkhead to a minimum elevation of +10.9
feet NAVD88, will be provided by others.
INTRODUCTION
The proposed project is a new duplex structure, in the City of Newport Beach. Figure 1,
downloaded from Google Maps (Bird's Eye View), shows the site in relation to the adjacent
properties, the boat dock, and the narrow navigation channel within Newport Bay. The
proposed finished first floor (FF) elevation of the residences is 10.0 feet NAVD88. The
residence will be designed with a concrete curb which effectively makes the FF at about
elevation +11 feet NAVD88. The site is fronted by a concrete bulkhead to about elevation
6.8 NAVD88 (shore protection). It is our understanding that the project includes additional
assessment of the bulkhead and either an addition to, or possible replacement of, the
bulkhead in the current location. The site is currently mapped by FEMA to be in the AE
Zone with a base flood elevation (BFE) of +8 feet NAVD88. It should be noted that a
portion of the site adjacent to the narrow navigation channel is also designated as a FEMA
AO Zone with potential flooding depth of 1 foot.
PA2021-094
Figure 1. Subject site, 3909 Marcus Avenue, adjacent properties, boat dock, and
Newport Bay channel.
DATA& DATUM
2
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 RdM
Surveying Inc., dated 9/28/20, and project plans were discussed with Bradford C. Smith
Architect. A site reconnaissance was performed in September 2020. There is no beach
fronting the shore protection/bulkhead at the site.
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.
Shoreline Erosion Hazard
There is no beach at the site. There is no typical shoreline fronting the site. A review of
PA2021-094
3
historical aerial photographs available on Google Earth dating back to 1995 show no beach
fronting the site. The high tide is located essentially at the shore protection/bulkhead. As
sea level rises, the high tide location will remain at the shore protection. Shoreline erosion
will not impact the proposed development over the life of the development.
Current Flooding Hazard
The National Oceanographic and Atmospheric (NOAA) National Ocean Survey tidal data
station closest to the site, with a long tidal record, is located at Los Angeles Harbor (Station
94106600). The tidal datum elevations are as follows:
Mean High Water
Mean Tide Level (MSL)
Mean Low Water
NAVD88
Mean Lower Low Water
4.55 feet
2.62 feet
0.74 feet
0.0 feet
-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 Nino 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, Los Angeles Harbor Tide station reports 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. The existing bulkhead at +6.8 feet NAVD88 is below the current
design maximum water level.
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." 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 el at, 2014). The 2018 CCC SLR Guidance states that the COPC
document is the "best available science." The COPC 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 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.
PA2021-094
Hlgl, imlssl II\
tow emissions
Hlgl!_em_l_l_~_IIS
1 ow em11s1ons
llgh ,missions
-------•-
OlO
1040
1010
sea-level rise meets
or exceeds ...
sea-level rise meets
or exceeds ...
LOW Medium -High Extreme ______ j_ ____ Av:~~~on! ____ ··-··-·--·-----.-1 -•'•-•A-•ec_,,o_n --,..-"'-''-A••_,.,_on"""II •····
0 .3 I 0.2 . 0.5 0.6
0.5 I 0.4 -0.7 0.9
0.7 I 0.5 -1.0 1.2
0.7
1.2
1.8
1.0
1.7
2.6
2060
1060 ---1---~: __ J ~~-:_ 1
1
: _____ ;; ___ ----····-~:--1---3.7_-t
1070 0.9 1 0.6 1.3 I 8 2.9
1070 1.2 I 0.9 · l.7 5.0 --~-------~-------2 2
2.1
2.8
2.5
3.4
3.3
3.6
4.3
4.5
5.3
5.4
6.7
6.0
7.1
liJW emlsslo11s 1080
I High emissions 2080
towemlsslvns 1090
Hlgl1 em•sslons 2090
Low emissions 1100
High emissions 1100
tow ,missions 21!0'
Hlgt, emtsslom 11IO'
1.0
1.5
1.2
1.8
L 0.6 -1.6
1.0 · 2.2
0.7 1.8
i.2 2.7 ---
1.3 I o.7 2.1 3.0
2:!_ -_ _( .1~
1.4 I o 9
2.3 : 1.6
3.2
2.2
3.3
4.l
3.1
4.3
6.4
8.0
9.9
11.5
4
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. The Newport
Beach City Council previously approved the use of the high estimate of the "low risk
aversion" scenario, which is 3.2 feet SLR by the year 2100. The design historical water
elevation at the for Newport Bay is elevation + 7. 7 feet NAVD88 (Moffatt & Nichol 1 % water
elevation). If 3.2 feet is added to this 7.7 feet NAVD88 elevation, then future design
maximum water level 10.9 feet NAVD88 is determined.
The "likely" sea level rise for the proposed project is 3.2 feet with a lower probability
(~0.5%) of SLR of about 5.4 feet. This SLR would account for future extreme bay water
level in the range of 10.9 feet NAVD88 (7.7 feet NAVD88 + 3.2 feet SLR) to 13.1 feet
NAVD88 (7.7 feet NAVD88 + 5.4 feet SLR) . The existing bulkhead is proposed to be
increased in height or replaced as part of the project. As stated before, the present
maximum (1 %) historical water elevation at the site, including El Nino effects, is~+ 7.7 feet
NAVD88. Based upon the proposed elevation of the shore protection 10.9 feet NAVD88,
the extreme Newport Bay water level will exceed the height of the existing walkway
bulkhead when SLR is 3.2 feet or greater. For the likely COPC SLR estimate range (high
emissions) the site is safe from overtopping flooding until about the year 2095. For SLR
greater than 3.2 feet the height of the bulkhead will need to be further increased. For the
0.5% SLR case this may occur after the year 2070. It should be noted that, if SLR is
higher, flooding 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.
The City of Newport Beach has recognized that in the future there will be a need to raise
the elevation of the bulkheads around Newport Bay. The City of Newport Beach Building
Department (CNBBD) has a standard drawing, and a regional plan for rehabilitating and
raising the bulkheads.
PA2021-094
5
Waves and Wave Runup
The potential surface gravity waves ( ocean swell) to arrive at this site is nil. Boat wakes
and wind waves are too small due to the narrow and fetch limited navigation channel to
overtop the raised bulkhead. In the future, with more than 3.2 feet of SLR, during the
highest high tides, bay water may overtop the bulkhead. The proposed increase and future
increase in the height of the bulkhead is a SLR adaptation strategy recommended in the
2018 CCC SLR guidance document.
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.
PA2021-094
6
CITY OF NEWPORT BEACH INFORMATION
Coastal Hazards Report (NBMC 21.30.15.E.2):
i. A statement of the preparer's qualifications;
Mr. Skelly is Vice President and Principal Engineer for GeoSoils, Inc. (GSI). He has
worked with GSI for several decades on numerous land development projects
throughout California. Mr. Skelly has over 40 years experience in coastal
engineering. Prior to joining the GSI team, he worked as a research engineer at the
Center for Coastal Studies at Scripps Institution of Oceanography for 17 years.
During his tenure at Scripps, Mr. Skelly worked on coastal erosion problems
throughout the world. He has written numerous technical reports and published
papers on these projects. He was a co-author of a major Coast of California Storm
and Tidal Wave Study report. Mr. Skelly also performs wave shoring and uprush
analysis for coastal development, and analyzes coastal processes, wave forces,
water elevation, longshore transport of sand, and coastal erosion.
ii. Identification of coastal hazards affecting the site;
As stated in this hazard analysis, the typical coastal hazards to consider
are shoreline erosion, flooding, and wave/wake impacts. There is no beach
fronting the site. Boat wakes and wind waves are too small within the narrow
channel, even with sea level rise (SLR), to flood the residence provided the
bulkhead is raised in response to SLR is maintained. There is a potential coastal
hazard offloading of the residence, with an effective FF elevation +11 feet NAVD88,
if SLR exceeds 3.3 feet. For SLR higher than the current estimates, adaptation
strategies such as waterproofing the structure to above the potential flood elevation,
and increasing the height of the bulkhead can be implemented, if required in the
future.
iii. An analysis of the following conditions:
1. A seasonally eroded beach combined with long-term (75 year)
erosion factoring in sea level rise;
There is no beach at the site. There is no typical shoreline fronting the site.
A review of historical aerial photographs available on Google Earth dating
back to 1995 show no beach fronting the site. The high tide is located
essentially at the shore protection/bulkhead. As sea level rises, the high tide
location will remain at the shore protection. Shoreline erosion will not impact
the proposed development over the life of the development.
2. High tide conditions, combined with long-term (75 year) projections
for sea level rise;
Using the SLR estimates over the project 75-year design life, the range in the
year ~2096 is between 3.2 feet (likely) and 5.4 feet (0.5% probability). This
is the sea level rise range for the proposed project. This SLR range would
PA2021-094
7
account for future extreme bay water levels in the range of 10.9 feet NAVD88
(7. 7 feet NAVD88 + 3.2 feet SLR) and 13.1 feet NAVD88 (7. 7 feet NAVD88
+ 5.4 feet SLR).
3. Storm waves from a one hundred year event or storm that compares
to the 1982/83 El Nino event;
No ocean waves can reach the site. Boat wakes and wind waves in the
narrow navigation channel will be too small to impact the residence provided
the bulkhead is maintained.
4. An analysis of bluff stability; a quantitative.slope stability analysis
that shows either that the bluff currently possesses a factor of safety
against sliding of all least 1.5 under static conditions, and 1.1 under
seismic (pseudostatic conditions); or the distance from the bluff edge
needed to achieve these factors of safety; and
There is no bluff fronting the site. This condition does not occur at the site.
5. Demonstration that development will be sited such that it maintains
a factor of safety against sliding of at least 1.5 under static conditions
and 1.1 under seismic (pseudostatic) conditions for its economic life
(generally 75 years). This generally means that that setback necessary
to achieve a factor of safety of 1.5 (static) and 1.1 (pseudostatic) today
must be added to the expected amount of bluff erosion over the
economic life of the development (generally 75 years);
There is no bluff fronting the site. There is no potential for sliding. This
condition does not occur at the site.
iv. On sites with an existing bulkhead, a determination as to whether the
existing bulkhead can be removed and/or the existing or a replacement
bulkhead is required to protect existing principal structures and adjacent
development or public facilities on the site or in the surrounding areas; and
The existing shore protection is necessary to protect the existing structure,
the adjacent properties, and the public facilities and infrastructure.
v. Identification of necessary mitigation measures to address current
hazardous conditions such as siting development away from hazardous areas
and elevating the finished floor of structures to be at or above the base floor
elevation including measures that may be required in the future to address
increased erosion and flooding due to sea level rise such as waterproofing,
flood shields, watertight doors, moveable floodwalls, partitions, water-
resistive sealant devices, sandbagging and other similar flood-proofing
techniques.
PA2021-094
8
The project is safe from the coastal hazard of flooding by the proposed
increase in elevation of the bulkhead, the elevation of the finished floor, and
the project design, which can accommodate future waterproofing, if
necessary. To further adapt to higher than expected SLR, the shore
protection/bulkhead can be further increased in height in the future. It is
important to point out that SLR will not impact this property alone. It will
impact all of the Newport Bay low lying areas. The public streets throughout
the Newport Beach coastal area, including Balboa Island and the Balboa
Peninsula, will flood with lower SLR well before the residence floods. It is
very likely that the community will soon adopt some of the SLR adaptation
strategies that are currently being considered by the City of Newport Beach.
These strategies involve raising, or adding/replacing the bulkheads, and
waterproofing. These are a site specific adaptation strategies.
CONCLUSIONS
• A review of aerial photographs over the last several decades shows no shoreline at
the site.
• The existing strucutre has not been subject to flooding, erosion damage, or wave
runup attack in the past.
• The proposed duplex residential structure will not be adversely impacted by
potential coastal hazards including a 3.2 feet sea level rise over the next minimum
75 years. Based upon the design of the residence, additional waterproofing can be
added, if needed, to mitigate hazards due to SLR greater than 3.3 feet. In addition,
the height of the shore protection can be further increased to exclude bay water.
The site will be part of a community wide response to mitigate SLR hazards.
RECOMMENDATIONS
Based upon the analysis and discussion herein, the proposed development is reasonably
safe from coastal hazards for the next 75 years including shoreline movement, waves and
wave runup, and flooding with future SLR for the next 75 years provided the
recommendations herein are followed. 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 City of Newport Beach
bulkhead modification/replacement plan will likely mitigate any SLR impacts on the project.
The proposed development will neither create nor contribute significantly to erosion,
geologic instability, or destruction of the site or adjacent area.
PA2021-094
9
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
PA2021-094
10
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, 2020, 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.
PA2021-094