HomeMy WebLinkAboutPA2021-305_20211221_Coastal Hazard_05-19-2021Geotechnical 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
May 19, 2021
Mr. & Mrs. Gannon
20 Bay Island
Newport Beach, CA 92661
SUBJECT: Coastal Hazard and Sea Level Rise Discussion for New Residence, 20 Bay
Island, Newport Beach, Orange County, California.
Dear Mr. & Mrs. Gannon:
In accordance with the request and authorization, GeoSoils, Inc. (GSI) is pleased to
provide this discussion regarding the potential coastal hazards, including the impact of
future sea level rise (SLR), on the proposed new residence at 20 Bay Island in
Newport Beach, California. Bay Island is a private community accessible only by golf carts.
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 and E.3, a review
of the proposed plans, and preparation of this letter report.
INTRODUCTION
The proposed project is new single-family residence, 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, Newport Bay, the existing community bulkhead to the west of the site,
and the golf cart access bridge to the island. The bulkhead is owned by the Bay Island
Community Association and is not on the site proper. The bulkhead was structurally
rehabilitated and increased in height to elevation +8.5 feet NAVD88 in 2014. Based upon
our review of the rehabilitation plans the height of the bulkhead can be increased in the
future without additional bayward encroachment. There is no beach at the site proper.
However, there is a wide community beach to the east about 250 feet from the site proper.
The proposed finished first floor (FF) elevation of the residence is at or above +10 feet
NAVD88. The site is currently mapped by FEMA to be in the AE Zone with a base flood
elevation of +8 feet NAVD88.
PA2021-305
2
Figure 1. Subject site, 20 Bay Island, adjacent properties, bulkhead, island access
bridge, and Newport Bay.
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 Apex
Surveying Inc. Proposed development elevations were taken from project plans provided
by Brandon Architects. A site reconnaissance was performed in early May 2021. The
beach to the east of the site showed no signs of ongoing erosion. The offsite bulkhead is
in good condition.
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 actual shoreline at the site proper. There is a community owned bulkhead to
the west and there is a community beach about 250 feet to the east of the site. The beach
is held in place by spurs off of the ends of the bulkhead. There is a beach wall at the back
of the beach between the grass common area and the property. The beach is a receiver
beach from dredging nearby. A review of aerial photographs available on the internet show
no change in the beach width from 1972 to 2019. Figure 2a is the beach in 1972 and
Figure 2b is the beach in 2019. There is no shoreline erosion occurring because there is
PA2021-305
3
no along the shore sand transport due to the spurs holding the beach in place. The boat
wakes are too small for significant movement of the beach sand offshore into the
navigation channels. As sea level rises the beach may become narrower and possibly
reestablish at a higher elevation. However, this would not be due to erosion but rather due
to an increase in the elevation of the bay.
Figure 2a. The beach to the east of 20 Bay Island in 1972.
Figure 2b. The beach to the east of 20 Bay Island in 2019 for comparison to Figure 2a.
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
PA2021-305
4
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.”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 3 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 3 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 3. Table from Kopp et al., (2014) and COPC 2018, providing current SLR estimates
and probabilities for the Los Angeles Harbor tide station.
PA2021-305
5
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 CCC
requires the consideration of the low probability (0.5%) high emissions SLR over the next
75 years. The 0.5% SLR in the year 2096 is about 6 feet of SLR. This SLR would account
for future extreme bay water level of 13.7 feet NAVD88 (7.7 feet NAVD88 + 6.0 feet SLR).
As stated before, the present maximum (1%) historical water elevation in Newport Bay,
including El Niño effects, is +7.7 feet NAVD88. Based upon the elevation of the offsite
bulkhead (8.5 feet NAVD88), the extreme Newport Bay water level will exceed the height
of the bulkhead when SLR is 0.8 feet or greater. For the likely COPC SLR estimate range,
the site is safe from flooding until about the year 2060. For the 0.5% SLR case this may
occur in before the year 2040. It should be noted that, if SLR is higher, flooding the site
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 at or above +10 feet NAVD88
the structure will be safe from flooding for SLR less than 2.3 feet. For the 5% COPC
SLR estimate range (high emissions) the structure is safe from flooding until beyond the
year 2070. For the 0.5% SLR case this may occur about the year 2060.
The Kopp et al. paper used 2009 to 2012 SLR modeling for the probability analysis, which
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 4 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 4.
Figure 4. Current global SLR plotted on SLR prediction curves (graphic taken from
TerraCosta Consulting).
PA2021-305
6
The recent global SLR measurement provided in Figure 4 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. The Bay Island bulkhead has been recently
rehabilitated and can be increased in height. In the future with sea level rise the beach wall
at the common area will need to be increased in height to prevent flooding of all of the 23
homes on the island. Flooding through the beach areas will result in flooding of the
access cart paths to the homes. The island will require some form of shore protection
before there is flooding of the residences.
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.
Boat 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.
PA2021-305
7
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.
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. He has extensive experience with coastal processes
in southern California. 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 costal 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 wide beach near
the site but is not part of the site. Boat wakes and wind waves are too small, even
with sea level rise (SLR), to potentially erode the beach or flood the proposed
residence provided the offsite bulkhead and beach wall are maintained to mitigate
SLR impacts. 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 and as such
the City of Newport Beach Building Department (CNBBD) has a standard drawing,
and a regional plan for rehabilitating and raising the bulkheads. There is no
potential coastal hazard of flooding of the development provided adaptation
strategies such as waterproofing the structure to above the potential flood elevation
(minimum elevation 10.9 feet NAVD88), and increasing the height of the offsite
bulkhead and beach wall are implemented if required, in the future.
PA2021-305
8
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 a stable beach near the site that is held in place by coastal
structures (spurs) and. There are no seasonal changes in the beach. As
SLR occurs the beach may become smaller and likely shift higher in
elevation based upon the equilibrium beach principal.
2. High tide conditions, combined with long-term (75 year) projections
for sea level rise;
Using the likely CCC SLR estimate over the project 75-year design life, the
SLR in the year ~2096 is 3.2 feet. There is a 0.5% probability that SLR could
be 6 feet in the next 75 years. This is the design sea level rise range (3.2 feet
to 6 feet) for the proposed project. This SLR would account for future
extreme bay water level of 10.9 feet NAVD88 to 13.7 feet NAVD88.
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.
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 the 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
PA2021-305
9
There is no bulkhead on the site. However, there is a recently rehabilitated
community bulkhead that protects most of Bay Island. Based upon our
review of the bulkhead design, the height of the bulkhead can be increased
to adapt to SLR with no additional bayward encroachment. The bulkhead
protects the cart paths, other residences, and improvements on Bay Island.
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.
The project is safe from the coastal hazard of flooding by the proposed
elevation of the finished floor, and the ability to waterproof the building, if
necessary. To further adapt to SLR, the community will likely protect the cart
paths and residences of the island, in the future. It is important to point out
that SLR will not impact this island alone. It will impact all of the Newport
Bay low lying areas. The public streets throughout the Newport Beach
coastal area, including the Lido Island, Balboa Peninsula and Balboa Island,
will flood with lower SLR well before the residence floods. It is very likely that
the Bay Island 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, beaches
and walkways that surround the bay, and waterproofing of residences. These
are a site specific adaptation strategies.
Bulkhead condition report shall include (NBMC 21.30.15.E.3):
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. Mr.
Skelly has extensive experience in shoreline erosion, bluff erosion, soils
engineering, and the design, permitting, and construction of shore protection
devices. Projects include levee engineering and design in San Francisco Bay,
seawall and marina engineering in Baja California Sur, coastal boardwalk design
and protection in Pacifica, and seawall projects throughout southern California.
ii. An analysis of the condition of any existing bulkhead including whether the
top elevation meets current City standards, the conditions of the sheetpiles
or panels, the condition of existing tiebacks and/or deadmen or similar, and
PA2021-305
10
any other relevant conditions;
The off site community bulkhead has been recently rehabilitated and is in good
condition. The bulkhead, at elevation +8.5 feet NAVD88, does not meet the current
City’s top of bulkhead elevation requirement (10.9 feet NAVD88). It should be
noted that very few bulkheads in the Newport Bay area meet this newer elevation
requirement. The actual design, not elevation, of the bulkhead does meet the City
standard.
iii. Recommendations regarding the need for repair, augmentation or
replacement of the bulkhead or any parts thereof;
Augmentation of the community bulkhead in the future will be necessary to protect
Bay Island regardless of the proposed development at the site. The increase in
height is an SLR adaptation strategy. In addition, it will likely be necessary to
replace the beach wall with a new bulkhead to protect the entire island.
iv. If augmentation or replacement in the existing alignment is necessary,
recommendations that will avoid seaward encroachment of the bulkhead;
The new shore protection can be located within the current bulkhead and beach wall
footprints.
v. If replacement is necessary and the existing bulkhead is not in alignment
with adjacent bulkheads, recommended alternatives that will relocate the
bulkhead in as much in alignment with adjacent bulkheads and as far
landward, as possible.
There are no other bulkheads on Bay Island to align with. If necessary, the beach
wall at the back of the beach can be replaced with an approved shore protection
device.
CONCLUSIONS
• The proposed residential structure with the implementation of the SLR adaptation
strategies (design for future waterproofing to above elevation +13.7 feet NAVD88
and the future protection of all the island residences by the community) will not be
adversely impacted by potential coastal hazards including a 6 feet sea level rise
over the next minimum 75 years. The site will be part of a community wide response
to mitigate SLR hazards.
• An augmented or even a new SPD for Bay Island is currently not needed but will be
critically needed to protect the residences below elevation 13.7 feet NAVD88 (7.7
feet NAVD88 + 6 feet of SLR) over time as sea level rises regardless of the
development on the site.
PA2021-305
11
• For the most part, in the remainder of the Newport Beach coastal area, including the
Balboa Peninsula, and other developed islands within the bay, flooding with a lower
SLR is likely to occur. Such flooding will occur well before any part of Bay Island,
including the subject property, will flood. It only requires a few of the protective
bulkheads around Newport Bay to remain at low levels to allow flooding of the
streets.
• 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 next 75 years or more.
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. 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 Bay Island 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.
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
PA2021-305
12
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,” Revised 2021 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.
PA2021-305