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HomeMy WebLinkAboutPA2021-285_20211130_Coastal Hazards Report_9-9-21Geotechnical 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 September 9, 2021 WO S8192 Mr. & Mrs. Bland 14975 Corona Del Mar Pacific Palisades, CA 90272 SUBJECT: Coastal Hazard and Sea Level Rise Discussion for New Residence, 125 East Bay Front, Newport Beach, Orange County, California. Dear Mr. & Mrs. Bland: In accordance with your 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 with a basement at 125 East Bay Front, on Balboa Island in Newport Beach, California. 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), the CCC SLR Guidance (November 2018), a review of City of Newport Beach Municipal Code (NBMC) 21.30.15.E.2, a review of the plans of the proposed residence prepared by Studio William Hefner , a site inspection, and preparation of this letter report. INTRODUCTION The proposed project is a new single-family residence on Balboa Island, 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 and Newport Bay. The proposed finished first floor (FF) elevation of the residence is +9.00 feet NAVD88. The proposed residence design is such that it can be waterproofed in response to SLR. This makes the effective FF the height of the waterproofed portion of the structure The site is fronted by a public boardwalk and a narrow intertidal beach along a Newport Bay channel. Balboa Island is protected by a City of Newport Beach owned and maintained concrete seawall/bulkhead. The site is mapped by FEMA to be in the Shaded X Zone with a 1% chance flood depth of less than 1 foot. The elevation of the public road flow line is about +6.8 feet NAVD88 which means that FEMA has a site specific base flood elevation (BFE) of less than +7.8 feet NAVD88. PA2021-285 2 Figure 1. Subject site, 125 East Bay Front , public boardwalk, adjacent intertidal beach and Newport Bay channel in 2020. 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 Pasco Laret Suiter & Associates, dated July 2021, and project plans were provided by Studio William Hefner dated 8/13/21. A site reconnaissance was performed in August 2021. The visible portions of the offsite walkway/bulkhead were observed to be in good condition. The City owned bulkhead has been increased in height to elevation 8.75 feet NAVD88. There is only a narrow intertidal beach fronting the bulkhead/walkway bay ward of the site and the bay front area. 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. PA2021-285 3 Shoreline Erosion Hazard There is an intertidal beach in the site area, but at very high tide there is no beach. There is no actual shoreline (beach above the high tide) fronting the site. A review of historical aerial photographs available on Google Earth show no beach above the high tide fronting the site above the maximum high tide line and the adjacent public walkway. 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 (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.” 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. PA2021-285 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 design historical water elevation at the for Newport Bay is elevation +7.7 feet NAVD88 (EICI 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 5.5 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 13.2 feet NAVD88 (7.7 feet NAVD88 + 5.5 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 City bulkhead shore protection 8.75 feet NAVD88, the extreme Newport Bay water level will exceed the height of the existing walkway bulkhead when SLR is 1.0 feet or greater. For the likely COPC SLR estimate range (high emissions) the bulkhead is safe from overtopping flooding until about the year 2060. For SLR greater than 1.0 feet the height of the bulkhead will need to be further increased. For the 0.5% SLR case this may occur about the year 2040. 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 proposed residence with an FF at 9.0 feet NAVD88 will be safe from flooding until about the year 2070 under the likely SLR. The proposed residence design allows for additional waterproofing of the structure to above the potential flood elevation. LOW Medium. High Extll'eme Risk Aversion Risk Aversion Risk Aversion 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 PA2021-285 5 Sea Level Rise Model Check The CCC SLR Guidance (CCCSLRG) is based upon the California Ocean Protection Council (COPC) update to the State’s Sea-Level Rise Guidance in March 2018. These COPC estimates are based upon a 2014 report entitled “Probabilistic 21st and 22nd century sea-level projections at a global network of tide-gauge sites” by Kopp, et al., 2014. The Kopp et al. paper used 2009 to 2012 SLR modeling by climate scientists for the probability analysis, which means the “best available science” used by the CCC is about 10 years old. The SLR models used as the basis for the COPC and CCCSLRG have been in place for a decade. The accuracy of any model can be determined by comparing the measured SLR (real data) to the model predicted SLR (model prediction). If the model cannot predict, with any accuracy, what will happen in over the approximate last 11 years, it is very unlikely that the model will increase in accuracy when predicting SLR over the next 100 years. Simply put, if the model is not accurate now, it will be even less accurate in the future. The National Oceanic and Atmospheric Administration (NOAA) has been measuring SLR globally and at Los Angeles Harbor. The NOAA global SLR shows a current SLR rate of 3 mm/yr. The rate can be used to calculate a sea level rise of 33.75 mm (0.111 ft) over the last 11.25 years (Jan 2010 to March 2021). The NOAA Los Angeles Harbor SLR rate is 1.03 mm/yr. The rate can be used to calculate a sea level rise of 11.6 mm (0.038 ft) over the last 11.25 years (Jan 2010 to March 2021). What is clear is that SLR in Santa Monica is currently about 1/3 of the global SLR. What is also interesting is that the NOAA website shows that SLR is not occurring in several locations and the ocean is actually getting lower in elevation in these areas. Clearly “one size fits all” is not the best available science. The COPC provided plots of the various SLR model projections over time starting in the year 2010. Figure 3 attached is the model projections taken directly from the EICI. To see which model is more accurately predicting SLR, the global data and the data for Los Angeles harbor can be plotted onto the curves. Figure 4 attached is an enlargement of a portion of Figure 3 to show the results. As stated before, the SLR in Santa Monica is ~1/3 of the global SLR over the same time period. The CCC Guidance SLR is over 5 times the measured SLR in Los Angeles. More importantly the current global SLR trend matches the USACE “Intermediate” SLR, which predict about 1.5 feet of SLR by the year 2100. The SLR for Los Angeles matches the USACE “Low” SLR, which predicts about 0.5 feet of SLR in the year 2100, an order of magnitude lower than the CCCSLRG (0.5 feet versus 6.8 feet). PA2021-285 6 Figure 3. EICI graphic summary of SLR model projections Figure 4. Comparison of real SLR measurements to SLR Model predictions. 4' C: 6 -GI ~ ,. ~ 7.0 & ■ 6.0 5.0 4.0 1.0 COPCHlgh COPCL0W CA CoHtiil ConseNilf'ICV USACE/Hflh USACE / lntermed~t. 11.SACE/tow Vermttf .ind Rahmstorf ~an and Range MARCH 2021 CCC SLR GUIDANCE LOS ANGELES 6.7 FT _____ _.,, • --. -------------- . -.-. -~ ....................... --:-............................................................. . 0.0 ••-C..======'------------------------.-----,,-------1 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 Y•ar ,0 MARCH 2021 .0 CCC GUIDANCE 2030 0.5% SLR 0.7 FEET .0 CCC GUIDANCE j 0.5% SLR 2021 PROJECTED ---. . . 0 0 0 YEAR • . PA2021-285 7 The CCCSLRG document recommends that a project designer determine the range of SLR using the “best available science.” The information provided above is more current than the CCC SLR Guidance. The checking of the models is the “best available science” for SLR prediction and is required to be used. Currently, the SLR model that the CCC is “requiring” to be used for development is incorrect by a factor of about 4 as to the amount of the SLR in Los Angeles, and has been demonstrated to reasonably result in an error of magnitude in the year 2100. Regardless of the amount of SLR, 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, raising, or replacing the bulkheads. Waves and Wave Runup The potential surface gravity waves (ocean swell), boat wakes and wind waves to arrive at this site is nil. While the site is near a navigation channel, the geometry of the small navigation channel fronting the site, and the presence of the boat docks, preclude waves from reaching the bulkhead. 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 PA2021-285 8 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 a no beach above the high tide fronting the site. Boat wakes and wind waves are too small, even with sea level rise (SLR), to potentially flood the residence provided the offsite bulkhead/walkway is increased to mitigated SLR. The City of Newport Beach has recognized that in the future there will be a need to raise the elevation of the bulkheads or replace them on Balboa Island and as such the City of Newport Beach Building Department (CNBBD) has a standard drawing, and a regional plan for rehabilitating, raising, ore replacing 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, and increasing the height of the offsite walkway/bulkhead are 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; PA2021-285 9 There is no beach above the highest tide at the site. 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 2.9 feet. There is a very low probability SLR estimate of 5.5 feet in 75 years that is not currently support by the measured SLR through March 2021. This is the design sea level range for the proposed project. This SLR would account for future extreme bay water level of 10.6 feet NAVD88 to 13.2 feet NAVD88 over the next 75 years. 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 There is no bulkhead on the site. However, the nearby City of Newport Beach bulkhead protects this property along with the other properties on Balboa Island, 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 PA2021-285 10 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, the ability to waterproof the building, if necessary in the future, and the presence of the nearby walkway/bulkhead. To further adapt to SLR, the City owned bulkhead is planned to be increased in height or replaced, 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, beaches and walkways that surround the bay, and waterproofing. These are a site specific adaptation strategies. CONCLUSIONS • The proposed residential structure with the implementation of the SLR adaptation strategies (waterproofing of the residence and the future raising of the City’s bulkhead) will not be adversely impacted by potential coastal hazards including probable sea level rise over the next minimum 75 years. 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 additional 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 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 PA2021-285 11 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 PA2021-285 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,” 2017 Edition NOAA, 2021, 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 San Diego, 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. PA2021-285