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HomeMy WebLinkAbout20191219_GeotechnicalGeotechnical C Geologic C Coastal C Environmental 5741 P alm er W ay C Carlsbad, California 92010 C (760) 438-3155 C FAX (760) 931-0915 C www.geosoilsinc.com December 5, 2019 South Bayfront W aterpointe, LLC c/o Brandon Architects 151 Kalmus Drive, Suite G-1 Costa Mesa, CA 92626 USA SUBJECT:Coastal Hazard and Sea Level Rise Discussion for New Residence, 400 South Bayfront, Newport Beach, Orange County, California. Dear Sirs: 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 at 400 South Bayfront, 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), 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 Brandon Architects, 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 +9.0 feet NAVD88. The site is fronted by a public boardwalk along a Newport Bay channel. Balboa Island is protected by a City of Newport Beach owned and maintained concrete seawall/bulkhead. The site is currently mapped by FEMA to be in the AE Zone with a BFE of +8 feet NAVD88. FEMA is proposing to adopt a new FIRM map for the area soon. While the future FIRM map is currently not effective, the proposed future FEMA FIRM does not change the site zone designation or the elevation of the site BFE. The City of Newport Beach adopted the elevation of +9 feet NAVD88 as the minimum finished floor (FF) height for new residential development. 2 Figure 1. Subject site, 400 South Bayfront , and adjacent bay channel in 2018. 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 Land Surveying, Inc., dated 5/15/18, and project plans were discussed with Brandon Architects. A site reconnaissance was performed in March 2019. The visible portions of the offsite walkway/bulkhead were observed to be in good condition. There is a small intertidal beach fronting the bulkhead/walkway near 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 shoreline fronting the site. There is a small intertidal beach bayward of the public walkway. Shoreline erosion will not impact the proposed development over the life of the development. 3 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 W ater 4.55 feet Mean Tide Level (MSL) 2.62 feet Mean Low W ater 0.74 feet NAVD88 0.0 feet Mean Lower Low W ater -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.” W hen 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 el at, 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 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. 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. In early 2019, the Newport Beach City Council approved the use of the high estimate of the “low risk aversion” scenario, which is 3.2 feet SLR by the year 2100. Interpolating between the years 2090 and 2100 provides a SLR of 2.9 feet in the year 2095. The design historical water elevation at the for Newport Bay is elevation +7.7 feet NAVD88 (1% water elevation). If 2.9 feet is added to this 7.7 feet NAVD88 elevation, then future design maximum water level 10.6 feet NAVD88 is determined. Interpolating the above table, the very low probability SLR (0.5%) for the year 2095, low emissions, SLR is ~5.0 feet, and the high emissions, SLR is 6 feet. Based upon this tabel there is a much lower probability (0.5%) of SLR of about 5.5 feet ((5.0+6.0)/2). This SLR would account for future extreme bay water level in the range of 13.2 feet NAVD88 (7.7 feet NAVD88 + 5.5 feet SLR). The existing offsite bulkhead is proposed to be increased by the City in the future. 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 shore protection, the extreme Newport Bay water level will exceed the height of the existing walkway bulkhead when SLR is 1.3 feet or greater. For the likely COPC SLR estimate range (high emissions) the site is safe from overtopping flooding until about the year 2060. For SLR greater than 1.3 feet the height of the walkway bulkhead will need to be further increased. For the 0.5% SLR case this may occur in about the year 2050. 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 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 5 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. 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 bulkheads around Balboa Island. The City of Newport Beach Building Department (CNBBD) has a standard drawing, and a regional plan for rehabilitating and raising 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. 6 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. W hile 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 tsunam i. 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 7 papers on these projects. He was a co-author of a major Coast of California Storm and Tidal W ave 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 small intertidal 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 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 on Balboa Island 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 water proofing 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; There is a stable intertidal beach near the site. There are no seasonal changes in the beach. As SLR occurs the intertidal beach may become smaller or disappear altogether. 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 ~2095 is 2.9 feet. There is a very low probability SLR estimate of 5.5 feet in 75 years that is not currently support bu the measured SLR through 2018. 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. 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 8 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 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, and the presence of the nearby walkway/bulkhead. To further adapt to SLR, the City owned bulkhead is planned to be 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, beaches and walkways that surround the bay, and waterproofing. These are a site specific adaptation strategies. 9 CONCLUSIONS •The proposed residential structure with the implementation of the SLR adaptation strategies (water proofing 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 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 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, “W aterfront Project Guidelines and Standards, Harbor design Criteria Commercial & Residential Facilities,” 2017 Edition NOAA, 2013, W eb 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.