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HomeMy WebLinkAboutPA2023-0140_20230728_Coastal Hazards Report dated 07-07-23Geotechnical 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 July 7, 2023 Mr. Geof Wickett 309 East Bay Front Newport Beach, CA 92662 SUBJECT:Coastal Hazard and Sea Level Rise Discussion for Garage/Addition, 309 East Bay Front, Newport Beach, Orange County, California. Dear Mr.Wickett: 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 garage addition at 309 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 more current SLR science from National Oceanographic and Atmospheric Administration (NOAA) and National Aeronautics and Space Administration (NASA), a review of City of Newport Beach Municipal Code (NBMC) 21.30.15.E.2, a review of the plans of the proposed garage/addition, a site inspection, and preparation of this letter report. INTRODUCTION The proposed project is a garage and second story addition to an existing 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. There is no proposed change in the existing residence finished floor (FF) elevation. The FF elevation of the habitable portion of the garage/addition is about +17.6 feet NAVD88. The proposed garage/addition and existing residence are designed such that they can be waterproofed to adapt to SLR. The site is fronted by a public boardwalk, a bulkhead to elevation +8.6 feet NAVD88, 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 +6.3 feet NAVD88 which means that FEMA has a site specific base flood elevation (BFE) of less than +7.3 feet NAVD88. 2 Figure 1. Subject site, 309 East Bay Front, public boardwalk, adjacent intertidal beach, and Newport Bay channel in 2022. 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 RdM Surveying, Inc., dated March 24, 2022, and project plans were provided by Oldham Architects. A site reconnaissance was performed in July 2023. 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 8.6 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 3 analysis including conclusions and recommendations, as necessary. 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 highest tide) fronting the site. A review of historical aerial photographs available on Google Earth show no shoreline/beach 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 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 Newport Bay. For this analysis the historical highest water elevation will be +7.7 feet NAVD88. Future Tide Levels Due to Sea Level Rise The 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” in the CCCSLRG is over 10 years old. The SLR models used as the basis for the COPC and CCCSLRG have been in place for about two decades. The accuracy of any model can be determined by comparing the measured SLR (real time data) to the model predicted SLR (model prediction). The NOAA has been measuring SLR globally, and specifically at Los Angeles Harbor. The NOAA Los Angeles SLR rate is 1.04 mm/yr as shown in Figure 2. The rate can be used to calculate a sea level rise of 24.8 mm ( 0.08 ft) over the last 23 years (2000 through 4 December 2022). If we assume that the SLR do not change significantly in the next 7 years (which is likely) the amount of SLR will be about 0.1 feet (30X1.04= 31.2 mm or ~0.1 feet). Figure 2. Latest measure SLR at Los Angeles from NOAA (2023). NOAA also provides plots of the most current SLR model projections (best available science) over time starting in the year 2000. Figure 3, is the model projections taken NOAA, which is more current SLR science than from the COPC used models. To see which model is more accurately predicting SLR, the data for Los Angeles can be either plotted onto the curves or estimated from the table below the curves. Figure 3. NOAA 2021 SLR projections for Los Angeles. 5 Recognizing that in the year 2000 the SLR zero line is 2.70 feet, and using the current Los Angeles SLR data (trends), SLR should be (2.70 + 0.1 feet) 2.8 feet in the year 2030. Looking at the table in Figure 3 for the year 2030 (6.5 years from now) reveals that Los Angeles SLR is tacking below the NOAA 2017 Low Model curve. The Low Model predicts a SLR rise total in the year 2100 of less than 1 foot. In contrast, the model the CCC was requiring to be analyzed is the high emissions scenario and the 0.5% probability. For the year 2030 the CCC required SLR is 0.7 feet which is 7 times greater than the 0.1 feet that is being measured. Over the 75 year life of the development this results in very significant difference in what the CCC requires and what is the current best available science. The CCCSLRG document recommends that a project designer determine the range of SLR using the “best available science.” The NOAA SLR information provided above is more current than the CCCSLRG. 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 more than a factor of 7 as to the measured amount of the SLR in Newport Beach. Clearly, the CCC required model has inflated SLR, is incorrect, and over time will likely become more and more inaccurate. The CCCSLRG document recommends that a project designer determine the range of SLR using the “best available science.” The NOAA SLR information provided above is more current than the CCC SLR Guidance. Figure 4 provides the NASA 2023 IPCC AR 6 models with the SLR estimates (in meters) for the Los Angeles NOAA station, and the model probabilities of those estimate to meet or exceed the 1995-2014 baseline. The likely SLR in the year 2100 (SSP3-7.0) is 0.54 meters (1.77 feet) and the unlikely (very low probability ~0.5%) in the year 2100 (SSP5-8.5) is 0.76 meter (about 2.5 feet). The NOAA and NASA IPCC AR 6 data/models are the currently best available science and are required by the CCCSLRG to be used. Finally, it is our understanding the COPC is revising their previous SLR estimates and that they are more consistent with the NOAA and NASA data/models. 6 Figure 4. NASA SLR tool IPCC AR6 estimates for Las Angeles NOAA tide station. Measurements and models from NASA show that SLR is tracking to be about 1.8 feet to 2.4 feet in the year 2100. Measurements and models from NOAA have SLR to be between 0.66 feet to 5.3 feet (NOAA-2107 Int-High) in the year 2100. The typical design life of residential development is about 75 years or about the year 2100. Based on the discussion above (the current best available science), the SLR range for the project is about 2 feet (likely) to 6 feet (unlikely). Using the design ocean water elevation 7.7 feet NAVD88, the potential range of Newport Bay water level in the year 2100 due to SLR is 9.7 feet NAVD88 (likely) to 13.7 feet NAVD88 (unlikely). 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. 7 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 tsunami innundation maps, https://www.conservation.ca.gov/cgs/tsunami/maps/orange ). 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. 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 43 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 8 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; 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 is ~2.0 feet. There is a very low probability SLR estimate of ~6 feet in 75 years that is not currently support by the measured SLR through June 2023. This is the design sea level range for the proposed project. This SLR would account for future extreme bay water level of 9.7 feet NAVD88 to 13.7 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 9 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 addition finished floor, the ability to waterproof the existing and proposed 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. 10 CONCLUSIONS •The proposed garage and elevated addition structure with the implementation of the SLR adaptation strategies (future waterproofing of the building and 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 existing and proposed 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 11 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 NASA, 2023, IPCC 6th Assessment Report Sea Level Projection Tool, https://sealevel.nasa.gov/ipcc-ar6-sea-level-projection-tool. NOAA, 2023, https://tidesandcurrents.noaa.gov/datums.html?id=9410660 _______ 2022 Sea Level Rise Technical Report,” dated February 2022. https://oceanservice.noaa.gov/hazards/sealevelrise/sealevelrise-tech-report.html. State of California Sea Level Rise Guidance 2018 Update, by Ocean Protection Council, dated in March 2018.