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HomeMy WebLinkAboutPA2022-0155_20221012_Coastal Hazards Report_10-03-22Geotechnical 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 October 3, 2022 Beach Life Rentals, LLC 316 Grand Canal Newport Beach, CA 92663 SUBJECT:Coastal Hazard and Sea Level Rise Discussion for New Residence and Accessory Dwelling Unit (ADU) at 316 Grand Canal, Newport Beach, Orange County, California. Dear Beach Life Rentals, LLC: 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 and ADU at 316 Grand Canal 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), CCC SLR Guidance (November 2018), a review of City of Newport Beach Municipal Code (NBMC) 21.30.15.E.2, a discussion of the proposed new residence and ADU plans, a site inspection, and preparation of this letter report. INTRODUCTION The proposed project is a new residence and ADU, 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, the public boardwalk, the City of Newport Beach owned bulkhead, a small intertidal beach, the boat dock, and the Grand Canal bisecting Balboa Island. The proposed finished first floor (FF) elevation of the residence is elevation 9.0 feet NAVD88. The structure will be designed with a concrete perimeter curb, which effectively makes the FF at about elevation +10.6 feet NAVD88. The development plans show a privacy wall on the landward of the public board walk that extends up to +10.4 feet NAVD88. The top of the City of Newport Beach owned bulkhead is at about elevation 8.6 NAVD88 (shore protection). The site is currently mapped by FEMA to be in the shaded X Zone. The elevation of the Abalone Drive flow line is about +6.5 feet NAVD88 and the public boardwalk elevation is about +6.7 feet NAVD88. 2 Figure 1. Subject site, 316 Grand Canal, adjacent properties, public boardwalk, intertidal beach, boat dock, and the canal. 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 and project preliminary plans were provided by Christian Rice Architects, Inc, the project designer. A site reconnaissance was performed in September 2022. The visible portions of the offsite bulkhead were observed to be in good condition. There is a small intertidal beach fronting the bulkhead and 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 actual shoreline fronting the site. There is a small intertidal beach bayward of the public walkway. The shoreline is essentially located at the City bulkhead. 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 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 November 2018 California Coastal Commission (CCC) SLR Guidance Update 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 which the CCC has adopted in November 2018. These 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 Bay. 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 CCC SLR adopted estimates (in feet) and the probabilities of those estimate to meet or exceed the 1991-2009 mean. 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. 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 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 over 10 years old. The SLR models used as the basis for the COPC and CCCSLRG have been in place for a couple decades. 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 the past, it is very unlikely that the model will increase in accuracy when predicting SLR over the next 75 years. Simply put, if the model is not accurate now, it will be even less accurate in the future. NOAA currently provides the “best available” SLR science (NOAA, 2022). NOAA has been measuring SLR globally and at Los Angeles Harbor. The NOAA Los Angeles Harbor SLR rate is 1.03 mm/yr. The rate can be used to calculate a sea level rise of 30.9 mm (0.1 ft) over the last 22 years and next 8 years (Jan 2000 to Jan 2030), a period of 30 years. NOAA also provides the latest SLR model curves and tables for the Los Angeles Harbor NOAA Station. Figure 3 provides the SLR model curves and tables for Los Angeles Harbor. 5 Figure 3. Taken from the USACOE SLR curve calculator program. Looking at the table in Figure 3, the SLR base value in the year 2000 is 2.70 feet. Adding 0.1 feet to the base SLR value yields the value 2.8 for the year 2030. The model that most closely predicts the currently measured SLR is the NOAA 2017 Low Model. This NOAA model predicts about 1.5 feet of SLR in the year 2100. Examining Figure 2 for the year 2030 and 0.1 feet of SLR, the closest probability category is the lower limits of the “Likely Range.” The CCCSLRG document recommends that a project designer determine the range of SLR using the “best available science.” The NOAA information provided above, and in NOAA 2022, is more current than the CCCSLRG. The NOAA information provides 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. Figure 2 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 addition, based upon this 2018 COPC SLR report, the 5% probability SLR for the project is estimated to be less than 4.1 feet and a 0.5% probability that SLR will be between 5 feet and 6 feet in the year 2097. This 0.5% SLR is based upon the interpolation of the low 6 estimates and the high estimates for 2090 and 2100, recognizing that the interpolation is exponential and not linear( (4.5 + 5.4)/2 = ~5 and (5.3 + 6.7)/2 = ~6 feet)). The design historical water elevation for Newport Bay is elevation +7.7 feet NAVD88. This actual high water record period includes the 1982-83 severe El Niño, and the 1997 El Niño events, and is therefore consistent with the methodology outlined in the CCCSLRG document. The “likely” sea level rise range for the proposed project is 1.3 feet to 3.2 feet with a lower probability (~5%) of SLR of about 4.0 feet. The likely SLR range would account for future extreme bay water levels in the range of 9 feet NAVD88 (7.7 feet NAVD88 + 1.3 feet SLR) and 10.9 feet NAVD88 (7.7 feet NAVD88 + 3.2 feet SLR). There is a 0.5% probability that bay water will meet or exceed 14.4 feet NAVD88 (7.7 feet NAVD88 + 6.7 feet SLR). The top of the city bulkhead is at elevation 8.6 feet NAVD88. As stated before, the present maximum historical water elevation at the site, including El Niño effects, is ~+7.7 feet NAVD88. Based upon the elevation of the bulkhead (+8.6 feet NAVD88), the extreme Newport Bay water level will exceed the height of the bulkhead when SLR is 0.9 feet or greater. For the likely COPC SLR estimate range (high emissions) the bulkhead is safe from overtopping beyond the year 2040. For SLR greater than 0.9 feet the height of the City bulkhead will need to be increased. For the 0.5% probability SLR case this may occur around the year 2040. The site has a proposed privacy wall that extends up to elevation ~+10.4 feet NAVD88, which will protect the site from bay waters coming directly over the bulkhead and onto the property. SLR will need to be greater than 2.7 feet for the bay water at the highest tide to flood over the top of the privacy wall. For the “likely” SLR this will be about the year 2090 and for the 0.5% SLR case this would be after the year 2060. Finally, it should be noted that, if the bay water is higher than the bulkhead, flooding will not occur constantly but rather only a few times a month, at the full moon and new moon, for a period of about 1 hour. The City of Newport Beach has recognized that in the future there will be a need to raise the elevation of the boardwalks and bulkheads around Balboa Island and Newport Bay. The City of Newport Beach Building Department (CNBBD) has a standard drawing, and a regional plan for rehabilitating and raising the bulkheads. Flooding from the bay and from the beach will result in flooding of the public roads in the Newport Beach area. Recent action by the City Council has raised the minimum private bulkhead elevation for new development to be 10.9 feet NAVD88. In addition, the updated Waterfront Project Design Guidelines and Standards require that the bulkhead can be design to accommodate an increase in height to +14.4 feet NAVD88 or greater. Based upon our inspection of the existing city bulkhead, the bulkhead system is capable of being extended in height to+14.4 feet NAVD88. Waves and Wave Runup The potential surface gravity waves (ocean swell), boat wakes and wind waves to arrive at this site is nil. The site is located behind the intertidal beach and the public w alkway. Tsunami Tsunami are waves generated by submarine earthquakes, landslides, or volcanic action. 7 Lander, et al. (1993) discusses the frequency and magnitude of recorded or observed tsunami in the southern California area. James Houston (1980) predicts a tsunami of less than 5 feet for a 500-year recurrence interval for this area. Legg, et al. (2002) examined the potential tsunami wave runup in southern California. While this study is not specific to the site, it provides a first order analysis for the area. The Legg, et al. (2002) report determined a maximum open ocean tsunami height of less than 2 meters. The maximum tsunami runup in the Newport Beach open coast area is less than 1 meters in height. Any wave, including a tsunami, that approaches the site in will be refracted, modified, and reduced in height by the Newport jetties, and as it travels into the bay. Due to the infrequent nature and the relatively low 500-year recurrence interval tsunami wave height, and the elevation of the proposed improvements, the site is reasonably safe from tsunami hazards. It should be noted that the site is mapped within the limits of the California Office of Emergency Services tsunami innundation map, Newport Beach Quadrangle (State of California, 2009). The tsunami inundation maps are very specific as to their use. Their use is for evacuation planning only. The limitation on the use of the maps is clearly stated in the PURPOSE OF THIS MAP on every quadrangle of California coastline. In addition, the following paragraph is taken from the CalOES Local Planning Guidance on Tsunami Response concerning the use of the tsunami inundation maps. Inundation projections and resulting planning maps are to be used for emergency planning purposes only. They are not based on a specific earthquake and tsunami. Areas actually inundated by a specific tsunami can vary from those predicted. The inundation maps are not a prediction of the performance, in an earthquake or tsunami, of any structure within or outside of the projected inundation area. The City of Newport Beach and County of Orange have clearly marked tsunami evacuation routes for the entire Newport Beach/Bay area. 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 8 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, in the future, 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 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 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 NOAA estimate over the project 75-year design life, the SLR in the year ~2097 is ~1.5 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 (1.5 feet to 6 feet) for the proposed project. This SLR would account for future extreme bay water level of 9.7 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 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 the Balboa Peninsula, 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 the Balboa Peninsula and Balboa Island, 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 residential and ADU structure with the implementation of the SLR adaptation strategies (waterproofing to elevation ~+10.6 feet NAVD88 and the future raising of the City’s bulkhead) will not be adversely impacted by potential coastal hazards including 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 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 NOAA, 2022, 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 N O A A 2 0 2 2 , S e a L e v e l R i s e R e p o r t e m a i l l i n k . https://oceanservice.noaa.gov/hazards/sealevelrise/sealevelrise-tech-report.html 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.