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HomeMy WebLinkAbout1120WestBayNewportHazardBulkheadGeotechnical 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 October 4, 2019 JB Collins Homes 2026 W est Balboa Blvd. Suite D Newport Beach, CA 92663 SUBJECT:Coastal Hazard and Sea Level Rise Discussion for New Bulkhead,1120 W est Bay Avenue, Newport Beach, Orange County, California. Dear JB Collins Homes: 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 bulkhead at 1120 W est Bay Avenue 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 bulkhead plans, a site inspection, and preparation of this letter report. INTRODUCTION The proposed project is a new bulkhead to protect an existing single-family residence, in the City of Newport Beach. There is currently no bulkhead at the site. Figure 1, downloaded from Google Maps (Bird’s Eye View), shows the site in relation to the adjacent properties, the boat dock, and the navigation channel within Newport Bay. The finished first floor (FF) elevation of the residence is about 9 feet NAVD88. The proposed vinyl pile bulkhead is designed with a concrete cap up to elevation +10.6 feet NAVD88. This is the minimum elevation recommended by the City of Newport Beach. The site is fronted by a patio to about elevation 8.0 feet NAVD88, a narrow beach, a boat dock, adjacent to a navigation channel in Newport Bay. The proposed bulkhead will be located at the bayward edge of the patio. The addition of the bulkhead will provide protection for the existing structure, the adjacent homes, and public street (W est Bay Avenue) from flooding through the subject site. The site is currently mapped by FEMA to be in the shaded X Zone, with no base flood elevation (BFE). It should be noted that the adjacent bay is mapped in the AE Zone with a BFE of +8 feet NAVD88, 7 to 8 feet bayward of the bulkheads located on the adjacent properties. 2 Figure 1. Subject site, 1120 W est Bay Avenue, adjacent properties, boat dock, and Newport Bay channel. 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 6/12/19, and bulkhead plans, prepared by PMA Consulting, Inc., were reviewed. A site reconnaissance was performed in June 2019. There is small beach fronting the low elevation patio at the site. It is our understating that the beach slightly erodes over time and is nourished when the adjacent boat dock area is dredged. 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. 3 Shoreline Erosion Hazard There is a small beach at the site. The beach is subject to erosion when the sand is transported bayward and down slope into the artificially deepened boat slips. Over time the berthing areas fill up with sand. To mitigate this infilling, the area is periodically dredged and the sand is placed back on the beach. However, between the dredging cycles the patio can flood, which jeopardizes the existing residences (including the adjacent residences), and public streets. The proposed bulkhead will not impact this process and therefore, not impact the shoreline. The highest tide shoreline is located essentially just bayward of the patio. As sea level rises, the amount of exposed beach will diminish. However, with SLR the highest tide shoreline location will relocate to the proposed bulkhead at the bayward edge of the patio. The future highest tide shoreline will remain at the proposed bulkhead and shoreline erosion will not impact the existing development over the life of the bulkhead. Current Flooding Hazard The National Oceanographic and Atmospheric (NOAA) National Ocean Survey tidal data station closest to the site, with a long tidal record, 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, Los Angeles Harbor Tide station reports 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. Currently, the patio at elevation +8 feet NAVD88 is vulnerable to flooding from wind and wake wave runup during extreme high tides. Flood water can pass through the site and impact the adjacent properties and public streets. 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 4 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, which was also adopted by the CCC in November 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. Figure 2. Table from Kopp et al (2014), COPC 2018, and CCC 2018 providing current SLR estimates and probabilities for the Los Angeles 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 addition, based upon this 2018 COPC SLR report, the 5 % probability SLR for the project is estimated to be 4.1 feet. The maximum historical water elevation at the Los Angeles tide station is elevation+7.72 feet NAVD88 on January 10, 2005. 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 CCC Sea-Level Rise Policy Guidance document. The Newport Beach City Council approved the use of “low risk aversion” scenario for development, which is 1.3 feet to 3.2 feet by the year 2100. If 1.3 and 3.2 feet are added to this 7.7 feet NAVD88 elevation, then future design maximum water levels of 9.0 feet NAVD88 and 10.9 feet NAVD88 are determined. The “likely” sea level rise range for the bulkhead project is 1.3 feet to 3.2 feet with a lower probability (~5%) of SLR of about 4.0 feet. This SLR range would account for future 5 extreme bay water levels in the range of 9.0 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 low probability that bay water will meet or exceed 11.7 feet NAVD88 (7.7 feet NAVD88 + 4.0 feet SLR). The proposed bulkhead will be up to elevation +10.6 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 proposed bulkhead, to elevation +10.6 feet NAVD88, the extreme Newport Bay water level will exceed the height of the bulkhead when SLR is about 2.9 feet or greater. This SLR is greater than the 2100 lower project sea level rise range of 1.3 feet but less than the higher estimate of 3.2 feet . For SLR greater than 2.9 feet, the height of the bulkhead will need to be increased. The design of the bulkhead is such that it can be increased in height. For the likely SLR case this may occur in about the year 2090. 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. 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 proposed bulkhead fronting the site. W ave runup and overtopping of the bulkhead at the site is calculated using the USACOE Automated Coastal Engineering System, ACES. ACES is an interactive computer-based design and analysis system in the field of coastal engineering. The methods to calculate wave runup and bulkhead overtopping implemented within this ACES application are discussed in greater detail in the 2004 Coastal Engineering Manual. A 0.75-foot high wave was used in the ACES wave runup and overtopping analysis with a current maximum historical water level of +7.7 feet NAVD88 (no SLR). This combination of wave and water level represents an approximate 100-year recurrence interval oceanographic condition under current sea level. Table I below is the computed output from the ACES program for the wave runup analysis. TABLE I 6 The calculated maximum wave runup under the current 100-year recurrence interval conditions is just about 0.8 feet above the historical 100-year water level (elevation 8.5 feet NAVD88). This shows why the patio and site are currently subject to flooding without the proposed bulkhead. However, this is below the proposed elevation of the bulkhead (+10.6 feet NAVD88). In the future, with more than 2 feet of SLR, boat wakes during the highest high tides may overtop the bulkhead. The bulkhead is designed such that the height of the bulkhead can be increased to a minimum of +12.5 feet NAVD88. The increase in bulkhead height is a SLR adaptation strategy recommended in the 2018 CCC SLR guidance document. 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. 7 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 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 coastal 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 beach fronting the site. Future boat wakes and wind waves will be too small, even with sea level rise (SLR), to flood the existing residence provided the bulkhead is maintained. There is currently a coastal hazard of flooding of the site, adjacent properties and the public street due to the low elevation of the site and runup from wakes and wind waves. The site may be flooded in the future if the bay waters exceed the bulkhead elevation +10.6 feet NAVD88. This may occur when SLR exceeds 2.9 feet. For SLR higher than the current estimates, adaptation strategies such as increasing the height of the proposed bulkhead can be 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 only a small beach fronting the site with no seasonal changes. The beach slowly erodes into the adjacent berthing areas due to the steep slope because the berth location is close to the shoreline. The berth is periodically dredged and the sand is put back onto the adjacent beach. The proposed project will not impact this process. The width of the beach may decrease with SLR. The future highest tide shoreline is essentially at the proposed bulkhead and this will not change with SLR. 2. High tide conditions, combined with long-term (75 year) projections for sea level rise; 8 Using the SLR estimates over a typical 75-year design life, the range in the year ~2095 is between 1.3 feet and 4 feet. This is the sea level rise range for the bulkhead project. This 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 11.1 feet NAVD88 (7.7 feet NAVD88 + 4.0 feet SLR). 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. Boat wakes and wind waves in the bay can impact the existing residence without the bulkhead in place. In the future, boat and wind waves will not impact the existing residence provided the bulkhead is constructed and maintained. 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 that 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 The proposed bulkhead is necessary to protect the existing residence, the adjacent properties, and the public facilities and infrastructure from flooding. 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, 9 flood shields, watertight doors, moveable floodwalls, partitions, water- resistive sealant devices, sandbagging and other similar flood-proofing techniques. The bulkhead is safe from the coastal hazard of flooding by the proposed elevation and the bulkhead design, which can accommodate future increas in height, if necessary. 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 •A review of aerial photographs over the last several decades shows no net shoreline erosion at the site. •The site has been subject to some flooding, from wake and wave runup in the past. •The proposed bulkhead will not be adversely impacted by potential coastal hazards including a 2.9 feet sea level rise over the next minimum 75 years. Based upon the design of the, the height of the proposed bulkhead can be increased to exclude bay water. The site will be part of a community wide response to mitigate SLR hazards. •No additional protective devices will be necessary to protect the existing development from any existing or anticipated future coastal hazards for the next 75 years or more. The proposed bulkhead can be increased in height without and further bayward encroachment. RECOMMENDATIONS Based upon the analysis and discussion herein, the proposed bulkhead and existing residence are 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 provided the recommendations herein are followed. 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 elevation of the bulkhead. 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 10 project. The proposed bulkhead 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, “W aterfront Project Guidelines and Standards, Harbor Design Criteria Commercial & Residential Facilities,” 2017 Edition NOAA, 2018, 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 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.