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HomeMy WebLinkAbout20210426_Coastal Hazard AnalysisPA2021-101 Geotechnical • Geolog ic • Coastal • Environmental 5741 Palm er Way • Carlsbad, California 92010 • (760) 438-3155 • FAX (760) 931-0915 • www.geosoilsinc.com March 22, 2021 Mr. & Mrs. Greer 263 Cedar Street Newport Beach, CA 92663 SUBJECT: Coastal Hazard Analysis, 237 Canal Street, Newport Beach, Orange County, California. Dear Mr. & Mrs. Greer: 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 residence at 237 Canal Street 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 CCC Sea-Level Rise (SLR) Policy Guidance document (adopted August 2015), a review of City of Newport Beach Municipal Code (NBMC) 21.30.15.E.2, a review of the proposed residence plans, a site inspection, and preparation of this letter report. SITE INFORMATION SITE DESCRIPTION & PROPOSED DEVELOPMENT The subject site is a quasi-rectangular lot approximately 28 feet by 85 feet in the City of Newport Beach. The proposed development is removal of the existing residence and construction of a new single family residence. The lowest habitable elevation of the proposed development is + 9.25 feet NAVD88. The site is adjacent to the Semeniuk Slough which is a former channel of the Santa Ana River that historically emptied into Newport Bay. The Semeniuk Slough is exposed to limited tidal influence thru a tidal culvert connected between the Santa Ana River and the slough. The slough is isolated from influence by both the ocean and the Santa Ana River for water elevations greater than 6.5 feet NAVD88. The proposed development is landward of the upper limit elevation of the slough. Figure 1 is a recent aerial photograph of the site downloaded from the internet. The site is located about 1/4 mile from the Pacific Ocean and over 1/3 mile from the levee that controls the maximum water elevation within the Semeniuk Slough. PA2021-101 2 Figure 1. Subject site and adjacent Semeniuk Slough. DATUM & DATA The datum used in this report is North American Vertical Datum of 1988 (NAVD88) which is 2.35 feet below NGVD29, and 4.49 feet below Mean High Water (MHW). The units of measurement in this report are feet (ft), pounds force (lbs), and seconds (sec). Site elevations relative to NAVD88 were taken from a site survey by Toal Engineering, Inc. Development plans were provided by Eric Aust, Architect. The site was inspected on March 17, 2021. The site inspection revealed that there were no signs of hydric soils at elevations above +6.5 feet NAVD88. The soils were sandy but contained no organic matter and there was no streaking of the subsurface horizons by organic matter above elevation +6.5 feet NAVD88. There was no evidence of inundation or any impact of water above elevation +6.5 feet NAVD88. Hydric soils need frequent inundation to develop anaerobic conditions. There are no signs of any inundation above elevation +6.5 feet NAVD88. Interviews with long time residents revealed that water elevations never exceed +6.5 feet NAVD88 including during the 1982 -83 El Nino weather with very high ocean waters and very strong rainfall (river flows). In conclusion, the areas above elevation +6.5 feet NAVD88 do not show signs of inundation or hydric soils and therefore do not depict a wetland type environment. PA2021-101 3 HAZARD ANALYSIS Typical coastal hazards include, wave and wave runup attack, erosion, and flooding. The Semeniuk Slough is a relatively narrow long body of water and not a large enough body of water for waves to develop. The slough is not subject to ocean waves. In addition, there is not active boating in the slough, so no wakes reach the shoreline. A slough by definition is a slow moving body of water with insufficient flow velocities to erode the shoreline. FEMA provides flood risk maps that typically cover the several years. Figure 2 below is the FEMA designation of the site and adjacent areas including the slough. The site is classified as an "area of reduced flood risk due to levee, Zone X." Figure 2. FEMA flood zone overlay. 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 Mean Tide Level (MSL) Mean Low Water NAVD88 Mean Lower Low Water 4.55 feet 2.62 feet 0.74 feet 0.0 feet -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, PA2021-101 4 wave group effects and El Nino sea level effects. The 1 % ocean water elevation at the Los Angeles Harbor Tide station is + 7.18 feet NAVD88. The highest water elevation in the Semeniuk Slough is 6.5 feet NAVD88. Currently, the street flow line is at about elevation +8 feet NAVD88. The potential hazard of site flooding, currently and for the next approximately 1 O years, is determined by FEMA. Figure 2 is the current FEMA FIRM , 06059C0264K, effective 3/21/2019, and identifies the area as mapped in ZONE X with little to no risk of flooding. There is no flooding risk because the site area is protected by levees and berms. In addition, the site grade is at elevation 9.0 feet NAVD88, which is above any potential flooding due to the river and or the ocean. The levees and berms that protect the area are part of the area infrastructure. These berms and levees will be maintained and enhanced, if necessary, to protect the areas that rely on them. There are very few tools available to determine how SLR may impact the potential for flooding of the site in the future. The United States Geological Survey (USGS) has also developed a model called the Coastal Storm Modeling System (CoSMoS) for assessment of the vulnerability of coastal areas to SLR and the 100-year storm, http://walrus.wr.usgs.gov/coastal processes/cosmos/. However, the use of CoSMoS is limited by the following disclaimer. Dis.claimer: This interactive mapping tool, incl uding its data and other information ("tool and data") are provided for informational purposes. The tool and data are not for the purpose of providing advice or guidance on issues or activities related to its content including, but not limited to, navigation, investment, development or permitting .The tool and data are based on model simulations, which are subject to revision and do not take into account many variables that could have substantial effects on flood extent and depth. Real world results will differ from results of the tool and data.Commercial use of this tool and data are prohibited. The use of CoSMoS for future flooding potential at this site, located substantially far from the coast and protected by levees, needs to be significantly qualified. Simply running the program with a specified SLR scenario may yield nonsensical results. The CoSMoS program output using the 100 year storm condition can show wave run up in areas that are absolutely impossible for wave runup to reach. Some of this is the result of the program's inability, in some cases, to recognize rapidly changing elevations, such as a berm or levee, or tide/flood control structures like on the Semeniuk Slough . These variables and others (such as elevation) have a substantial effect on the flood extent and depth as explained in the disclaimer above. Figure 3 is the CoSMoS output example that shows the site is just becoming vulnerable to flooding with 125 cm (4.1 feet) of SLR. This would translate to a maximum ocean water level of 4.1 feet SLR + 7 .18 feet NAVD88 (historic highest water) or ~11.3 feet NAVD88. It should be noted that the source of water for this flooding potential is not from the Pacific Ocean. In addition, the model doesn't account for the water level limit of 6.5 feet NAVD88 in the Slough . PA2021-101 Figure 3. CoSMoS output for 4.1 feet SLR. omrlli!lo,.Y-fyt",9M,;,a; 1HA + \'/il,~COO Huard (ro dl!l) llScrn +W.r."li!OOO H.U¥dl25onst.R•\\a·,~ 2':>0cmlB.2ftl S00cm06.•ft) 7S0cm(24.6ft} 5 The CoSMoS model clearly has shortcomings, particularly for this section fo Newport Beach and the subject site. These shortcomings are acknowledged in the software disclaimer. The modeling is more informative when applied to the open coastline than it is to a site that is more than 1400 feet from any coastal body of water. In addition, the disclaimer states that the modeling should not be used for permitting. However, ignoring the programs shortcomings, it is clear that with increasing SLR much of Newport Beach is vulnerable based upon elevations alone. The site, being in a slightly higher section of Newport Beach, will be unreachable by the public roads (Pacific Coast Highway) long before the development may be subject to flooding, if no regional adaptation plan is implemented. This vulnerability is not just at this particular site or development. Most of Newport Beach and surrounding communities (Huntinton Beach, Seal Beach, Long Beach, etc.) are vulnerable to flooding. This vulnerability existed long before SLR became such an important issue. The entire area is basically a huge in filled flood plain. Figure 4 is a 1927 aerial photograph that shows the site and area before the coastal structures and tidal level controls Semeniuk Slough. PA2021-101 6 Figure 4. Subject site and area on November 3, 1927. The CoSMoS modeling assumes that no regional action will be taken to mitigate the SLR flooding impacts in the future, in spite of the fact that the levees and tide gate have been built and have been maintained over the years. That is to say the CoSMoS analysis assumes that the City of Newport Beach resiliency program will not include the continued protection of the majority of the City. There is no form/type of shore protection that can be installed for any development at this site or area, in general, that will insure that there is access to the development regardless of how high the FF elevation is. The public roads will be underwater long before the proposed structure can flood due to SLR, if regional adaptation action is not taken. Project SLR 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 COPC 2018 guidance has been identified by the CCC as the best available science with regards to SLR in the November 2018 CCC SLR Guidance Update. This update included SLR estimates and probabilities for Los Angeles, the closest SLR estimates to Newport Beach. The report provides SLR estimates based upon various carbon emission scenarios known as a "representative concentration PA2021-101 7 pathway" or RCP. Figure 5 provides the March 2018 COPC data (from the COPC Table 28 and 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. Piobiiblllsdc Project(onii (In re"JJ (ba~edo~kopp et al. 20/4) H++ scenario MEDIAN LIKELY RANGE HN-20 CHANCE HN-200 CHANCE (Sweet et al. ! 5096 probability l 66% probability sea-level rise meets sea-level rise 1 or e;l(ceeds... is between .. , or exceeds ... --------~-+----+-------- ' ~fs~ Medium -High eimeme r J_ Aversion ----..--•'-''-A•_.,._,on_..,..•_'''-A-•er-•lo_n ""I zoio -· --0.3 I 0.2 o.5 0.6 0.7 1.0 1040 o.5 I 0.4 0.1 o.9 1.2 1.1 2017) ··• ••·• , ··• 'Sin_gff' scenur,o -H!91l emI11Ion1 __ _102,0__[ __ o_.7_~_o._5 --f-1._o __ 1_.2 __ 1 ___ 1._8 2.6 loii emI11Ion1 /060 1 o.8 o.5 1.1 1.4 2.2 Hlglt emll)IOns 1060 l--1.0 __ 1 _o_?_ _____ ..1:~-___ 1.7 2.5 3.7 tow emIs1Ion1 1070 o.9 o.6 u 1.0 2.9 lllgh emlsslOlll I tow emI1sIon1 ~1_g11~m11s1on1 I tow eml11Ion1 I Hlqh emissions ,_.:.. __ · tow emI11Ion1 fHlgh emllll~lll tow cffilSIIOlll I fllgll eml,1I0111 1070 2080 1080 ·-2090 1090 l!00 1100 1110· 1110' 1.2 0.8 1.7 2.2 3.3 5.0 ------------• -------------➔------------·------·· ----- 1.0 1.5 1.2 18 1.3 2.2 1.4 2.3 0.6 1.6 2.1 3.6 1.0 2.2 4.3 6.4 4.5 07 1.2 1.8 2.7 2.8 2.5 3.4 5.3 8.0 0.7 2.1 3.0 5.4 I u 3.2 4.1 6.7 9.9 Jog· : · 2.2-31 6.0 I 1.6 3.3 4.3 7.1 11.5 -----------•-----1----1 Figure 5. Best available science SLR estimates from Table 28, COPC 2018. The City of Newport Beach recognizes that there are areas in the community that are vulnerable to flooding due to SLR. The coastal portions of the City was established in an area that was already vulnerable to flooding. The City is in the process of updating their LCP, and based upon the history of the community, the "do nothing" or "retreat" adaptation strategies, the City resiliency program will continue the maintenance and augmentation of levees and tide gate as an adaptation strategy to mitigate SLR impacts. The LCP update includes plans for SLR resiliency. These plans call for the enhancement and buttressing of the existing levees and upgrading the existing storm drain infrastructure. The Santa Ana River banks will be raised as needed to adapt to SLR. This can be accomplished with something as simple as a parapet on top of existing flood control structures. GROUNDWATER & SLR In general, ocean tides impact groundwater elevations when the site is very near the ocean. The further away the site is from the ocean, the impact of the tide on the groundwater becomes less and less. A scientific paper in the Journal of Hydrology: Regional Studies (Hoover, et al., 2015) provides a study of the impact of sea level rise on groundwater for three California coastal sites: Arcata , Stinson Beach, and Malibu Lagoon. The paper, available online , concludes that "additional groundwater emergence/shoaling PA2021-101 8 due to tidal forcing seems unlikely to be a major factor." The study at the Malibu Lagoon included data on well (groundwater) tidal response that suggest only modest response. The report states that significant damping of tidal response with distance from the shoreline, with about 15% of the tidal signal visible in a well 60 meters (200 feet) from the shore and about 1 % of the tidal signal visible in a well 115 meters (380 feet) from the shore. The report concludes that direct marine inundation will be the dominant mechanism of inundation of low lying areas of the California Coast. This would be in areas where the level of the ocean is above the ground surface elevation and there is a path for ocean waters to travel into the area. The study also points out that in many low-lying coastal areas transient events will produce more severe conditions than SLR impacts. Heavy rain can cause short-lived increases in groundwater levels from direct infiltration and up gradient areas. The project site is about 1400 feet from the Pacific Ocean. At this distance, the groundwater is not noticeably impacted by the tides or ocean water levels. With an increase of up to 6 feet of SLR in 75 years, the future maximum groundwater elevation at the site would be the typical groundwater elevation plus, at most, 0.06 feet (1 % of 6 feet SLR). The proposed lowest floor is above this elevation. To prevent future groundwater issues, we recommend that all below grade foundations be waterproofed. The lowest habitable finished floor is at about ~+9.25 feet NAVD88 and with 6 feet of SLR in 75 years from today, the natural groundwater will still be well below that elevation at the site. 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 PA2021-101 9 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 CalOES maps model the inundation of a tsunami with an approximate 1,000 year recurrence interval (0.1 % event). The Science Application for Risk Reduction (SAFRR) tsunami study headed by USGS investigated a tsunami scenario with a 200-240 year recurrence interval. The SAFRR modeling output is shown in Figure 6 and reveals that the site is not within the more probable (0.4% event) tsunami inundation zone. The City of Newport Beach and County of Orange have clearly marked tsunami evacuation routes for the entire Newport Beach/Bay area. Figure 5. SAFRR tsunami modeling output for the site. Coastal Hazards Report shall include (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 PA2021-101 10 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; The typical coastal hazards to consider are shoreline erosion, flooding, and wave impacts. No ocean swells reach the site and no wakes will impact the development over the next 75 years. There is no potential for shoreline erosion at the site. The water elevation in the Semeniuk Slough cannot exceed elevation 6.5 feet NAVD88. iii. An analysis of the following conditions: 1. A seasonally eroded beach combined with long-term (75 year) erosion factoring in sea level rise; As discussed in this report the there is no historical erosion at the site. There was no observable erosion over the 90+ years of historical photographs (Figure 4) reviewed to analyze erosion. 2. High tide conditions, combined with long-term (75 year) projections for sea level rise; Using the CCC SLR estimate over the project 75-year design life, the range in the year ~2096 is between 3.2 feet and 6.1 feet. This is the sea level rise range for the proposed project. This SLR range would account for future ocean water levels in the range of 10.9 feet NAVD88 (7.7 feet NAVD88 + 3.2 feet SLR) and 13.8 feet NAVD88 (7.7 feet NAVD88 + 6.1 feet SLR). Because the water elevation in the Semeniuk Slough cannot exceed elevation 6.5 feet NAVD88 high tides and SLR will not impact the project. 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 that setback necessary to achieve a factor of safety of 1.5 (static) and 1.1 (pseudostatic) today PA2021-101 11 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 fronting the site. 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 mitigates the coastal hazard of flooding by elevating the finished floor (FF) above the flow line of the adjacent streets. The proposed effective finished floor is at elevation 9.25 feet NAVD88. The Canal Street flow line at ~8.0 feet NAVD88 is well below the effective FF elevation. Waterproofing can be added in the future to an elevation above the potential flood elevation. The future addition of waterproofing up to about elevation above the flood potential is a SLR adaptation strategy that will protect the structure beyond the year 2096. Many of the public streets around the Newport Bay area are at elevation +8 feet NAVD88 or LOWER. The public streets will flood due to SLR long before the residence will be impacted by SLR. In the future if necessary, the residence can be retrofitted with waterproofing to an elevation above the flooding potential elevation along with flood shields and other flood proofing techniques. It is very likely that the community will adopt SLR adaptation strategies that are currently being considered by the City of Newport Beach. These strategies involve raising levees, replacing or adding to bulkheads, beaches and walkways that surround the bay. These are regional adaptation strategies. CONCLUSIONS • Using the latest SLR projections, the average (0.5%) SLR over the next 75 years is about 5.7 feet. It is possible, but not probable, that SLR could be 6.1 feet in 75 years. It is likely SLR will be less than 3.3 feet in the year 2100. PA2021-101 • • 12 The site is in a slightly higher section of Newport Beach as compared to Balboa Island and the Balboa Peninsula, and the streets and adjacent areas are protected from flooding by levees. The water level in the Semeniuk Slough can be controlled regardless of SLR. The lowest finished floor elevation (not garage floor) is ~1.25 foot above the highest adjacent streets flow lines. This elevation is sufficient to mitigate the vulnerability of the development to the unlikely event in the future emergent groundwater with SLR. There is no need for shore protection over the life of the development. In the future, depending upon the rate of SLR, the community will need to increase the height of the existing flood control systems in the area. This is a reasonable adaption strategy including the raising of the Santa Ana River levees and controlling flooding of Pacific Coast Highway. The Newport Beach community will necessarily implement this type of strategy. Not adapting to SLR in this manner will result in the inundation of large areas, and several communities and cities. RECOMMENDATIONS • The design and materials of the proposed development should be such that waterproofing may be retrofitted in the future, if necessary. • To prevent future groundwater issues, we recommend that all below grade foundations be waterproofed. 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 PA2021-101 13 PA2021-101 14 REFERENCES Aerial Fotobank, San Diego web site www.landiscor.com. 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 Legg, Mark R., Borrero, Jose C., and Synolakis, Costas E., Evaluation of tsunami risk to southern California coastal cities, in The 2002 NEHRP Professional Fellowship Report. State of California, County of Orange, 2009, "Tsunami Inundation Map for Emergency Planning, Newport Beach Quadrangle," 1 :24,000 scale, dated June 1.