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HomeMy WebLinkAboutCoastal_Hazards_4-5-2017Geotechnical • Geologic • Coastal • Environmental 5741 Palmer Way , Carlsbad, California 92010 • (760) 438-3155 • FAX (760) 931-0915 • www.geosoilsinc.com April 5, 2017 Mr. Dave Gunderson 409 North Bay Front Newport Beach, CA 92662 SUBJECT: Coastal Hazard Analysis, 409 North Bay Front, Newport Beach, Orange County, California. Dear Mr. Gunderson: 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 409 North Bay Front on Balboa Island in Newport Beach, California. The purpose of this report is to provide the hazard information for your permit application requested by the City of Newport Beach in the March 20, 2017 Notice of Incomplete Coastal Development Permit Application Filing. Our scope of work includes a review of the California Coastal Commission (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 & E.3, a review of the proposed residence plans, a site inspection, and preparation of this letter report. The CCC Sea-Level Rise Policy Guidance provides a SLR range, over the time period from 2000 to 2100, of 17 inches to 66 inches. SITE INFORMATION The shoreline fronting this site is located on north side of Balboa Island, inside Newport Bay, on a channel fronted by adjacent islands. The site is fronted by a public boardwalk and a City of Newport Beach owned concrete bulkhead. Figure 1 shows the site and current FEMA flood zone designation Zone AE, with a base flood elevation of +8 feet NAVD88. It is our understanding that the soon to be released (proposed 2018) FEMA maps do not change the flood zone designation at this location. Figure 2 is a photograph taken from the bay back towards the property and shows the beach and Balboa Island. This beach is a feeder beach and is regularly nourished by dredging projects (channel maintenance) in the area. The proposed project involves the demolition of the existing residence and the construction of a new residence essentially within the footprint of the existing residence. The beach fronting the project site is located far enough inside Newport Bay that it will not be subject to waves from the ocean. Small wind waves may reach the shoreline fronting the site, only if the wind is coming from the north. However, the fetch is very limited and only waves on the order of inches high can be generated and reach the shoreline. Boat wakes are another source of waves. Finally, it is unlikely that a tsunami could propagate into the Newport Bay such that it would significantly impact this site. PA2017-045 2 Figure 1. Google Earth image with the FEMA flood zone overlay. Note the relatively wide beach as a result of frequent nourishment from dredging of the bay navigation channels. Figure 2. The subject site, beach, adjacent bay area, and Balboa island. The proposed new residence has a lowest finished floor at +9.0 feet NAVD88. The perimeter walls of the residence will be constructed with a concrete curb to about elevation 9.5 feet NAVD88, with the ability to add additional waterproofing to higher elevations as needed to adapt to SLR. PA2017-045 3 HAZARD ANALYSIS There are three different potential oceanographic hazards identified at this site; shoreline erosion, flooding, and waves (wind waves, boat wakes, and tsunami). 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, if necessary. Erosion Hazard In an effort to determine typical changes in the shoreline position, aerial photographs from the early 1970s to 2017 were reviewed. Due to the differences in tide levels and oblique angles of the photos, it is difficult to determine the exact location of the shoreline. However, a visual comparison of the most recent and the oldest photographs shows little or no change in the shoreline position {beach width) over the last three decades. The future shoreline changes over the next 75 to 100 years can be assumed to be the same as in the previous few decades. It is unlikely that there will be any significant change in Newport Bay with regards to the dredging program, the vessel traffic, and waves in the next 75 to 100 years. It should be noted that the tides or water level at the site do not erode the beach. The erosion is caused by currents and waves/wakes within the bay. A future rise in sea level will not contribute significantly the erosion rate at the site for this reason. The actual inter-tidal shoreline may move landward as sea level rises. Current Flooding Hazard The current water levels (tides) within Newport Bay are well documented. The National Oceanographic and Atmospheric National Ocean Survey tidal data station closest to the Project site is located about 1 mile away at the Newport Bay Entrance Station (NOAA, 2013). The current (last tidal epoch) elevations in feet are as follows: MEAN HIGHER HIGH WATER (MHHW) = 5.25 MEAN HIGH WATER (MHW) = 4.49 MEAN TIDE LEVEL (MTL) = 2.62 MEAN SEA LEVEL (MSL) = 2.59 MEAN LOW WATER (MLW) = 0.74 NORTH AMERICAN VERTICAL DATUM-1988 (NAVO)= 0.0 MEAN LOWER LOW WATER (MLLW) = -0.18 The present maximum historical water elevation at the site on Balboa Island, including El Nino effects, is -7.2 feet NAVD88. At -9.5 feet NAVD88, the FF elevation (with curb) of the proposed single family residences will be 4.25 feet above the MHHW line and 2.3 feet above the El Nino water elevation. As a result, the development is to be located at an elevation that would not expose it to flooding from even the highest current tides. PA2017-045 4 Future Tide Levels Due to Sea Level Rise There have been a number of studies that provide some predictions as to the amount of future SLR in Newport Bay. The City of Newport Beach contracted Everest International Consultants, Inc. (EICI) to produce an assessment report on the Balboa Island seawall(s) (EICI, 2011). This report provides a comprehensive discussion of future SLR and the potential for flooding of Balboa Island. In addition, the U.S. Army Corps of Engineers (USACOE) has provided a guideline for incorporating sea level change in civil works projects (USACOE, 2013). The 2013 USACOE guideline provides a high, an intermediate, and a low SLR estimate. The CCC Sea-Level Rise Policy Guidance document was approved in August 2015. The CCC has adopted the National Research Council 2012 SLR estimates of 16.56 inches to 65.76 inches over the time period from 2000 to 2100. Figure 3 compares many of the current SLR estimates including the USACOE, the CA Coastal Conservancy, CA Ocean Protection Council, the predictions of leading climate scientists (Vermeer and Rahmstorf), and the NRC/CCC adopted estimates. It should be noted that the CCC maximum SLR (the black line) curve is higher than the other SLR projections plotted at all times until the year about 2090. The CCC minimum SLR line is roughly the same as the USACOE/lntermediate dashed red line. It is clear that while there is some agreement over the next 30 years, beyond 30 years from today there is little agreement on SLR projections as evidenced by the large range of SLR in the year 2100. • a,oc...,. .. ~-Sea level Rise Prolections for Calffornia (NRC 2012) TIME SOUTH OF CAPE 6.0 -• ·a_~~ --r Ji-'-'='--+-==~--, ·-·-,...,, ~· PERIOD* MENDOCINO :; l.O ............ _ ...,._,~. by 2030 by 2050 by 2100 2-121n (4-30cm) 5-24in (12-61 cm) 17-66in l::x (42-167cm) I ' ·---··-··-·-----1 I . ... Ji.-~lii:il~!!::±~::.:.::.:::.::.::::.:::::.::::.::.:::::::::::::::::::::~---~~~~~___J ,.,. ,.,. ,010 ,, .. .... Figure 3. Sea level rise predictions by various agencies including the CCC. PA2017-045 5 The proposed effective finished floor is at elevation -9.5 feet NAVD88 with the alley and public walkway flow lines at less than 6.0 feet NAVD88. As stated before, the present maximum water elevation at the site including El Nino effects is 7 .20 feet NAVD88. Based upon the elevation of the area drainage, the Bay water level already exceeds the height of the drainage adjacent to the residence. The top of the adjacent public bulkhead is elevation +7.8 feet NAVD88. The bay water will exceed the elevation of the bulkhead under maximum still water elevation when SLR is about 0.6 feet. This could occur in the next few decades. The historical bay water elevation will exceed the effective FF elevation when SLR is greater than 2.3 feet. As depicted in Figure 3, the residence will not flood during El Nino high tides until SLR reaches 2.3 feet above present levels. Using the most aggressive SLR estimate that is in the year 2055, and under the lowest SLR estimate, not until well after the year 2100. The addition of the waterproofing up to a higher elevation, as needed, a SLR adaptation strategy that will protect the structure from flooding beyond the year 2092 (75 years from today). Figure 3 also illustrates that the residence FF elevation is safe from flooding until approximately 2070 under the highest COPC SLR rate. In addition, the actual site is safe from flooding until about the year 2100 using the COPC low estimate. Finally, Figure 3 shows that under some SLR predictions the residence will be safe beyond the year 2100. The 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. This flooding hazard analysis does not include any temporary flooding associated with wind/wave runup. Potential flooding associated with wind/wave runup is considered in the next section. 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 beach fronting the site. Both of these types of waves are dampened by the moored vessels and dock systems that are located in front of, and adjacent to, the site. Boat wakes are small due to speed restrictions and wind waves will be less than 0.75 feet in height with periods of less than 4 seconds. Wave runup and overlapping for the site is calculated using the US Army Corps of Engineers 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 overlapping 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 overlapping analysis with a water level of +7.0 feet NAVD88 and a beach/site elevation of 8.5 feet NAVD88. 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. PA2017-045 TABLE I ~:;ic.:::·: .. ::·l~~:~:;::~::~::;::T;7=1 Item Unit Va,lue 1roooth Slope -------------··--------------·-··---------·-··-----~-------·------.,·------------"·--·---------·-·-------Hunup and Incident Wave Heig}1t Hi: ft 0.750 Overtopping Wave Period T: sec 4.000 ····--··-·--······-······--------- COTAN of Nearshore Slope COT(¢): 20.000 Water Depth at Structure Toe ds: ft 10. 2€8 COTAN of Structure Slope COT(8): 0.000 Structure Height Above Toe hs: ft 11.5€8 Wave Hunup R: Onshore Wind Velocity U: Deepwater Wave Height H0: Relative Height ds1'H0: Wave Steepness H0/(g'l"'Z): Overtopping Coefficient «: Overtopping Coefficient Qstar0: Overtopping Rate Q: ft ft/sec ft ft'3/s-ft 0 .830 0.000 0.816 12.494 0.001586 0.077000 0.050000 0.000 409 North Bay Front Bulkhead Overtopping 3 ft SLR 6 The calculated maximum wave run up under the current 100-year recurrence interval conditions is just about 7 inches above the 100 year water level. This is just above the elevation of the bulkhead but well below the proposed FF elevation. Any wake/wave run up waters that reaches this elevation will have a low velocity, no energy, and no erosion capacity. It should be noted that under future SLR conditions the wake/wave run up will be about the same 7 inches above the water elevation. Looking at Figure 3 and adding 7 inches for wave run up shows that the residence will be safe from flooding beyond the year 2060. There is no potential hazard from wakes/waves to the proposed development until a least the year 2060. 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 run up 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 run up 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. PA2017-045 7 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. CCC SLR GUIDANCE INFORMATION Step 1. Establish the projected sea level rise range for the proposed project's planning horizon using the best available science, which is currently the 2012 NRC Report. Using the CCC SLR estimate over the project 75-year design life, the range in the year -2092 is between 1.25 feet and 4.5 feet. This is the sea level rise range for the proposed project. This SLR range would account for future extreme bay water levels in the range of 8.45 feet NAVD88 (7 .2 feet NAVD88 + 1.25 feet SLR) and 11. 7 feet NAVD88 (7 .2 feet NAVD88 + 4.5 feet SLR). Step 2. Determine how physical impacts from sea level rise may constrain the project site, including erosion, structural and geologic stability, flooding, and inundation. The discussion herein explains that it is unlikely that if a wave/wake overtops the bulkhead and floods the walkway, that the residence will flood over the project life (75 years). The lowest finished floor elevation at+9.5 NAVD88, with the ability to add waterproofing up to higher elevations, as needed, is above the historical high bay water plus 4.5 SLR. This is at the upper limit of the project SLR range over the project design life. Balboa Island public streets will flood due to SLR long before the site will be subject to flooding. Step 3. Determine how the project may impact coastal resources, considering the influence of future sea level rise upon the landscape as well as potential impacts of sea level rise adaptation strategies that may be used over the lifetime of the project. The project will not impact coastal resources considering sea level rise. To adapt to SLR PA2017-045 8 the existing public bulkhead can be increased in height to +12 feet NAVD88. 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 on Balboa Island, Lido Island, and Balboa Peninsula will flood with lower SLR well before the subject site floods. It is very likely that the community will adopt some of the SLR adaptation strategies that are currently being considered by the City of Newport Beach. These strategies involve raising/replacing the bulkheads, beaches, and walkways that surround the bay. This is a regional adaptation strategy. The project has a site specific SLR adaptation strategy to waterproof the structure up to elevations above the flood potential and to deploy flood shield for the doors. There are currently several very effective temporary flood control systems such a Quick Dam. In 40 or more years, when there is a chance that some form of flood shield at the residence doors may be needed, the flood control technology will have greatly improved. Step 4. Identify alternatives to avoid resource impacts and minimize risks throughout the expected life of the development. The project does not impact resources and minimizes flood risk through the project design, and future implementation of community and site specific SLR adaptation strategies. Step 5. Finalize project design and submit CDP application. The project architect will incorporate this report into the project design. CITY OF NEWPORT BEACH INFORMATION Bulkhead condition report shall include (NBMC 21.30.15.E.3): 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. Mr. Skelly has extensive experience in shoreline erosion, bluff erosion, soils engineering, and the design, permitting, and construction of shore protection devices. Projects include levee engineering and design in San Francisco Bay, seawall and marina engineering in Baja California Sur, coastal boardwalk design and protection in Pacifica, and seawall projects throughout southern California. ii. An analysis of the condition of any existing bulkhead including whether the top elevation meets current City standards, the conditions of the sheetpiles or panels, the condition of existing tiebacks and/or deadmen or similar, and any other relevant conditions; PA2017-045 9 The City of Newport Beach bulkhead has been visually inspected by the undersigned and it appears to be in conformance with the City bulkhead standards. No maintenance of the bulkhead is needed at this time. iii. Recommendations regarding the need for repair, augmentation or replacement of the bulkhead or any parts thereof; No maintenance recommendations are needed. A future height increase is currently being consider by the City. iv. If augmentation or replacement in the existing alignment is necessary, recommendations that will avoid seaward encroachment of the bulkhead; In the future, if augmentation is needed as part of the regional SLR adaptation program, the bulkhead can be increased in height without any bayward encroachment. v. If replacement is necessary and the existing bulkhead is not in alignment with adjacent bulkheads, recommended alternatives that will relocate the bulkhead in as much in alignment with adjacent bulkheads and as far landward, as possible. Replacement is not necessary. 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 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; As stated in this hazard analysis, the typical coastal hazards to consider are shoreline erosion, flooding, and wave/wake impacts. There is a beach fronting the site that is subject to periodic nourishment from channel dredging. Boat wakes PA2017-045 10 and wind waves are too small, even with sea level rise (SLR), to flood the residence over the next 75 years. There is a potential coastal hazard of flooding which is shared with the entire Balboa Island community. iii. An analysis of the following conditions: 1. A seasonally eroded beach combined with Jong-term (75 year) erosion factoring in sea level rise; The beach has been essentially stabilized by the beach nourishment efforts. As sea level rises, the width of the beach may narrow unless the nourished beach is reestablished at a higher elevation. 2. High tide conditions, combined with Jong-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 -2092 is between 1.25 feet and 4.5 feet. This is the sea level rise range for the proposed project. This SLR range would account for future extreme bay water levels in the range of 8.45 feet NAVD88 (7.2 feet NAVD88 + 1.25 feet SLR) and 11. 7 feet NAVD88 (7 .2 feet NAVD88 + 4.5 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. The analysis herein shows that boat wakes and wind waves will not impact the residence. 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. PA2017-045 11 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 a City of Newport Beach owned bulkhead fronting the site. The bulkhead cannot be removed in that it is part of a continuous bulkhead system that prevents flooding of Balboa Island and most properties in the area. The bulkhead is required to protect existing development, both public and private. 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 well above the flow line of the adjacent streets. To adapt to SLR, the bulkhead can be increased in height from +7.8 feet NAVD88 to +12 feet NAVD88 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 SLR adaptation strategies that are currently being considered by the City of Newport Beach. These strategies involve raising/replacing the bulkheads, beaches and walkways that surround the bay. This is a site specific adaptation strategy. Finally, 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. CONCLUSIONS • A review of aerial photographs over the last several decades shows no shoreline retreat in front of the site. No shoreline erosion is expected in the future. • The existing residence has not been subject to flooding, erosion damage, or wave runup attack in the past. PA2017-045 12 • The proposed improvements with waterproofing are above any potential coastal hazard including a 4.5-foot sea level rise over the next minimum 75 years. • 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 provided the elevation of the existing bulkhead is raised to -+12 feet NAVD88 in the future. CONCLUSIONS 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 moderate future SLR. The proposed development will neither create nor contribute significantly to erosion, geologic instability, or destruction of the site or adjacent area. It is very unlikely that any additional form of shore protection other than the bulkhead height increase will be needed in the next 75 years with SLR. It should be noted that future flooding hazards due to SLR are shared by all development on Balboa Island. SLR impacts will be a regional problem and only solved by a regional management plan. 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 PA2017-045 13 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. NOAA, 2013, 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 State of California, County of San Diego, 2009, "Tsunami Inundation Map for Emergency Planning, Newport Beach Quadrangle," 1 :24,000 scale, dated June 1. USACOE, 2013, "Incorporating Sea Level Change in Civil Works Programs," ER 1100-2- 8162, dated 31 December PA2017-045