AECOM 510 419 6000 tel 2101 Webster Street 510 419 5355 fax Suite 1900 Oakland, CA 94612 www.aecom.com Memorandum To Wendy Goodfriend, BCDC Page 1 CC Claire Bonham-Carter, AECOM Subject Bay Farm Island Focus Area From Jeremy Mull, P.E., Michael Mak, P.E., Kris May, Ph.D., P.E. Date June 25, 2014 1. INTRODUCTION AND PURPOSE Bay Farm Island was selected as a focus area for more detailed sea level rise (SLR) exposure analysis and adaptation strategy development as part of the ongoing work by the San Francisco Bay Conservation and Development Commission (BCDC) and Alameda County Flood Control and Water Conservation District (ACFCWCD) to better understand sea level rise, storm surge, and shoreline vulnerabilities in Alameda County. Under a separate project named the Adapting to Rising Tides (ART) Transportation Vulnerability and Risk Assessment Pilot Project (ART) (BCDC et al. 2011), this area was identified as vulnerable to inundation by SLR and coastal storm surge that could impact critical transportation assets and other adjacent assets that support the region. Additional sea level rise inundation maps were created for the Alameda County Sea Level Rise Shoreline Vulnerability Assessment (in progress), which includes the Bay Farm Island focus area within the mapping extent. These inundation maps were examined to better understand the timing of when sea level rise is expected to impact the shoreline and inland areas. The inundation maps showed that an extensive portion of Bay Farm Island, including facilities associated with the Oakland International Airport, would be inundated with as little as 12 inches of SLR (see Figure 1). Prior to identifying key vulnerabilities within the focus area, a first step was taken to verify the accuracy of the initial inundation maps and confirm the likelihood of inundation shown during 12 inches of SLR. Refinements to the inundation maps were made where necessary and are presented in this memorandum. The purpose of this memorandum1 is to identify the key areas of vulnerability that exist within the focus area and assess the sources, mechanisms, and timing of inland inundation and flooding to inform the development of adaptation measures, some of which are presented in this memorandum. This technical memorandum should be considered in tandem with other ongoing work in Alameda County, including the current Metropolitan Transportation Commission (MTC) Climate Adaptation Pilot Study, which is conducting a similar level of vulnerability assessments in other areas that have been identified as vulnerable. The following sections provide a description of the Bay Farm Island Focus Area (Section 2), an assessment of exposure to inundation and flooding (Section 3), the verification of topographic features and elevations at several locations (Section 4), identification of key areas of vulnerability (Section 5), recommendations for the timing of adaptation measures (Section 6), potential adaptation measures (Section 7), and conclusions and next steps (Section 8). 1 This memorandum is funded with qualified outer continental shelf oils and gas revenues by the Coastal Impacts Assessment Program, Fish and Wildlife Service, U.S. Department of the Interior. Figure 1. Initial Inundation Maps Showing Inundation of Bay Farm Island with 12 inches of SLR Note: Inundated areas, including the Oakland International Airport, are shaded in blue. B.4-2 2. FOCUS AREA DESCRIPTION Bay Farm Island is a relatively low-lying peninsula that extends from the eastern shoreline of San Francisco Bay. It was historically an island surrounded by several marsh complexes but is now connected to the San Francisco Bay shoreline by artificial land fill and a majority of the marshes have been developed with fill. Located within the City of Alameda and the City of Oakland, key assets include the Oakland International Airport, Oakland Raiders headquarters, the Chuck Corica Golf Complex, and several residential neighborhoods. Bay Farm Island is currently protected from coastal flooding and inundation by several standalone structures and tide gates along the shoreline. In addition, there is a system of levees that protects the shoreline in several areas, most notably around the Oakland International Airport. Placements of rip-rap and revetments protect areas of the shoreline from erosion. The key areas in this assessment include the northern and eastern portions of the Bay Farm Island shoreline along San Francisco Bay and San Leandro Bay, where inundation is first expected to occur during sea level rise and storm surge events. Several sections along this shoreline are known low-lying areas that are already vulnerable to flooding during storm surge events. The section of the shoreline investigated in this focus area is highlighted in Figure 2. Key areas of inundation that needed to be verified during the assessment are presented in Section 4. Figure 2. Focus Area Shoreline B.4-3 3. INUNDATION AND FLOODING EXPOSURE In the discussion that follows, a clear distinction is made between the terms inundation and flooding. Permanent inundation occurs when an area is exposed to regular daily tidal inundation. A permanently inundated area can no longer be used in the same way as an inland area due to the frequency of its exposure to sea water. In contrast, flooding occurs when an area is exposed to episodic, short duration, extreme tide events of greater magnitude than normal tide levels. Inland areas may be temporarily flooded during an extreme tidal event while maintaining at least a portion of their functionality once the floodwaters recede. However, sensitive assets may suffer irreversible damage if exposed to any amount of water, even temporarily. The term flooding, as it is used throughout this memorandum, is therefore a temporary condition that results from a storm event rather than the permanent inundation due to daily high tides. To assess portions of the shoreline that are exposed to inundation and flooding within the Bay Farm Island focus area, six sea level rise inundation mapping scenarios were examined (Table 1). Inundation maps were created for each of the scenarios using the methodology developed by the National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center (Marcy et al. 2011). The scenarios were developed by adding different amounts of sea level rise onto the elevation of the existing conditions daily high tide level (represented by the Mean Higher High Water (MHHW) tide). The MHHW reference water levels used in this analysis were derived from MIKE21 model output from a regional San Francisco Bay modeling study completed as part of the Federal Emergency Management Agency (FEMA) San Francisco Bay Area Coastal Study 2 (DHI 2011). The modeling study spanned a 31-year period from January 1, 1973 to December 31, 2003. The MHHW tidal datum was calculated using the portion of the model output time series corresponding to the most recent National Tidal Datum Epoch (1983 through 2001), which is a specific 19-year period adopted by NOAA to compute tidal datums. In accordance with the most up-to-date sea level rise projections, the following scenarios were evaluated for the present study: 12 inches, 24 inches, 36 inches, and 48 inches of sea level rise above existing MHHW. In addition to these scenarios, 72 inches and 96 inches above MHHW were also evaluated, but these water levels are outside the range of current scientific predictions for sea level rise within this century and, therefore, do not correspond with permanent inundation scenarios that are likely to occur before 2100 (NRC 2012). These scenarios are included to evaluate important extreme flooding scenarios that could happen during storm surge events with lesser amounts of sea level rise. The initial inundation maps for this focus area were developed by AECOM as part of the Alameda County Sea Level Rise Shoreline Vulnerability Assessment for BCDC and ACFCWCD. The maps show the extent of inland inundation, inundation depths, and the depth of “overtopping potential” along the shoreline. Overtopping potential refers to the condition where the water surface elevation associated with a particular reference water level exceeds the elevation of the shoreline asset. The depth of overtopping potential at each shoreline segment is calculated by taking an average of several depths over the length of the segment. Note that the overtopping calculations represent an average of inundation depths over a specified length of the shoreline and do not show depths less than 0.5 feet to account for the limits in the vertical accuracy of the DEM and the sea level rise inundation mapping process. Therefore in some areas on the inundation maps, inundation over a shoreline feature may be shown with no corresponding overtopping depth, if the overtopping depth is 2 www.r9coastal.org B.4-4 less than 0.5 feet. This assessment is considered a planning-level tool only, as it does not account for the physics of wave runup and overtopping. It also does not account for potential vulnerabilities along the shoreline protection infrastructure that could result in complete failure of the flood protection infrastructure through scour, undermining, or breach after the initial overtopping occurs. The mapping scenarios are listed in Table 1. The revised inundation maps created for this assessment are presented in Attachment A. Table 1. Sea Level Rise Inundation Mapping Scenarios Applicable Range for Mapping Scenario Reference Water Level Mapping Scenario (Reference +/- 3 inches) Scenario 1 MHHW + 12 inch MHHW + 9 – 15 inch Scenario 2 MHHW + 24 inch MHHW + 21 – 27 inch Scenario 3 MHHW + 36 inch MHHW + 33 – 39 inch Scenario 4 MHHW + 48 inch MHHW + 45 – 51 inch Scenario 5 MHHW + 72 inch MHHW + 69 – 75 inch Scenario 6 MHHW + 96 inch MHHW + 93 – 99 inch It is important to understand that the reference water levels listed for each mapping scenario can occur due to a variety of hydrodynamic conditions by combining different amounts of sea level rise with either a daily3 or extreme high tide.