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The development of the inundation depth rasters follows the methodology adopted by the NOAA Coastal Services Center to map sea level rise inundation for the Sea Level Rise and Coastal Flooding Impacts Viewer (NOAA, 2012). The process used to develop the inundation rasters is also known as a modified bathtub or linear superposition method.
Mean Higher High Water (MHHW) values:
The MHHW tide level was selected to represent the typical daily high tide. The MHHW tide level for existing conditions was computed using model hindcast data corresponding to the most recent National Tidal Datum Epoch (1983 through 2001), which is a specific 19-year period adopted by NOAA to perform tidal computations. The MHHW tide level is defined as the average of the higher high tides of each day recorded during the National Tidal Datum Epoch. The FEMA MIKE21 model output water level data was provided in 15-minute time steps, as described in the Regional Coastal Hazard Modeling Study for North and Central San Francisco Bay and the Regional Coastal Hazard Modeling Study for South San Francisco Bay (DHI, 2011, 2013). In the north and central Bay, the water level and wave hindcast period extended from January 1, 1973 to December 31, 2003 (31 years). In the south Bay, the water level and wave hindcast period extended from January 1, 1956 to December 31, 2009 (54 years). Model output was extracted at a series of 20 output points along the Alameda County shoreline. The extraction points were selected to adequately characterize the spatial variability of water levels throughout the study area.
Detailed Mapping Process:
Topographic/bathymetry Digital Elevation Model (DEM) -2m resolution:
Ground elevations were based on a DEM mosaic created from products created for the California Coastal Mapping Program (CCMP). The USGS managed the LiDAR data collection in south San Francisco Bay. The South Bay LiDAR data were collected in June, October, and November 2010. NOAA managed additional LiDAR collection for CCMP in northern San Francisco Bay from February to April 2010, as well as additional LiDAR collection in 2011 to provide enhanced coverage of many coastal shoreline areas. The USGS and NOAA LiDAR data were delivered in point-cloud format at 1-meter point spacing, and complied as a composite 2-m resolution DEM for use in the normalized shoreline task.
MHHW water surface grid - 2m resolution
Process:
Step 1: Add desired amount of SLR to MHHW water surface DEM to create new MHHW + SLR grids.
Step 2: Subtract DEM values from water surface grid to compute initial inundation depth grid
Step 3: Evaluate hydrologic connectivity for each SLR scenario. Create DEM to show inundation extent using Spatial Analyst (Single Output Map Algebra)
Step 4: Evaluate hydrologic connectivity of the inundation extent DEM using Spatial Analyst (Generalization - Region Group) to identify hydrologically connected grid cells.
Step 5: Extract connected inundation surface using Spatial Analyst (Extract by Attributes)
Step 6: Identify low-lying areas greater than one acre using Spatial Analyst (Extraction - Extract by Attributes). This is computed by identifying groups of connected grid cells that are greater than 1,011. 1 grid cell = 43.06 ft^2. 1 acre = 43,560 ft^2. 1 acre = 43,560 ft^2/ 43.06 ft^2= 1,011 grid cells.
Step 7: Create depth grid for connected inundation areas using Spatial Analyst (Extract by Mask)
Step 8: Optimize visualization by showing SLR surfaces clipped to land extents using Spatial Analyst (Reclassify and Extract by Mask). Step 8 was computed for output rasters from Step 6 and Step 7.
Step 9: Create polygon layers from raster data.
Coordinate System: State Plane California III FIPS 0403 Feet
Horizontal Datum: North American Datum 1983
Vertical Datum: North American Vertical Datum of 1988
The development of the inundation depth rasters follows the methodology adopted by the NOAA Coastal Services Center to map sea level rise inundation for the Sea Level Rise and Coastal Flooding Impacts Viewer (NOAA, 2012). The process used to develop the inundation rasters is also known as a modified bathtub or linear superposition method.
Mean Higher High Water (MHHW) values:
The MHHW tide level was selected to represent the typical daily high tide. The MHHW tide level for existing conditions was computed using model hindcast data corresponding to the most recent National Tidal Datum Epoch (1983 through 2001), which is a specific 19-year period adopted by NOAA to perform tidal computations. The MHHW tide level is defined as the average of the higher high tides of each day recorded during the National Tidal Datum Epoch. The FEMA MIKE21 model output water level data was provided in 15-minute time steps, as described in the Regional Coastal Hazard Modeling Study for North and Central San Francisco Bay and the Regional Coastal Hazard Modeling Study for South San Francisco Bay (DHI, 2011, 2013). In the north and central Bay, the water level and wave hindcast period extended from January 1, 1973 to December 31, 2003 (31 years). In the south Bay, the water level and wave hindcast period extended from January 1, 1956 to December 31, 2009 (54 years). Model output was extracted at a series of 20 output points along the Alameda County shoreline. The extraction points were selected to adequately characterize the spatial variability of water levels throughout the study area.
Detailed Mapping Process:
Topographic/bathymetry Digital Elevation Model (DEM) -2m resolution:
Ground elevations were based on a DEM mosaic created from products created for the California Coastal Mapping Program (CCMP). The USGS managed the LiDAR data collection in south San Francisco Bay. The South Bay LiDAR data were collected in June, October, and November 2010. NOAA managed additional LiDAR collection for CCMP in northern San Francisco Bay from February to April 2010, as well as additional LiDAR collection in 2011 to provide enhanced coverage of many coastal shoreline areas. The USGS and NOAA LiDAR data were delivered in point-cloud format at 1-meter point spacing, and complied as a composite 2-m resolution DEM for use in the normalized shoreline task.
MHHW water surface grid - 2m resolution
Process:
Step 1: Add desired amount of SLR to MHHW water surface DEM to create new MHHW + SLR grids.
Step 2: Subtract DEM values from water surface grid to compute initial inundation depth grid
Step 3: Evaluate hydrologic connectivity for each SLR scenario. Create DEM to show inundation extent using Spatial Analyst (Single Output Map Algebra)
Step 4: Evaluate hydrologic connectivity of the inundation extent DEM using Spatial Analyst (Generalization - Region Group) to identify hydrologically connected grid cells.
Step 5: Extract connected inundation surface using Spatial Analyst (Extract by Attributes)
Step 6: Identify low-lying areas greater than one acre using Spatial Analyst (Extraction - Extract by Attributes). This is computed by identifying groups of connected grid cells that are greater than 1,011. 1 grid cell = 43.06 ft^2. 1 acre = 43,560 ft^2. 1 acre = 43,560 ft^2/ 43.06 ft^2= 1,011 grid cells.
Step 7: Create depth grid for connected inundation areas using Spatial Analyst (Extract by Mask)
Step 8: Optimize visualization by showing SLR surfaces clipped to land extents using Spatial Analyst (Reclassify and Extract by Mask). Step 8 was computed for output rasters from Step 6 and Step 7.
Step 9: Create polygon layers from raster data.
Coordinate System: State Plane California III FIPS 0403 Feet
Horizontal Datum: North American Datum 1983
Vertical Datum: North American Vertical Datum of 1988
This flood zone data should not be used for flood insurance purposes, but rather hazard mapping. Flood Insurance Rate Map (FIRM) data retrieved directly from the Federal Emergency Management Agency (FEMA) should be used for insurance programs.From FEMA:The FIRM is the basis for floodplain management, mitigation, and insurance activities for the National Flood Insurance Program (NFIP). Insurance applications include enforcement of the mandatory purchase requirement of the Flood Disaster Protection Act, which '... requires the purchase of flood insurance by property owners who are being assisted by Federal programs or by Federally supervised, regulated or insured agencies or institutions in the acquisition or improvement of land facilities located or to be located in identified areas having special flood hazards,' Section 2 (b) (4) of the Flood Disaster Protection Act of 1973.
In addition to the identification of Special Flood Hazard Areas (SFHAs), the risk zones shown on the FIRMs are the basis for the establishment of premium rates for flood coverage offered through the NFIP. The FIRM Database presents the flood risk information depicted on the FIRM in a digital format suitable for use in electronic mapping applications. The FIRM Database serves to archive the information collected during the Flood Risk Project.The National Flood Hazard Layer (NFHL) data incorporates all FIRM databases published by the FEMA, and any Letters of Map Revision (LOMRs) that have been issued against those databases since their publication date. It is updated on a monthly basis. The FIRM Database is the digital, geospatial version of the flood hazard information shown on the published paper FIRMs.The FIRM Database depicts flood risk information and supporting data used to develop the risk data. The primary risk classifications used are the 1-percent-annual-chance flood event, the 0.2-percent-annual-chance flood event, and areas of minimal flood risk. The FIRM Database is derived from Flood Insurance Studies (FISs), previously published FIRMs, flood hazard analyses performed in support of the FISs and FIRMs, and new mapping data, where available. The FISs and FIRMs are published by FEMA.The NFHL is available as State or US Territory data sets. Each State or Territory data set consists of all FIRM Databases and corresponding LOMRs available on the publication date of the data set.
The specification for the horizontal control of FIRM Databases is consistent with those required for mapping at a scale of 1:12,000. This file is georeferenced to the Earth's surface using the Geographic Coordinate System (GCS) and North American Dataum of 1983.
Date flood hazard areas were last updated:
Alameda County - 2019-08-10