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FEMA Leverages NOAA Water Level Data for CCAMP OPC Study Along California Coast
Nicole Metzger, Oceanographer and Justin Vandever, Coastal Engineer, BakerAECOM
Since 1854, the San Francisco tide station has recorded the rise and fall of the tides along the California coast. This station, and many others in California, continue to play an important role in ensuring safe navigation, understanding coastal oceanography, monitoring large-scale atmospheric-oceanic phenomena, measuring tsunami events, and tracking long-term sea level change. Water levels recorded at coastal tide stations represent the combined effects of mean sea level, astronomical tide fluctuations, storm surge, and other influences such as El Niño effects. As part of FEMA’s ongoing efforts to study coastal flood hazards along the California coastline, the agency has partnered with the National Oceanic and Atmospheric Administration (NOAA) to obtain comprehensive water level records from 15 long-term tide stations and many other short term stations within the study area. In collaboration with FEMA’s study contractor, BakerAECOM, the study team conducted a suite of water level analyses in support of the CCAMP OPC study, including determination of extreme stillwater elevations (SWEL) for mapping purposes and reconstruction of a 50-year record of hourly stillwater levels (SWL) for use in a response-based analysis of total water levels (TWL) at the shoreline, which includes calculation of wave setup and runup.
 
The first component of the onshore analyses of coastal processes is the determination of prescribed statistical water levels, such as the 1% and 0.2% annual chance SWELs. Currently, coastal studies that establish return period relationships for SWELs along the coast and inland bays use a technique referred to as the regional L-Moment method.  This statistical analysis tool combines data from multiple tide station records to develop estimates of extreme SWELs. The approach taken for the analysis was to first extract annual maximum water level values from observed tide station records. Using these data, multiple frequency distributions were tested to obtain the best fit to the data, and results were then compared to those of for adjacent tide stations along the coast and to group stations into “homogeneous regions.” Data from all tide stations within a homogenous region were then analyzed in a combined statistical analysis so that major flood events that were only experienced or recorded at some sites in the region would have an influence on the regional frequency curve. By combining data within a region, more robust estimates of extreme tide levels can be made than by independent site-specific analysis alone. Estimates of the following return period SWELs were made at each of the 15 long-term tide stations in the study area: 50-, 20-, 10-, 4-, 2-, 1-, and 0.2-percent annual chance. These estimates of extreme SWELs will then be applied spatially along the coast to specific shoreline reaches for mapping onto Flood Insurance Rate Maps (FIRM).
 
The second component of the onshore analyses of coastal processes is the reconstruction of a 50-year hourly SWL record for use in the response-based analysis of TWLs at the shoreline. The hourly SWL values will be combined with the 50-year hindcast of nearshore wave conditions to estimate a complete 50-year TWL hindcast (from January 1, 1960 to December 31, 2009) at each wave analysis transect within the study area. The entire 50-year time series of wave, tide, and shoreline interactions is then statistically analyzed to determine the 1-percent annual chance TWL to establish Base Flood Elevations (BFE) for FIRM mapping. Since the majority of the tide station records along the California coast do not continuously span the 50-year hindcast period, a methodology was developed to fill temporal gaps in the tide station records based on comparisons with data records from adjacent stations.
 
The analysis was conducted by first deconstructing the observed water level records into their astronomical (“predicted”) and residual (“storm surge”) components. Correlations were then developed among all tide stations such that the missing residual component at one station could be estimated using the observed residual from a nearby station. The estimated residual at the station of interest was then combined with the predicted tide to provide the best estimate of the actual water level at the time of the data gap. Data gaps were filled at each of the long-term tide stations within the study area to provide comprehensive records of SWLs for use in subsequent analyses. The complete reconstructed hourly 50-year SWL records at each tide station were assigned to specific shoreline reaches for input to the TWL analysis.
 
Water level records provide the base input to any coastal flood study and the CCAMP OPC Study benefited greatly from FEMA’s partnership with NOAA to fulfill these two crucial analysis tasks. In addition to supporting the coastal flood hazard mapping as part of the current RiskMAP program, these data products will benefit other groups and agencies conducting work along the California coastline for years to come.
 

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