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11/10/2008 Grace Le, Redwood City Redwood Shores Levee Proposal October 7, 2008 Page 3 ~~~-~ ... 1.5 Conduct Preliminary Engineering Assessment ConductCoasffilAnalvses This task will include obtaining wind and water level information and performing wind- wave analysis. Locally generated wind-waves that approach the site will be estimated, and used to estimate the run-up for different wave heights and water levels. Specific tasks include the following. . Estimate water levels for different return period events (for exam pie 1 OO-y r water levels) based on available data for the region. . Obtain wind data representative of the site and conduct statistical analy sis to estimate different return period wind events (for example 25-,50-, and 100-yr wind). . Estimate wind wave characteristics for different return period events based on wave generation theory. The most recent bathymetry for SF Bay will be used in the analysis. . Conduct wave run-up analyses to estimate water levels associated with various wind wave events for different areas along the shoreline. Wind wave and run-up analysis will be used to determine the run-up levels for the surveyed cross- sections. . Estimate the amount of overtopping for different shoreline alignm ent segments. for different combinations of water levels and waves; Identifv Deficiencies We understand that the re may be 2 approaches to identify required levee crest elevations, based on the relative significance of water levels and waves. The standard 3 feet freeboard above 100-year return period water level. which is FEMA's standard for riverine levee certification may be applicable for certain segments of the levee, and the 1% annual chance flood event which is FEMA's standard for coastal dikes may be applicable for other segments. For this task. we will conduct a statistical coastal engineering analyses to estimate the elevation of the 1% annual chance flood event and associated wave run-up for the 7 levee segments identified by the City at a minimum. The method of estimating the 1 % annual chance flood event will be the draft guidelines that have been issued by FEMA, Region IX yet. For the segments along Belmont and Steinberger Sloughs, there is little to no wave action, and we do not expect am biguity there in determ ining which method is more appropriate. For the segments exposed to wave action from the Bay (along the SBSA and Area H levees) we will estimate wind waves for different levels of storm activity and compute the 1% flood event based 0 n a combination of tidal water levels and magnitude of storms. Preoare Mitiaation Concepts If the analysis determines that elevation deficiencies in the levee system exist, we will develop 1 to 2 schemes to raise the levee crest. The concept level schemes will not M:I2007 Proposals\07401 Redwood City Levee CertIGLe-10-07-08.doc 14908 MUFF #304