10-13-08 Drainage and Flooding Committee minutes

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k lot z .~. ass 0 cia t e s imperviousness, while the percentage imperviousness IS approximated by the DLU percentage. 3.4 Hydraulic Models 3.4.1 FEMA Hydraulic Models Hydraulic models (specifically HEC-RAS models) for most primary watercourses are available as a result of the TSARP program. These hydraulic models were used to delineate what are now officially designated as FEMA base floodplains along major watercourses, such as most of the watercourses defined as primary watercourses in this CWDS report. These HEC-RAS models are termed for his study as FEMA models and channels for which such FEMA models are available are termed FEMA channels. Table 3-1 tabulates the FEMA channels; these channels are also shown in Exhibit 3-2. The FEMA models are used in the following ways: To determine existing channel capacity in a FEMA channel: This is accomplished by executing the FEMA model and by determining at what points the 1 DO-year storm event rises above the lower of 1) the top of bank or 2) the ground levels in areas immediately beyond the berms forming the banks of the channel. This latter condition may occur when the natural ground is below the bank level due to depressed topography beyond the channel. Top of bank elevations were determined from the HEC-RAS cross section data (which in turn are based upon survey data) while ground levels beyond the channel banks were determined from LiDAR-determined topography. To determine necessary channel widening to increase the channel conveyance to handle the 1 DO-year flood: This is accomplished by a trial and error process which gradually increases the channel width until the required discharge is conveyed within a channel 3-6 Klotz Associates Project No. 0127.008.000 January 2009 La Porte Citywide Drainage Study City of La Porte