Drainage_and_Flooding_Report-dated_10_03_2008

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Mr. Ron Bottoms, City Manager October 1, 2008 Page 4 of 31 o t z associates Encroachment into floodways cannot occur unless it can be certified by a professional engineer that such encroachment shall not result in increase in base flood levels. The following are criteria and standards considered particular pertinent to the CWDS; potential changes or modifications are recommended to the City for inclusion into its storm drainage design criteria and standards for new development and redevelopment. • The design frequency for storm sewer design, drainage ditch, and drainage channels • Sheet flow provision • Construction in special flood hazard areas • Bridge clearances • Capacity of storm sewer systems • Capacity of open ditch systems • Allowable depths of ponding in streets and adjacent properties • Amount of minimal detention • Regional detention as alternative mitigation • Types of mitigation • Encroachment certification These issues are addressed in the following subsections; specific recommendations are made when appropriate. Recommendations are made for the purpose of application to new development or redevelopment; application to existing developments and facilities is not intended. Criteria or standards which might be considered for existing development requires case -by -case evaluation and cannot typically be imposed, required, or achieved once development has already occurred. The City criteria currently require that storm sewer facilities convey the 3-year storm. Ditches and culvert and other crossing structures for drainage areas more than 100 acres are to be designed for the 25-year event. These levels of storm sewer design capacity are consistent or even slightly more stringent than some other communities, including the City of Houston. However, since the City does experience continuing flooding and drainage problems with small storm events, consideration should be given to the possibility of generally using or using in selected situations a 5-year frequency storm event as the basis for sewer design.