10-13-08 Drainage and Flooding Committee minutes

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k lot z 4~~ ass 0 cia t e 5 into the low lying areas, as illustrated in Figure ES-7 for Ditch B112-00-00. Out-of-bank or adversely high in-channel flooding is evaluated for the lOO-year flood frequency. A lOO-year design frequency criterion is used in the City as well as by other municipalities and drainage districts throughout most of the Houston area for assessing flooding impacts for severe storm events. Channel improvements are accomplished by one of following: widening of the channel, lining the channel with concrete to reduce flow resistance, or a combination of these two methods. Table ES-l lists the proposed widening projects while Figures ES-l through ES-6 show where channel widening is proposed. Widening is accomplished by generally excavating the soil in mid and lower portions of the channel to make full use of the channel right-of.;.way as approximately defined by the existing approximate top width of the channel. Slopes for non lined channels are set to a maximum of 3:1 (i.e., 3 horizontal to 1 vertical) as defined by City design criteria. In many locations, current land use will preclude significant channel widening and improvement in the channel capacity will require lining of the channel, as currently exists in many of the major ditches and channels in the City. The data of Table ES-l indicates which channels are recommended for actual widening and those chalmels which, because of limited space for channel widening, would be recommended for concrete lining. In some instances, large drainage pipes might be used as an alternative to channel widening (e.g., see Project 28 in Table ES-l). Increase in downstream flows is an adverse impact from channel improvements; this impact can be mitigated with detention ponds (i.e., "mitigation ponds") specifically designed to capture the flow increase and temporarily detain the increase until it can be released without adverse impact. All channel widening or lining projects include as part ofthe project a detention pond to mitigate (hence the name "mitigation pond") the adverse downstream impacts arising from the enhanced discharge capabilities of the improved channel. Potential sites for the mitigation detention ponds are shown in Figures ES-l through ES-6. In some cases, optional locations for a mitigation pond for one particular channel improvement are shown; in such cases, the actual mitigation pond EN-7 Klotz Associates Project No. 0127.008.000 January 2009 La Porte Citywide Drainage Study City of La Porte