10-13-08 Drainage and Flooding Committee minutes

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k lot z .~. ass 0 cia t e 5 4.2.2 Mitigation of Channel Improvements Channel improvements, such as widening, will usually result in increases in the peak discharge that occurs downstream during the design flood condition because the floodwaters move more effectively in the improved channel. If these increases are significant, an adverse impact to the downstream regions can result. To prevent such adverse impacts, mitigation of increases is required. In addition, if these increases are seen at downstream points outside the City, controversy over the proposed improvements might result. Consequently, construction of mitigation ponds associated with channel improvements should be expected. For planning purposes, mitigation ponds would be expected to be located in the approximate vicinity of where the channel improvements occur. However, the mitigation volume would not necessary have to be provided in a single pond, but could be divided among several smaller ponds. Underground detention might in part be used to provide some of the required mitigation (though typically, underground detention is more expensive than surface detention unless land acquisition costs for surface detention are quite high and the surface above the underground detention can be used for high-valued purposes; such alternatives could be examined in detailed design). For planning purposes, the required mitigation volume is estimated as the volume of excavation needed to construct the channel capacity improvement. Construction of mitigation will introduce additional cost for channel capacity improvement. Locations where mitigation ponds (or detention or diversion ponds might be located) are discussed below in regard to regional detention issues. 4-6 Klotz Associates Project No. 0127.008.000 January 2009 La Porte Citywide Drainage Study City of La Porte