10-13-08 Drainage and Flooding Committee minutes

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k lot z 4~. ass 0 cia t e s by a surface relief swale or a combination relief swale and stonn outfall pipe improvement, with typical width of 8 feet and depth of 3 feet. Detailed drainage analysis should be done before the construction of sheet flow paths and land acquisition is started. 4.3 Identified Types of Remedies for Future Conditions Future conditions are characterized by more or less maximum development of land; details for evaluating runoff from such lands are described in Section 3. The present discussion focuses upon the rationale for selection of potential remedies for drainage and flooding when future development increases the runoff from various areas draining to either FEMA-modeled channels or non-FEMA-modeled channels. 4.3.1 Assumed Detention Strategies for Development Future development in a watershed will, generally, increase imperviousness and consequent runoff beyond that which currently exists. The remedies for existing conditions (discussed above) are intended to resolve current drainage problems under the broad-based assumption -that significant increases in runoff due to development do not occur. Increases in runoff due to development, on the other hand, can be dealt with in two ways: on-site mitigation or regional mitigation. On-site mitigation is control of runoff from a development site in such a way that the peak discharge does not increase above the level that existed before the development of the site. City drainage criteria require that such control is achieved as part of future development. The on-site mitigation is typically accomplished using on-site surface detention, but other methods such as subsurface detention and low-impact development teclmiques can be used. The key factors in use of on-site mitigation are 1) that the mitigation is accomplished before runoff leaves the site so that peak discharges in 4-9 Klotz Associates Project No. 0127.008.000 January 2009 La Porte Citywide Drainage Study City of La Porte