Drainage_and_Flooding_Report-dated_10_03_2008

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Mr. Ron Bottoms, City Manager k I o t z I 'kF, I a s s o c i a t e s October 1, 2008 Page 10 of 31 communities in the Houston metropolitan area tend to be in the range 0.4 to 0.8 ac-ft/ac. New developments should use such storage factors. Engineering review of specific conditions in the City and possible adjustment of these factors need to be considered. For mitigation by detention, the City specifies storage coefficients which depend upon developed area for areas less than 10 acres (as noted in section 2.1 above). For larger areas, HCFCD criteria (10 to 100 acres) or review by the City Engineer (greater than 100 acres) are applied. We would recommend that regional detention alternatives be allowed (but not necessarily required) where conditions would make regional detention feasible and effective, for which the regional detention facility would have to provide for the proposed development area in question the following information: • The allocated storage in the regional detention facility for the developed area would at a minimum have to meet the storage criteria as currently defined in City criteria for on -site detention basins. • Conveyance of storm waters from the developed area to the regional detention must be shown to the satisfaction of the City Engineer that such conveyance could be constructed and will be maintained by the development owner (or other identified party acceptable to the City) and that the conveyance would not adversely affect existing flood levels. Common mitigation techniques are detention ponds, over -sizing of sewers, and, less frequent but of increasing interest, subsurface detention systems. While such detention storage measures are a heavily relied upon mitigation techniques, the City should give consideration to the use of what are termed low impact development (LID) techniques, which are on -site detention devices or systems which limit runoff at or very near the point where runoff begins. Bio-retention, swales, rain barrels, special types of pavement and surfacing, and similar techniques can be used to promote infiltration and lessen direct runoff. ft is recommended that low impact development techniques be allowed to be used to the satisfaction of the City Engineer that a proposed technique or collection of techniques will, with frugh likelihood, not only achieve mitigation which equals or exceeds that required for mitigation if a detention pond were to be used but also will, with high likelihood, function as intended in light of required maintenance for such techniques. The demonstration must recognize the potential reduction in