Drainage and Flooding Report January 2009

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iotz 11.41 associates would be either at one of the two sites, or, possibly smaller ponds would be used at each of the optional sites. Detailed engineering analyses would be required to identify the optimal number, configuration and sizes of individual ponds. In all situations, the pond configurations, locations, and sizes shown are only approximate. In special situations, mitigation may be avoided if it can be demonstrated that adverse impacts or are absent or of very little consequence. An example of this possibility is provided by the improvements for Ditch F216-01-00, for which three alternatives have proposed (Projects 26, 28, and 30 in Table ES-1). Project Options 26 and 28 include a mitigation pond; however; the channel improvement extends to the confluence of F216-01-00 and F216-00=00, which lies quite close to Galveston Bay. Thus locations downstream of the channel improvements' potential impacts are quite limited in extent, and because of the relatively large flows that occur in F216- 00-00, the increases in flow due to improvements along F216-01-00 may be so small, in a relative sense, that they are inconsequential. Hence, mitigation of the channel improvement may be unnecessary. Detailed engineering analysis would be required to confirm this speculation. It is recognized that using channel lining to improve channel conveyance capacity is not desirable from an environmental or permitting perspective. On the other hand, widening of the channel with 3:1 or flatter side slopes will often result in a channel width that will significantly impact adjacent properties, including in some instances actual residences and consequent requirements for possible buy-out of affected residences. Therefore, the option of off -channel diversion detention storage is sometimes considered. Table ES-1 identifies proposed detention ponds for diversion use while Figures ES=1 through ES=6 show approximate locations of the diversion ponds. Pond configurations, locations, and sizes shown are only approximate. When optional locations are available for a mitigation pond, the optional locations are shown. EN Klotz Associates Project No. 0127.008,000 La Forte Citywide [drainage Study January 2009 City of La Porte