10-13-08 Drainage and Flooding Committee minutes

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k lot z 4~~ ass 0 cia t e s In developing channel capacity improvements, channel widening is gIVen preference over detention since widening is typically less expensive than other improvement options. However, in some instances, diversion to a detention pond may become the preferred choice or part of a preferred choice because of either its relative cost, the potential use of the detention site to serve multiple uses, or other special characteristics of the project. Thus, for example, two alternatives (Projects 23 and 25) are proposed for Ditch BI06-00-00 (also know as Big Island Slough); see Figures ES-2 and ES-3. Project 25 proposes a concrete lined channel, mitigation pond, and an approximately 207 acre-foot diversion pond. Project 25 requires diversion detention in order to limit channel lining to between Spencer Highway and the confluenc-e ofB106-00-00 and BI06-02-00. . Estimated construction-costs for this project are $11.7 million (see Table ES-I). Project 23 proposes a larger diversion pond, with 228 acre-feet of storage, in approximately the same location as that for Project 25 but no channel improvements. The cost of Project 23 is only $8.3 million. The larger pond size of Project 23 achieves the same net result as the combined channel improvement and smaller pond of Project 25. Between these two alternatives, the alternative without channel improvement is the preferred alternative; the Project 23 choice, which does not include the channel lining, is clear because the diversion ponds of both projects are located at the same approximate location. ,- For flooding along Ditch BI12-00-00 (Willow Springs Bayou) in the western "panhandle" of the City, two alternative solutions have been identified. Flooding in this panhandle segment of B112-00-00 is due not primarily to over-bank flooding, but, rather, due in large measure to flooding of low lying areas beyond the channel banks even before the water levels rise to the top of channel. As illustrated by the representative channel section view in Figure ES-7, some areas beyond the channel bank are low relative to top of the channel. Rising flood waters in the channel can move out the channel through low points in the channel bank or by backup into sewers into these low lying areas. The widening necessary to keep channel flows low enough to prevent flow from the channel moving into low lying areas is large (some 175 feet if 30-foot channel maintenance berms are included; see Project 14 in Table ES-1). Therefore diversion of EN-9 Klotz Associates Project No. 0127.008.000 January 2009 La Porte Citywide Drainage Study City of La Porte