Drainage and Flooding Report January 2009

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k10tz associates i �■ A City Wide Drainage Study (C S) for the City of La Porte (City) was undertaken to identify, develop and recommend drainage improvements to address drainage problems and lessen flooding and its impacts across the City. Reasons for existing drainage and flooding problems include 1) insufficient flow capacity in ditches and channels, 2) ponding of waters in streets and adjacent properties; 3) undersized storm sewers, 4) temporary blockage of storm water inlets by debris, 5) backup of storm waters in sewers, and 6) lack of overland or sheet flow paths. Also contributing to the drainage problems are natural effects common to coastal areas: Relatively small ground slopes making it difficult to rapidly drain away runoff waters; tides and storm surges causing rising water levels which impede drainage; and frequent but severe storm events with large amounts of rain falling in short periods of time. Future drainage problems can, on the other hand, result if the runoff from future land development is not controlled. Flooding is a fact of life in coastal areas and control of flooding in coastal areas presents significant challenges. The strategy used to address drainage and flooding issues had two components: 1) remedy of current drainage and flooding problems; and 2) mitigation of future drainage problems. The bases of our recommendations are summarized in the Engineering Summary following this Executive Summary. Details of the engineering analyses leading to the recommendations are provided in the main body of this report. To address the current drainage and flooding problems, 1) channel improvements, 2) detention ponds for flood flow diversion, 3) storm sewer upgrades, and 4) development of relief swales (Le., directed sheet flow pathways) options are evaluated. Improvements proposed in prior studies by others were incorporated into the proposed solutions of this study when appropriate. Relief swales are a very cost effective (i.e., low cost/benefit ratio) drainage improvement. Relief swales reduce or limit ponding of runoff waters in streets and low lying areas for small to ....._... _.. ...... ....... __.._. ES — 1 Klotz Associates Project No. 0127.008.000 La Porte Citywide Drainage study January 2009 City of La Porte