Drainage and Flooding Report January 2009

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k I o t zi FI lassociates A City Wide Drainage Study (CWDS) for the City of La Porte (City) was undertaken to identify and describe existing and future drainage and flooding problems across the City and devise solutions for the identified problems. Sources of drainage and flooding problems in the City are several and vary with location in the City. Some channels and major drainage ditches have insufficient capacity for conveyance of the runoff from severe storm events. Sometimes low lying developed areas adjacent to but beyond the boundaries of a ditch or channel can become flooded even if the channel itself is not full because water levels in the channel are above the level of the adjacent low lying areas. Poinding in low lying areas is sometimes caused by lack of surface pathways to rapidly drain away storm waters which are not captured and carried away by storm sewers. Storm sewers draining to a channel or ditch may not always have sufficient capacity to prevent collection of waters in streets and adjacent properties. Storm sewer system capacity can be limited by pipe size, insufficient numbers of inlets, debris -blocked inlets, or backup of water in the storm sewers due to high water levels in receiving channels. Storm drainage problems in the City are also in part due to natural effects common to coastal areas: Ground slopes are relatively small, making it difficult to rapidly drain away storm runoff waters. Tidal effects worsen drainage conditions, with storm tidal rises or storm surges causing rising water levels in the channels and bayous near the coast and limiting how well water can drain. The Texas coastal area is also subject to frequent but severe storm events with large amounts of rain falling in short periods of time, often overpowering drainage systems. This planning report presents the result of the CWDS and provides recommendations for improved drainage infrastructure to reduce flooding and its impacts in the City. Three letter reports have been previously developed as part of the work leading to this CWDS report. Letter Report No. 1, dated March 10, 2008 described the City's existing drainage infrastructure and general data collection activities. The City lies in three major watersheds: Armand Bayou, Clear Creek, and San Jacinto/Galveston Bay. Assembled data and previous reports by others (see Appendix A in the main report following this summary) were used to provide a preliminary EN - 1 Klotz Associates Project No, 0127.008.000 La Porte Citywide Drainage Study Januaty''009 City of La Porte