Drainage_and_Flooding_Report-dated_03-10-08

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Mr. Ron Bottoms March 10, 2008 Page 2 of 15 k I o t z associates city is drained by storm sewer drainage systems and part of the city is drained by open ditch or roadside ditch drainage systems. Much of the areas identified as prone to drainage or flooding conditions are older areas of the City. These older areas were constructed to older standards and usually without much consideration given to handling overland surface flows also called sheet flows. Newer areas of the City generally have better drainage facilities. The City outfalls into major watersheds including: Armand Bayou, Clear Creek, and Lower San Jacinto/Galveston Bay (see Exhibit 2). The topography of the City is generally flat and average about 24 feet above sea level (see Exhibit 3). Tidal variations can significantly affect drainage in portions of the City. The city has approximately 36.1 miles of Harris County Flood Control (HCFCD) channels, and must comply with applicable drainage requirements established by rules and regulations of the HCFCD for discharges to HCFCD ditches and channels. The City has approximately 3 miles of coast contiguous to Galveston Bay. The United States Army Corp of Engineers (USACE) has authority over navigable waters, which include coastal waters and tidally effected waterways draining to these coastal waters with coordination required with these authorities. Data Collected This primary focus of the study tasks of the CWDS addressed by this letter report is the collection and evaluation of drainage data. The following summarizes the data collection activities. GIS Data The City has also provided Klotz Associates with their GIS data related to drainage issues, including repetitive loss information, storm sewer layout, culvert data, information on severely flood -damaged homes, inlets and manhole locations, and floodplain maps. Floodplains delineated and recognized by the Federal Emergency Management Agency (FEMA) floodplains are shown in Exhibit 4. Repetitive loss data provided by the City is shown in Exhibit 5B. LiDAR (light detection and ranging technology) elevation data are also readily available from HCFCD; the elevation contours developed by Klotz Associates from these data are shown in Exhibit 3. The LiDAR data were also processed using GIS software to delineate drainage areas and sheet flow paths for the 100 flood event for various watershed areas in the City; these drainage areas and flow paths in them are shown in Exhibits 6 to 10. These data assist in understanding and describing overland flow conditions and directions. As evident from Exhibits 6 through 10, the drainage is