06-03-09 Drainage and Flooding Committee minutes

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runoff based on inlet capacity alone. Eleven (11) streets intersect Fleetwood Drive with forty-four (44) inlets for all residential streets and one (1) inlet on Fleetwood Drive, total number of inlets is forty-five (45). Three-year runoff for most of the contributing areas is about 5.0 (cfs) with the highest equal to 8.1 (cfs) and lowest equal to 2.2 (cfs). Overall, total inlet flow capable entering the storm sewer can be estimated to be 225.0 (cfs) (45 inlets x 5 (cfs)/inlet = 225.0 (cfs)). With a 3-year overall runoff of 180.0 (cfs) for 106.6 acres (194 (cfs) for C = 0.60) the total inlet capacity is adequate. 2.4 Storm Sewer Capacity Analysis Existing storm sewer sizes were field measured along the main trunk line on the east side of Fleetwood Drive; using LiDAR manhole rims and curb inlet tops were combined with field measurement to determine elevations. Field survey collected for design of overland sheet flow swales was checked against LiDAR at the outfall pipe to determine accuracy of the LiDAR data to +/-0.30 feet. From this, a storm sewer model was constructed in Hydraflow Storm Sewer Extension of AutoCAD Civil 3D 2008. This software was chosen to easily evaluate different alternatives. Existing storm sewer sizes were obtained from the City of La Porte and field verified, (See Exhibit 2). The outfall pipe is a sixty-inch (60-inch) corrugated metal pipe (CMP) with invert elevation six-inches (6-inches) above HCFCD Unit B106-02-00 channel flow line. Typically, storm sewers are designed with velocities of 6.0 feet per second (fps), based on this, the capacity of a 60-inch CMP has a cross sectional area of 19.6 (sf) capable of producing 118.0 (cfs) (6.0 fps x 19.6 (sf) = 118 (cfs)). This is inadequate to carry the peak runoff for Fairmont Park East at 180.0 (cfs) for the 3-year event. Verification of these results were performed in a Hydroflow model which calculated an existing capacity of 116.0 (cfs), (See Appendix A). 2-3 Klotz Associates Project No. 0127.009.000 May 2009 Fairmont Park East & Brookglen Drainage Study City of La Porte