03-23-09 Drainage and Flooding Committee meeting minutes

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2.4 Overall, total inlet flow capable of entering the storm sewer is 225 cfs (45 inlets x 5 cfs/inlet = 225 cfs). With a three-year overall runoff of 180 cfs for 106.6 acres (194 cfs for C = 0.60) the total inlet capacity is adequate. Storm Sewer Capacity Analysis The existing storm sewer sizes were field measured along the main trunk line along the east side of Fleetwood Drive and then LiDAR was used to determine elevations of manhole rims and curb inlet tops. Field survey collected for design of overland sheet flow swales was checked against LiDAR over the outfall pipe and it was determined that the LiDAR data is accurate 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. The existing storm sewer sizes were obtained from the City of La Porte and field verified, (See Exhibit 2). On first observation, the outfall pipe is a sixty -inch (60-inch) corrugated metal pipe (CMP) that outfalls at flow line six -inches (6-inches) above the flow line of HCFCD Unit B 106-02-00. Typically, storm sewers are designed with velocity of 6.0 feet per second (fps) and based on this the capacity of a 60-inch CMP with cross sectional area equal to 19.6 sf is 118.0 cfs (6.0 fps x 19.6 sf = 118 cfs). This capacity is inadequate to carry runoff of 180 cfs. This was verified in the Hydroflow model, which calculated the capacity of the existing system to be 116 cfs, (See Appendix A). The design capacity of a storm sewer system is based on the water surface elevation in the receiving drainage channel and is considered the starting water surface elevation for calculating the hydraulic grade line (HGL) elevation along the pipe network. It is common engineering practice to set the starting water surface elevation at the top of the storm sewer system's outfall pipe. The existing storm sewer capacity was analyzed based on the top of pipe for the starting water surface elevation, equal to an elevation of 14.69 1-5 Klotz Associates Project No. 0127.009.000 Fairmont Park East & Brookglen Drainage Study May 2009 City of La Porte