06-03-09 Drainage and Flooding Committee minutes

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runoff for drainage areas contributing flow to each storm sewer inlet. Drainage boundaries were developed using LiDAR and verified by site investigation, aerial photography. Runoff coefficients were calculated for residential areas by selecting several sample lots and measuring the percent of impervious cover verses ground cover. These calculations include the area encompassed by the side property lines and from the back property line to the center of roadway. Runoff coefficients were determined for representative areas to be between 0.55 - 0.60, 0.55 is used in the runoff calculations. Time of concentration for each inlet was calculated using the City of La Porte nomagraph. These inlet times were compared to the City of Houston method for consistency. Overland slope comparisons provided lower inlet times and higher flows. Detailed analysis of the existing storm sewer is estimated and should be verified during design using topographic survey and/or as-built record drawings. To verify the capacity of the existing storm sewer, a 3-year storm event was used to calculate runoff similar to historic methods and was determined to be about 180.0 cubic feet per second ((cfs)) for a 106.6- acre area using a runoff coefficient of 0.55. A 5-year storm even generates about 225.0 (( cfs)) for the same area. Peak discharges for rainfall frequencies between the 3-year even up to the 100-year even are included in Table 1. 2.3 Inlet Capacity Analysis The City requires inlets and storm sewers to be designed for a minimum 3-year rainfall frequency. Two City of Houston Type-B or BB curb inlets collect drainage from each residential street. From the City of Houston Design Manual, Type B or BB inlets have a capacity of 5.0 (cfs) each. Accordingly, each street has a capacity to drain 10.0 (cfs) of 2-2 Klotz Associates Project No. 0127.009.000 May 2009 Fairmont Park East & Brookglen Drainage Study City of La Porte