02-28-11 Drainage and Flooding Committee minutes

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klotz*associates Date: August 30, 2010 Re- analysis of Drainage Impacts to 8106 -02 -00 and B106 -00 -00 Drainage Improvements for Fairmont Park East City of La Porte, Texas Key Map 579C; Pct 2 HCFCD Project No. 2005922, Unit B106 -02 -00 Klotz Associates Project No. 0127.012.001 Background and Purpose Fairmont Park East (FPE), which includes approximately 110 acres of land, is bordered on its south side by stream B106-02-00 and on its west side by a pipeline right-of-way and Old Hickory Drive (see Exhibit 1). B106 -02 -00 empties into Big Island Slough (B106- 00 -00) at a point approximately 3,500 feet downstream of Fleetwood Drive, the outfall point for runoff from the FPE development. Under pre - project conditions, runoff from the development empties into B106-02-00 through a single outfall pipe. Under post - project conditions, an enlarged outfall pipe and a swale will be added, thereby increasing flows into the stream. The outfall location will be the same between pre - and post- project conditions. The differences between the outfall hydrographs between pre- and post - project conditions were described in our December 2009 report. The purpose of the analysis described below was to determine the impact of the proposed drainage improvements on flows within streams B 106 -02 -00 and B106- 00 -00. This analysis replaces the draft results that were presented during an August 10, 2010 meeting at HCFCD. Analysis The HEC -HMS hydrologic model from TSARP was used for the analysis. TSARP subbasin B106C was divided into seven (7) subareas with FPE being one of the subareas. For subareas C1 thru C6, unit hydrograph parameters TC and R were calculated and input into HEC -HMS to compute runoff hydrographs. For the FPE subarea, hydrographs were taken from the XP- SWMM models described in our December 2009 report. For consistency with the hydrographs from XP -SWMM, and unlike TSARP procedures, storm peak centers in HEC -HMS were set at 50% compared to the typical 67% used in TSARP procedures. Exhibit 2 shows the resulting HEC -HMS schematic. This same schematic applies to both pre- and post - project conditions and both the 10% (10 year) and 1% (100 year) storm events.