10-13-08 Drainage and Flooding Committee minutes

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k lot z 4~. ass 0 cia t e s 3.3.3 Correlation Models For drainage areas not covered by nor included in the available HEC-HMS models, a correlation model was developed. Data from the HEC-HMS models were used to compute the unit runoff (peak cfs per acre of drainage area) as a function of channel L and level of development DLU; see Figure 3-1, 3-2, and 3-2 for the 10-, 50-, and 100- year event correlations developed, respectively. These correlations, with a different correlation for different storm event frequencies (e.g., 10-, 50-, and lOa-year storm events), were used to compute peak discharges for areas which did not have a HEC-HMS model. Discharges at frequencies other than 10-,50-, or 100-year levels were computed by interpolation. 3.3.4 Hydrograph for Correlation Models The complete hydro graph for a drainage area with peak discharge described by a correlation model was approximated by the Malcom hydro graph used in the HCFCD small watershed method. The discharges defining the Malcom hydro graph are determined by three paranleters: time to peak Tp, total volume of runoff V, and peak discharge Qp. Only two of these three parameters are independent, the three parameters being defined by the following equation: Qp = V/(1.39 Tp) In application of this method for the present study, Qp is determined by the peak discharge correlation (discussed above) and the volume of runoff. The volume of runoff is determined from the intensity-duration-frequency behavior for a lOa-year 24-hour duration rainstorm event and an imperviousness determined by the land development parameter DLU. The fraction of rainfall which becomes runoff is set equal to the 3-5 Klotz Associates Project No. 0127.008.000 January 2009 La Porte Citywide Drainage Study City of La Porte