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SECTION 3 Flood Control Euaination <br />Table 3 -3 <br />Rainfall Data Used for the Model Validation <br />Date <br />Daily Rainfall Total (inches) <br />Validation Conducted for <br />Day? (Y/N) <br />December 27, 2005 <br />1.25 <br />N <br />December 28, 2005 <br />0.88 <br />Y <br />December 29, 2005 <br />0.17 <br />N <br />December 30, 2005 <br />2.56 <br />Y <br />December 31, 2005 <br />0.94 <br />Y <br />January 1, 2006 <br />0.16 <br />N <br />January 2, 2006 <br />0.37 <br />N <br />January 3, 2006 <br />0.12 <br />N <br />Total: <br />6.45 <br />Several combinations of model adjustments were evaluated to obtain the best match to observed <br />conditions (i.e., to reduce differences between simulated and observed freeboard). The model <br />adjustments that were evaluated during the model validation process are shown in Table 3 -4. The <br />best results (i.e., closest to observed data) were obtained when the model input parameters were <br />adjusted to reflect the use of effective impervious percentage areas instead of mapped <br />impervious percentage areas (i.e., lowering the impervious percentages). Effective impervious <br />percentage areas were estimated based on Roger Sutherland's paper (provided as Appendix C) <br />titled Methodology for Estimating the Effective Impervious Area of Urban Watersheds (1995). <br />His method includes five different equations for estimating the effective impervious area from <br />mapped impervious area. The five equations are based on how directly connected the mapped <br />impervious areas are to the piped storm drainage system. Equation #1 (below) was used to <br />calculate effective impervious area for the purpose of conducting this model validation. <br />Average subbasins which are predominately sewered with curbs and gutters, have no <br />infiltration facilities, and the residential rooftops are not directly connected to the <br />drainage system: <br />Effective Imp. % 0.1 * (Mapped Imp. %) ­ <br />Although the use of equation #1 (for average connected basins) and associated revised <br />impervious surface estimates resulted in somewhat better model results (i.e., closest to observed <br />data), the drainage area upstream of the calibration site was determined to be all curb and gutter, <br />and the impervious area was estimated to be mostly connected. As the calibration data came <br />from only one point in the basin, and as it was observed as opposed to measured, it was decided <br />that it would be better to use the more realistic input parameters and err on the conservative side <br />(i.e., model - simulated flows higher than observed flows). The entire River Road Santa Clara <br />basin varies with respect to whether streets have curb and gutter, but without better calibration <br />data the variations were not taken into account with the equation used to calculate effective <br />impervious surface and thus accounted for in the model. In other words, one consistent equation <br />was used to convert mapped impervious areas to effective impervious areas for the entire basin. <br />0: 25695978 Eugene RR -SC Final Basin P1an\Nlaster P1an\FINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 3-7 <br />