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IML <br />S = dimensionless slope of terrain <br />Tch = channel flow time (min) = KLo.770S-0.185 <br />K= 0.0078 <br />L=channel flow length (feet) <br />S=dimensionless main channel slope <br />The Reservoir Storage Coefficient (R) is necessary to develop the runoff hydrograph <br />from the sub -basins in a Clark Hydrograph method. The R value is adjusted to <br />calibrate the XPSWMM model against the Rational Method, and the peak flow at the <br />hydrograph is matched to the peak flow estimated from rational method. <br />Supplemental Table A presents the watershed data for each drainage area. <br />Hydraulic Analysis <br />An XPSWMM model was built of the existing drainage conditions on the La Porte <br />Municipal Airport site. All culverts and natural channels were modeled, with all flow <br />exiting the system via the storm sewer located in Spencer Highway. Figure 3 <br />displays the layout of the XPSWMM model. As -built drawings dated 03/01/1994 were <br />used to model the Spencer Highway storm sewer line, which outfalls into the Big <br />Island Slough channel to the west of the site. <br />Page 119 <br />