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L = overland flow length (feet) <br />N = dimensionless retardance coefficient = 0.15 <br />S = dimensionless slope of terrain <br />Tch = channel flow time (min) = KLo.770S-0.385 <br />I:BI�IZIyE.: <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 from the sub - <br />basins in a Clark Hydrograph method. The R value is adjusted to calibrate the XPSWMM model <br />against the Rational Method, and the peak flow at the hydrograph is matched to the peak now <br />estimated from rational method. <br />Hydraulic Analysis _ La Porte Airport Property <br />An XPSWMM model used to evaluate existing conditions of La Porte Municipal Airport was <br />modified to analyze the two offsite areas separately. <br />Figure 3-XPSWMMModel Layout <br />For the 5-year and 10-year storm events, the tailwater was modeled with the water surface at the <br />top of the outfall pipe (10 x 6 feet box outfall) into the Big Island Slough. For storm events larger <br />La Porte Municipal Airport — Offsite Drainage Analysis <br />HT&J, LLC, January 2018 Page 8 <br />