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The maximum conveyance was determined for the channel using two separate methods. The first <br />was calculating flow at bankfull elevation using Manning's Equation with a roughness coefficient <br />of 0.035. This method resulted in an approximate maximum conveyance of 155 CFS. However, <br />this does not take into account the downstream control of the 30-inch RCP that conveys the flow <br />from the channel into the Spencer Highway storm sewer system. Using the XPSWMM model of <br />the existing airport drainage system, it was determined that the outfall to Spencer Highway controls <br />the flow down to only approximately 34 CFS when the channel is flowing full. <br />Four study points were chosen to evaluate any overflow that occurs between the airport site and <br />the neighborhood west of Farrington Drive. Figure 6 presents the location of the study points, <br />located at Archway Drive, Crestway Drive, Meadow Place Drive, and Meadow Crest Street. <br />Figure 6 Farrington Drive Study Points <br />The water surface elevation at different storm events was compared to the elevation at Farrington <br />Road, and from that differential the overflow rate was calculated. The existing XPSWMM model <br />was expanded to calculate the overflow rate at each study point. Links were added to simulate <br />runoff leaving the channel and flowing over Farrington Drive. The Links were given a width <br />matching the distance to the upstream study point, so that all overflow along Farrington Drive was <br />accounted for in four stream segments. The following tables present the comparison of road <br />La Porte Municipal Airport — Offsite Drainage Analysis <br />HT&J, LLC, January 2018 Page 11 <br />