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6.0 DRAINAGE MODIFICATIONS AND EROSION CONTROL <br /> 6.1 Design Storm and Runoff Characteristics <br /> Drainage improvements have been designed on the basis of the 100-year, <br /> 1 -hour, storm event (1 .3 inches of precipitation) . The temporal <br /> distribution of the rainfall was adjusted so that the majority of the <br /> precipitation would occur within the third 15-minute interval . <br /> Peak flows for the School Section and Johnson Pits were determined <br /> I using the Kinematic Wave Analysis of the Corps of Engineers HEC 1 computer <br /> program. Input included the rainfall distribution indicated) above, <br /> representative flow lengths and slopes and runoff curve numbers determined <br /> using soil maps of Moffat County obtained from the Soil Conservation <br /> Service. Curve numbers used in the analysis ranged from 86 to 91. <br /> The peak discharge for drainage improvements was calculated using the <br /> HEC 1 for the School Section Pit. Major basins and basin areas are shown <br /> on Fig 6. 1. Figure 6.3 shows a few of the subbasins that will remain <br /> after the larger basins are diverted. Figure 6.2 shows photographs of the <br /> areas of some of the proposed channels. Several of the curve numbers and <br /> input variables used in the Chen Report were reviewed, field verified, and <br /> adjusted as needed before running HEC 1 . The resulting correlations <br /> between the Chen runs and those prepared for this report were extremely <br /> poor. The HEC 1 version used by Chen was one of the original versions <br /> adopted for use on a personal computer. Since that time several revised <br /> versions of HEC 1 have been produced. <br /> t -7- <br />