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<br />Several factors effect the peak discharge from a <br />basin. Basin shape, area, and slope; percentage of <br />impervious area within the basin; and the soil type all <br />have an effect on the hydrograph produced by the basin. <br />As such, sub-basin parameters required as input to HEC- <br />1 were rainfall. subbasin area. percent imperviousness, <br />the SCS curve number, and Snyder's tp and Qp uni t <br />hydrograph coefficients. Where channel routing was <br />included in the model, typical channel cross sections <br />for each routing reach and Manning's Un" values were <br />required as input. <br /> <br />Subbasin delineation and areas were determined <br />using topographic information from USGS 7.5 minute <br />quadrangle maps (reference 4). Snyder's tp and Qp unit <br />hydrograph coefficients were calculated based on the <br />Colorado Urban Hydrograph Procedure criteria (reference <br />5). Percent imperviousness was taken to be two percent <br />for all sub-basins which did not contain portions of <br />Otis proper, based on inspections of the "Washington <br />County Soil Survey" maps. Percent imperviousness for <br />those basins containing portions of Otis proper were <br />calculated based on insepction of aerial photography. <br /> <br />Generally, soils within the basins above Otis <br />belong to SCS hydrologic soil group B, with smaller <br />amounts belonging to group C, and very minor amounts <br />belonging to groups A and D. Area-weighted curve <br />numbers based on soil type and. land use were developed <br />for each sub-basin by using the methodology described <br />in Chapters 7 and 9 of Section 4 of the SCS National <br />Engineering Handbook (reference 6). For those areas of <br />dryland farming where fallow techniques were used, an <br />average curve number for fallow and average cover <br />conditions for small grains was used. <br /> <br />" <br />, <br />;, <br /> <br />Because both the northwest and southwest basins <br />tend to be somewhat long in relation to their width, <br />the modified PuIs channel routing routine available in <br />HEC-1 was used to route flows through various reaches <br />of both the northwest and southwest basins. The <br />modified PuIs method is a routing technique used for <br />accounting for the effects of transient channel storage <br />on peak discharge and time-to-peak. Typical cross- <br /> <br /> <br />10 <br />