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Ray Lazuk <br />March 19, 2014 <br />Page 5 <br />watershed boundaries and dimensional parameters for the drainage basins were generated <br />from USGS 7.5- minute quadrangle maps along with information and site understanding <br />developed during the field reconnaissance phase of this project. The key watershed <br />parameters are shown in Table 1 below. <br />3.2 Basin Soils and Infiltration <br />The HEC -HMS model uses the SCS Curve Number method to approximate runoff during storm <br />events. This is an empirical method, based on an extensive database of soils maps and site- <br />specific rainfall -runoff studies, which uses cumulative precipitation, soil cover, and land use to <br />estimate precipitation excess from a storm event. <br />A composite Curve Number was calculated for each drainage basin using soil data from the <br />Holy Cross Soil Survey (USDA, 2003). The Curve Numbers used in the model range from 72 to <br />77. The average Curve Number was 74, which corresponds to Hydrologic Soil Group C with a <br />woods -grass combination cover type (USDA, 2004). Much of the soils at the site are consistent <br />with Hydrologic Soil Group B, which has a lower runoff potential than Hydrologic Soil Group C. <br />However, the Curve Numbers used are only slightly conservative, because much of the <br />drainage area at the site is above timberline with shallow or exposed bedrock and high runoff <br />potential consistent with Hydrologic Soil Group D. <br />The Curve Number for each basin was used to calculate the potential retention, a measure of <br />the ability of a drainage basin to abstract and retain storm precipitation thereby reducing the <br />amount of excess precipitation that runs off the site. The initial loss for each basin was set to <br />five percent of the basin's potential retention. This initial loss estimate is based partially on site <br />runoff observations made during several 2012 storm events. <br />