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BASIN INPUT PARAMETERS <br />Basin <br />Area <br />(Sq. Mi.) <br />Area <br />(acres) <br />Minimum <br />Infiltration <br />(inJhr.) <br />Percent <br />Impervious <br />UCR <br />1.64 <br />1,050 <br />0.18 <br />17.5 <br />HC <br />0.61 <br />390 <br />0.19 <br />17.5 <br />EBRB <br />0.90 <br />576 <br />0.30 <br />9.5 <br />EBRT <br />1.63 <br />1043 <br />0.20 <br />21 <br />TPB <br />4.20 <br />2,688 <br />0.28 <br />6.5 <br />ADOPTED INFILTRATION RATES <br />FOR SOIL HYDROLOGIC GROUPS <br />Hydrologic Group <br />A <br />B <br />C <br />D <br />Infiltration Rate (in/hr) <br />0.40 <br />0.25 <br />0.10 <br />0.05 <br />subbasin. The soil survey indicates the Hydrologic Soil Group for the soils in the drainage <br />basins tributary to the Henderson Mill facilities. The infiltration rates used in our analysis for <br />each of the four soil groups documented in the drainage basin are summarized in the table <br />below. These infiltration rates are near the average of the minimum infiltration rates <br />recommended for these soil groups in the USBR Flood Hydrology Manual. An initial <br />abstraction of 0.0 inch per hour was also used in our analysis to simulate saturated soil <br />conditions. <br />The Group A soils are considered to have high infiltration rates and low runoff <br />potential and were generally located in areas of the valley floor and slopes that were covered <br />by glacial till and alluvial deposits. The Group B soils are considered to have moderate <br />infiltration rates and moderate runoff potential and included floodplain deposits along the <br />valley floor, alluvial terrace deposits, glacial moraine deposits, and soils covering the <br />mountain slopes and ridges. Group C soils are characterized by a moderate permeability <br />and rapid runoff. Group D soils have high runoff potential and include canyon walls, bedrock <br />outcrops, cliffs, shallow talus deposits, glacial cirques, and developed mining areas. A <br />weighted average infiltration rate was developed for each subbasin based on the areas of <br />each soil group identified in each subbasin. The resulting infiltration values used in the HEC- <br />1 model are summarized in the table below. It is important to note that the soils in the <br />Henderson basins are relatively deep compared to other mountain basins, and the vegetation <br />cover is dense, primarily consisting of grass and heavy timber. <br />8 <br />