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TABLE 1 <br />Northfield Project Curve Numbers <br />Subwatershed Hydrologic Hydrologic Curve <br />(refer to Site Cover Type Condition Soil Group Number <br />Draina a Ma <br /> Undisturbed <br />A Pinyon, juniper, or both Fair D 80 <br /> with ass understo <br /> Undisturbed <br />B Pinyon, juniper, or both Fair D 80 <br /> with ass understo <br /> Disturbed <br />C Pinyon, juniper, or both Poor C 85 <br /> with ass understo <br />2.9 Hydrograph Response Shape <br />The unit hydrograph methodology is used in SEDCAD4. A double triangle dimensionless unit <br />hydrograph shape is used to emulate hydrograph response. The hydrograph responses available in <br />the SEDCAD4 model are slow, medium, and fast. A slow response corresponds to a forested area or <br />an area with a number of obstructions. A fast response is anticipated for disturbed soils that have <br />been subjected to compaction by heavy equipment. For undisturbed subwatersheds on the Northfield <br />Project (subwatersheds A and B as shown on the Site Drainage Map), a medium hydrograph <br />response was used. A fast hydrograph response was used for Subwatershed C (see Site Drainage <br />Map) where proposed site disturbances will occur. <br />3.0 SEDIMENTATION CALCULATIONS <br />The sediment calculations used to model the Northfield Project site were performed utilizing the <br />SEDCAD4 computer model. The SEDCAD4 model calculates sediment yield using derivatives of <br />the Universal Soil Loss Equation (LISLE). Subwatershed C (see Site Drainage Map) is the only <br />subwatershed at the Northfield Project site where disturbances are proposed and, therefore, <br />sedimentation calculations were only performed for this subwatershed. The input parameters used in <br />the sedimentation analyses are presented in the following sections. <br />3.1 Particle Size Distribution <br />The particle size distribution of subwatershed soils is entered into the SEDCAD4 computer model <br />along with particle specific gravity and submerged bulk specific gravity. For the Northfield Project <br />site, soil types and classifications were determined from NRCS soil maps and tables. The soils are <br />generally silty sand and lean clay with silt. Although sieve and hydrometer analysis data for site <br />soils were not available, a typical particle size distribution for a sandy lean clay soil was used for <br />modeling the facilities subwatershed where disturbances are proposed. Prior to final design of <br />drainage and sediment control structures, site-specific soil samples should be analyzed to check <br />particle size distribution and erodible particle sizes. For specific gravity and submerged bulk specific <br />gravity, SEDCAD4 default values of 2.5 and 1.25 were used, respectively. <br />4 <br />