Laserfiche WebLink
floodplain and backwater effects. Since the SEDCAD4 model uses a simplified routing technique <br />and does not account for floodplain storage, use of the 241-point distribution would have resulted in <br />an over-design ofrun-off from the design storms used for the Northfield Project drainage. Therefore, <br />the 49-point distribution was used for all run-off calculations. <br />2.5 Subwatershed Area <br />Each Subwatershed, or drainage basin, used in the SEDCAD model has a specific surface area that is <br />available to collect rainfall and produce stormwater run-off. For the Northfield Project, <br />subwatersheds were delineated based on available area topography of undisturbed slopes. <br />Subwatershed boundaries are shown on the attached Site Drainage Map. Prior to the final design of <br />drainage and sediment control structures, Subwatershed delineations should be checked following site <br />grading activities. <br />2.6 Time of Concentration <br />Time of concentration is used to estimate the timing of peak flows at the subwatershed outlet. The <br />time of concentration value is estimated from the longest flow path length (and associated slope) <br />from the top of the subwatershed to the outlet. Normally, the time of concentration consists of an <br />overland flow component (calculated by the MRCS upland curve method) and a channel flow <br />component (calculated using the SEDCAD4 routing calculator). Flow paths for each subwatershed <br />are shown on the Site Drainage Map and time of concentration values for each subwatershed are <br />provided on the SEDCAD4 printouts in the Appendix B. <br />2.7 Muskingum Routing <br />Muskingum routing is used to route flow from an up-gradient to adown-gradient location including <br />routing between structures or routing from a subwatershed outlet to its structure, when the outlet of a <br />subwatershed is not located at the structure. Muskingum parameters are represented by K and X. <br />The K parameter accounts for the time it takes to proceed from an up-gradient to adown-gradient <br />location. The X parameter is functionally related to the average stream reach velocity and accounts <br />for the attenuation of the hydrograph. At the Northfield Project, since designed structures were <br />generally located at the subwatershed outlets, Muskingum routing was not used. <br />2.8 Curve Number <br />Curve number is a function of land use and hydrologic soil group. The curve number will relate the <br />amount of rainfall to the amount of run-off for a given area. For the Northfield Project, curve <br />numbers were determined from tables provided in the SEDCAD4 model for Arid and Semiarid <br />Rangelands. Hydrologic soil group information was obtained from NRCS soil surveys. In <br />undisturbed subwatersheds, the hydrologic soil condition was considered to be fair and in <br />subwatersheds with proposed grading disturbances the hydrologic soil condition was considered <br />poor. A summary of curve numbers assigned to the Northfield Project subwatersheds is provided in <br />' Table 1. <br />3 <br />