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• TIME OF CONCENTRATION, Tc <br />Time of concentration is calculated by using the SCS upland method. All <br />hydraulic lengths, drainage heights and slopes are measured directly <br />from a 1"=200' scale map. The values calculated for each structure are <br />shown on a subwatershed basis in the SEDCAD+ printouts contained in <br />Appendix A, Diversion and Collection Ditch Hydrology Calculations. <br />CURVE NUMBER, CN <br />Curve numbers were obtained from the surface water control system plan <br />submitted by Rocky Mountain Geotechnical (RMG) and contained in the <br />Southfield Permit. RMG utilized procedures set forth in Procedures for <br />Determining Peak Flows in Colorado, Technical Release No. 55, USDA-Soil <br />Conservation Service, 1977. Based on soil type and vegetation cover, <br />curve numbers were determined for disturbed and undisturbed conditions <br />as follows: <br />Drainage Structure <br />Permanent Diversion ill <br />West Refuse Collection Ditch <br />North Refuse Collection Ditch <br />*South Refuse Collection Ditch <br />Sedimentation Pond No. 4 <br />Land Conditions Curve Number <br />Undisturbed 69 <br />Disturbed 80 <br />Disturbed 80 <br />Disturbed 80 <br />Undisturbed 69 <br />Undisturbed 69 <br />Water Surface 100 <br />*The south refuse collection ditch drainage basin contains both <br />disturbed and undisturbed subwatersheds. A curve number was determined <br />for each. <br />As previously mentioned, computer (SEDCAD+) printouts for Permanent <br />Diversion ljl and all the refuse collection ditches are contained in <br />• Appendix A, Diversion and Collection Ditch Hydrology Calculations, of <br />this report. These printouts contain all hydrologic input values and <br />calculated peak flow values on a subwatershed basis for each structure. <br />-7R- Revised - February, 1990 <br />