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<br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br /> ESTIMATION OF RUNOFF CURVE NUMBERS (CN) <br /> FROM BLACKHAWK FLOODPLAIN INFORMATION TECHNICAL ADDENDUM (BFITA) <br /> CN q, Revised S la CNfrom <br /> est. from est from BFITA Revised from BFITA from BFIT A BFITA <br /> P I, S I, Q DA Tp I, qo Q (inches) qDvalues qo values qllvalues <br />North Clear Creek 3.04 0.5 2.5 80 1.28 33.88 7.5 2799 2640 1.2074702 2.69 0.54 79 <br />Chase 2.13 0.4 2 83 0.80 4.17 0.95 1705 1160 0.5460095 2.87 0.57 78 <br />Gregory Gulch 2.13 0.35 1.75 85 0.90 3.47 0.95 1587 1240 0.7014076 2.3 0.46 81 <br /> <br />.... <br />Note that Chase and Gregory values from the BFIT A are not proportional to their <br />respective Drainage areas. Re-evaluate the curve numbers by back calculating from the <br />hydrograph data and compare to values derived from the fa values <br /> <br /> <br />P = rainfall values from BFIT A pages 2-8 and 2-9 <br />I. "" Initial abstraction, from the hyetograph on BFlT A pages 2-8 and 2-9 <br />S = retention variable, computed from NRCS empirical fotmula; la = 0.2 . S <br />CN est from I, = the NRCS CN from the formula CN = lOOO/(S+IO) <br />Q = inches of runoff using NRCS formula Q - (P-0.2S)^21(P+0.8S) <br />DA = drainage area from BFIT A pages 2-3,5,6 <br />Tp = time to peak measured from hydrographs on BFIT A pages 2-8, 9 <br />'lp = peak q, cfs, from NRCS fonnula Qp = 484*A*Qffp, using values derived from la estimate <br /> <br />BFIT A q, = the peak q, cfs, from the hydrographs on BFIT A pages 2-8. 9 <br />Revised Q (inches) = the runoff value that yields the BFITA qp using the same A and Tp values taken from hydrographs <br />Revised S = the retention variable back calculated from Revised Q = (P-0.2S)^2J(P+0.8S) <br />Ia from BFIT A Qp values = the la calculated using the Revised S in the formula Ia = 0.2 . S <br />CN from BFITA q, values' theCN calculated using the Revised S in the formula CN = 1000/(S+IO) <br />