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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 /> ESTIMATION OF RUNOFF CURVE NUMBERS (CN) <br /> FROM BLACKHAWK FLOODPLAIN INFORMATION TECHNICAL ADDENDUM (BFITA) <br /> CN <lp Revis<<! 5 la CN from <br /> est. from est from BFITA Revised from BFIT A fromBFITA BFIT A <br /> P I, 5 I, Q DA Tp I. q, Q (inches) qo values qD values qD values <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 Sf!T 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 la values <br /> <br /> <br />P : rainfall values from BFIT A pages 2-8 and 2-9 <br />I, = Initial abstraction, from the byetograph on BFIT A pages 2-8 and 2-9 <br />S "" retention variable, computed from NRCS empirical formula; fa = 0.2 . S <br />CN est from I. ~ the NRC5 CN from the formula CN ~ 1000/(5+10) <br />Q ~ inches ofrunolTusing NRC5 formula Q ~ (P-0.25)^2/(P+0.85) <br />DA "" drainage area from BFIT A pages 2-3,5,6 <br />Tp ~ lime to peak measured from hydrographs on BFITA pages 2,8. 9 <br />'lp = peak q. cfs, from NRCS formula (Jp = 484. A .Qrrp, using values derived from I. estimate <br /> <br />BFITA q, ~ the peak q, cfs, from the hydrographs on BFITA pages 2-8,9 <br />Revised Q (inches) = the mnoffvalue that yields the BFITA qp using the same A and Tp values taken from hydrographs <br />Revised 5 = the retention variable back calculated from Revis<<! Q: (P-O.25)^21(P+O.85) <br />la from BFITA ~ values == the la calculated using the Revised S in the formula la = 0.2. S <br />CN from BFIT A q, values = the eN calculated using the Revised 5 in the formula CN ~ I 000/(5+ I 0) <br />