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Forward Landfill HYDROLOGY <br /> Runoff 07/17/2014 <br /> TASK: Select SCS Curve Numbers of 87 and 89 for bare ground(dirt) <br /> conditions based on Hydrologic Soil Groups C and D. Use CN=88. <br /> Variable Unit 25Yr 1,000 YR <br /> 24 Hour 24hr <br /> P Inches 2.83 4.89 <br /> Vr Ac-Ft 16.1 16.1 <br /> DA Acres 39.1 39.1 <br /> C 0.59 0.73 <br /> CN 88 88 <br /> S 1.36 1.36 <br /> la 0.27 0.27 <br /> Q 1.67 3.56 <br /> VR AC-FT 5.4 11.6 <br /> SCS Curve Number Method <br /> The SCS curve number method is a simiple,widely used and efficient method for <br /> dctermining the approxient amount of runoff from a rainfall even in a particular area. <br /> Although the method is designed for a single storm event,it can be scaled to find average <br /> annual runofTvalues.The stat requirments for this method are very low,rainfall amount <br /> and curve number.The curve number is based on the area's hydrologic soil group,land <br /> use,treatment and hydrologic condition.The 2 former being of greatest importance. <br /> The general equation for the SCS curve number method is as follows: <br /> The initial equation(1)is based on trends <br /> (P-IA2 observed in data from collected sites,therefore it is <br /> Q=�; •ar an emperical equation instead of a physically based <br /> (P-1a)+cJ equation.After further empirical evaulation of the <br /> trends in the data base,the initial abstractions.Is, <br /> could be defined as a percentage of S(2).With this <br /> Q=runoff(in) assumption,the equation(3)could be written in it <br /> P=raArtae(n) more simplified form with only 3 variables.The <br /> S=poteroal maxanum retention parameter CN is a transformation of S,and it is <br /> after ru otf begins used to snake interpolating,averaging.and <br /> la=tribal abs""ons weighting operations more linear(4). <br /> (a=0.2 S f2) With the following chart,the amount or runoff <br /> can be fond if the rainfall arttount(in inches)and <br /> curve number is known. <br /> (P+0.8 S) There arc two advantages of using L.-THIA over <br /> a manual method.One.the availablity of the data. <br /> L:THIA provides the rainfall data for any area in <br /> __ 1000 -10 (4) die United States.Two,L-THIA completes this <br /> S CN caluculation for every rainfall event for thirty years <br /> and then reports the average annual runoff value. <br /> Hydrology Runoff Page 5 of 6 <br />