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<br /> <br />4, COMPUTATIONAL INSTRUCTIONS <br /> <br />Step-by-step instructions are providcd below for computing the boundaries of flood <br />hazard zones on alluvial fans using log-Pcarson Type III analyses in accordance with <br />Bulletin No. 17B (Reference 3). <br /> <br />a. Determine Flood Discharge-Frequency Distribution <br /> <br />For the source of flooding at the apex of each alluvial fan, a complete flood dis- <br />charge,frequency distribution should be determined using log-Pearson Type III analyses in <br />accordance with Bulletin No. l7B. The determination of flood discharges in arid regions, <br />where alluvial fans are most frequently found, should be elosely coordinated with the PO <br />to ensure agreement on methodology. <br /> <br />The skew coefficient, standard deviation, and mean of logarithms of discharges <br />must be determined for the flooding source at the apex of the fan, When an analysis ac- <br />cording to Bulletin No. 17B is done, these statistics are known. For most alluvial fans, <br />however, these statistics will not be available, Therefore, flows of various rccurrence in- <br />tervals should be computed from appropiate regional methods, and the synthetic log-Pear- <br />son Type III parameters should be derived. <br /> <br />Dcrivation of Skew Coefficient. Derive the skew cocfficient using the ollowing <br />cquations: <br /> <br />G = -2.50 + 3.12 Log [(Q.01/.IO)/(Q.IO/50)] (I) <br /> <br />Using the skew coefficient computed above and the K values for the skew as <br />shown in Bulletin No. 17B, the standard deviation should be derived according to the <br />following equations: <br /> <br />S = [Log(Q.O 1 /.50)/(K.oJ - K .50)] (2) <br /> <br />(2) Derivation of Mean of Logarithms, Using the values determined in Equations <br />and 2, the mean of logarithms should be dcrivcd according to the following equation: <br /> <br />x = log (Q.50) - K.sO(S) (3) <br /> <br />where S and X are the standard deviation and mean rcspectively; Q 01' Q,IO' and <br />Q 50 are discharges with 0.01, 0.10 and 0.50 exceedance probabilitics; and ROland K 50 <br />are Pearson Type III deviates for rcspcctivc cxceedancc probabilities of 0.01 and 0.50 a'nd <br />skew coefficicnt G, Equation (I) above is an approximation appropriate for use between <br />skew values of +2.5 and -2.0. <br /> <br />b. Compute Transformation Variables <br /> <br />To permit solutions by use of log-Pearson Type III analysis and Bulletin No. 17B, <br />the log-Pearson Type III parameters must be transformcd. <br /> <br />Variables for transforming these parameters should bc computed as follows: <br /> <br />C-13 <br />