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<br />. <br /> <br />yhich hydrographB for periods antecedent to and subsequent to a staDdard <br />project stom of l2O-hours duration are represented by 1'10ws actually <br />observed in the ballin, vith such adjustmenh 11.8 IIIIlY be deemed appropriate <br />arter a general study of flood aDd stom characteristics in the region <br />involved. <br /> <br />e <br /> <br />1-06. BASIC FLOOD ESTDfATES IN I'I.AN1lIID AND DESIGN OF ProJrel'S. <br /> <br />a. General. Flood lIlllgllitudes il< a particular drainl&e baein <br />are governed by combinations of ma:oy variable factors, the IIOst 1JIIpOrtlU1t <br />being the quantity, intensity, sequence and areal distribution of precipi- <br />tation, the int'11tration capacity of the so11, and natural and artificial <br />storage effects during floods. compenllating variations in these several <br />factors Ullually serve to reduce flood rUDotf rates and volumes to values <br />far beloy those that would result ,from critical combinations of such factors. <br />l{bererelatively lo~period records of strewn flow are aV&ilab~e, statistical <br />analyses of the record provide a valuable lIIC&llS of est1lllating the lll8gnitude <br />of flood floys that may be expected yith frequencies bearing a reasonable <br />relationship to the period of observation. Statistical studies involv1ng <br />consideration of floy records of Il\lIllcrous strelUllS in a region of reasonably <br />cc;uparable hydrologic and meteorological influences provide IIIOre re,liable <br />est1lllates of maximum flood potentie.lities and average frequencies of c0/llll0nl7 <br />observed flood lll8gnitudes than can be obtained from studies of individual <br />station records. Hoyever, because of the Dumber and range of variation <br />in, independent variables involved and the vide range between 1'10od lIl86Di- <br />tudes that would result from opt1mum combu1&tions of critical flood- <br />producing factors as compared vith combinations generally observed, statis- <br />tical analyses of actual stream flow records seldOlll, if ever, provide a <br />reliable indication of extraordinary flood potentialities ot a specific <br />drainage basin. Accordingly, in the pllllming and dellign ot major engineering <br />projects for the regulation of stream flow, statistical analyses ot etream <br />flow records must be supplemented by hypothetical flood est1lllatee based <br />on scientUic studies of principal 1'lood-producing factors 1ndiv1dually aDd <br />in various combinations. <br /> <br />e <br /> <br />b. In accordance vith the preceding discussion, three clas,es <br />of basic flood est1.mates are required in general tlood control pl...".ing <br />and design investigations: <br /> <br />(1) Statistical Analyses of Stream Flow Records, including <br />flood-frequency est1lllates (preferably on a regionalized basis) and variOUll <br />correlations 'ot 1'lood characteristics and h~rologic features ot the drain- <br />age basin. <br /> <br />(2) Standard Pro ect Flood Abhrev. S Eat1.mates repre- <br />eenting flood d1scharges that _y e expecte from t e IIOst severe combina- <br />tion of IIleteorologic and hydrologic cOl:ld1tiODS that are considered re&llon- <br />ably characteristic of the geographical resioD iDvolved, excluding e-xtremely <br />rare combinatioDs. <br /> <br />1+ <br /> <br />e <br />