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<br />. <br /> <br />e <br /> <br />e <br /> <br />,e- <br /> <br />J <br /> <br />b. Infiltration losses assUllled Ul computilll ruDOff trOIS tile BPS <br />should correspond to those considered reasonably Ukely to occur durlcg <br />stOnDS or such ~tude. est1Jllated on the bade ot data obtained tro. the <br />an&lya s or rainf&ll-nmorr relations Ul aajor tlooda of record. <br /> <br />c. The same principles referred to li>ove should be rolloved in <br />the selection or unit hydrograpbs, in flood routing computations, and Ul <br />other c&lculations related to the develoi8l!!nt ot the SPY hydrographl. <br /> <br />d. Appropriate &llOW'llllces sbould be made tor ....riations ill <br />the &real distribution and sequence ot ra1ntAll over the buUl dur1ng tbe <br />SPS. The SPS rainfall estimate should be prepared Ul IN1table tom to <br />permit signiricant variations in areal distribution, sequence, aDd Ultenaities <br />to be taken into consideration in comput1ng runoff bydrosrapbs. <br /> <br />e. In some cases, particularly vitb respect to very large <br />drainage basins, the Standard Project Flood hydrograph est1Jllate may, ot <br />necessity, be based on a study of actual bydrographs or stages ot record, <br />or on other procedures not involvicg directly a SPS est1Jllate. In sucb cases, <br />the general principles and objectives presented tor the development ot sn <br />estimates from raintall and run-orf criteria should serve as a guide. <br /> <br />f. It is apparent from a study ot depth-area-duration data tor <br />major stonns, and general conJidaration ot the relative opportunities for <br />drainage basins of various sizes, shapes and locations to be subjected to <br />rainf"all occurrences of SPS category, that the stati6~1c&l probability of <br />SPF occurrence vould vary vith size of draill88e area and otber hydrometeoro- <br />logical factors. For these, and a number of other reasODll that m1ght be <br />Ulted, it is not considered teas1ble to .881gB .c1t1c frequency estimates <br />to SPF dete1'lll1nations 1n general. <br /> <br />g. Where only the highest peak discharge ot . SPF hydrograpb <br />is pertinent to specific project studies, the SPY IIIIlY be rete~d to by <br />that peak discilarge as . matter ot convenience. <br /> <br />1-05. STANDARD P!l).rECT FLOOD SERIES <br /> <br />Practically all detailed studies ot UDUsual storms completed <br />to date have been limited to durations ot 120 hours or less. In the develop- <br />ment of Itandard project flood criteria tor very large drainage basins or <br />those in vhich reservOirs serve to modulate ruDOtt rates to a substantial <br />extent, it is necessary to consider runoff resulting from raintall over. <br />substantially longer period than 120 hours. In some caselI, runoff tor a <br />period ot veeks or months prior to the occu~nce ot an extraord1nar:r <br />120-haur stonn must be taken into account in studies related to specitic <br />projects. It is apparent that the computation ot hypothetical hydrograpbs <br />covering such an extended lengtb ot t1llle, folloving procedures involvilll <br />stonn raintall data and application ot the unit hydrograph technique, vould <br />be Vf!I"'J lshorious and subject to many uncertainties &8 to proper basic <br />assucptions and criteria. These difficulties can be circumvented satisfac- <br />torily in most cases by developing a "standard project flood series" in <br /> <br />3 <br />