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<br />e <br /> <br />e <br /> <br />e <br /> <br />EM 1110-2-1405 <br />31 Aug 59 <br /> <br />transpose the respertivp storms to the basin under study, select the estimate that would r{"present <br />critical rainfall duration-depth-area relations for the particular drainage area during yarious <br />seasons of the ~Tear. <br />Estimate the maximum quantity and rate of contribution to flood runoff that might result <br />from mplting snow in conjunct.ion with the critical storm rainfall for the snow season. <br />Taking into account the minimum infiltration capacities likely to prevail during the various <br />seasons, and the maximum eontribution from melting snow during the snow season, seleet thE' <br />design storm estimate that would result in the critical runoff hydrograph for the project involved. <br />I n some cases it may be necrssary to compute hydrographs for two or more rainfall estimates t.o <br />determine the critical combination of volume and rate of runoff for the project.. <br />b. In aceordanee with cooperative arrangements between the Corps of Engineers and the Weather <br />Bureau, special design-storm estimates based on comprehensive studies generally as indicated in the <br />preceding subparagraph will be prepared by the Hydrometeorological Section of the Weather Bureau <br />insofar as time and facilities permit, upon requests from the Chief of Engineers., which in turn originate <br />from Dist.rid and Division Engineers. Such studies arc necessarily limiteel to important projpcts <br />where reasonabl.'T satisfactor.'" design-storm estimates cannot be deduced from data available for other <br />projects in the region. <br />Upon requpst, "preliminary" estimates of probable maximum precipitation, based on a,vailable <br />information and rplatively brief meteorological analysis, will be furnished by the Hydrometeorological <br />Srdion of Ow Wrather Bun'au, for important projects for which comprch('nsive studies cannot be l'om- <br />pletpcl in time for till' required usp. I n view of the limited staff in the Hy'drometoorological Seption of <br />the 'Vf'ather Bureau, it is necessary that District and Division Engineer Offices originating requ('Sts for <br />spl'eial studies furnish every possible assistance in the compilation of basic data. Rr-quests for prelimi- <br />nary estimatps will be submitted through the Chief of Engineers, and should be accompanied by sub- <br />stantially tll(' following data: <br />Bri('f dcseription of drainage basins concerned and spp('ific designft,tion of areas for which <br />(>stimates are desired. <br />Vi('inity map showing position of basin with respect to state boundarips, geographical features, <br /> <br />('Le. <br /> <br />Basin map showing important tributaries and the location of stl'eam-gaging and precipitation <br />stations in and immediately adja('ent to basin. <br />For basins in mountainous regions, a generalized topographic map of basin, indicating by colors <br />the area between various ranges of elevation, sueh as 500'-1,000', 1,000'-1,500', etc. <br />For basins located west of the] 03d meridian, an up-to-date isohyetal map of average annual <br />prceipitation over the basin. <br />A brief hydrologic summary for the basin, reviewing the records of stages and discharges, sea- <br />sonal variations in rainfall and runoff and a tabulation of highest stages and diseharges of r{'eord <br />foJ' PRrh season of the year. <br />List of the fivp largest storms of record in th", basin and five or more of the largest storms in <br />~pneral vicinity. Pr{'liminary isohyetal maps and representative mass-rainfall curvefit for stations <br />IlrRI' storm centers, together with all pertinent available data, should be submittpd for storms that <br />have not bren previously inv{'stigated under the Corps of Engineers' general storm study program. <br />c, 'I'll(' Hydrometeorological Section, at the request of the Chief of Engineers, prepared a reporl <br />entit.!<,d "Seasonal Variation of the Probable :\laximum Precipitation East of the 105th :\Ieridian for <br />Arpas from 10 to 1,000 Square :\Iiles and Durations of 6, ]2, 24, and 48 hours, dated April] 956."" This <br />report provides a means for obtaining the probable maximum precipitation by' months and for an all <br />seasonal cnvPlopc for the indicated areas and durations, The generalized estimates also providp a <br />useful guide in dpveloping probable maximum rainfall estimates for areas considerably larger than 1,000 <br /> <br />21 <br />