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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />in a rainfall-runolTcomputer model (HEC -lor IIEC-HMS) for thc uppcr Dolores River basin <br /> <br />\....ere developed based on analysis of an actual gcncral rainllood event. The computer model used <br /> <br />observed rainfall to reconstitutc the flood hydrograph ("compuled flood hydrograph") compared <br /> <br />with the observed flood hydrograph. (See References 4-e and 4-h.) <br />6.2 Previous model development. An IIEC.I rainfall-runofTcomputer model was <br /> <br />dcveloped for the Dolorcs River at Dolorcs as part of Ihe 1976 Flood Hazard Infonnation Study <br /> <br />(see References 4-c and 4-h). Thc September 1970 general rain and flood event \....as analyzed to <br /> <br />devclop the unit hydrograph and routing parameters for thc IIEC -I model. A 2-day stonn <br /> <br />isohyetal map, hourly rainfall data from ncarby precipitation stations, and thc observed hourly <br /> <br />flow hydrograph for the D(}lores Rivcr at Dolorcs wcrc available for the analysis. Unit <br /> <br />hydrographs lor uppcr mainstem Dolores River and West Dolorcs River in the model were based <br /> <br />on the S.clIrve method developcd by the Los Angeles DistricI COIT's of Enginecrs C'L.A. <br /> <br />Method"). The HEC-1 modcl was used to compute a Standard Project Flood on the upper <br /> <br />Dolores River basin. Thc 1 % flood hydrograph on thc uppcr Dolores River used in the 1976 <br /> <br />study was a ratio of the Standard Projcct Flood hydrograph. Appendix A, Table 2 contains the <br /> <br />parameters used lor development of the 1976 IIEC-I model. <br /> <br />6.3 Current Flood Insurance Study model. An I1EC-IIMS computer model was used to <br /> <br />develop the 1% chance flood hydrographs used for this Flood Insurance Study (see Referencc 4- <br /> <br />c). The lIMS modcl is more detailed than the earlicr IIEC-I model. with marc subbasins; it also <br /> <br />computes IS.minute flood hydrographs. T'he lHv(S model was used to compute the 1% chancc <br /> <br />llood hydrographs needed to dewlnp the I % chance llood plain for the study area. Again. the <br /> <br />Septcmber 1970 gcneral raintlood evcnt was used to calibrate the new lHvlS model, with a <br /> <br />different division of subbasins. For the 1976 study. the "detailed sludy area" was divided into <br /> <br />16 <br />