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<br />. <br /> <br />. <br />STATE OF COLORADO <br />o <br /> <br />COLORADO WATER CONSERVATION BOARD <br />Department of Natural Res.ources <br />823 State Centennial 8uilding <br />1313 Sherman Street <br />Denver, Colorado 60203 <br />Phone. (303) 866-3441 <br /> <br />March 28, 1983 <br /> <br />Ri<h,JH! D. LUlllll <br />Governor <br /> <br />J. William McDonald <br />Dir('clor <br /> <br />Dr. John Liou <br />Federal Emergency Manageme~t Agency <br />Region VIII <br />Division of Natural and Technological Hazards <br />Denver Federal Centec, Building 710 <br />Denver, Colorado 80225 <br /> <br />David w. Walker <br />Depuly Director <br /> <br />Dear Dr. Liou: <br /> <br />The Colorado Water Conservation Board has conducted a review <br />of the draft Flood Insurance Studies for El Paso County, Colorado <br />and the City of Colorado Springs, Colorado. We have discussed <br />our concerns with your consultant, Camp, Dresser & McKee, and we <br />would like to share them with you. <br /> <br />The Eicst general area of concern is hydrologic computa- <br />tions. For reasons of cost saving and ease of local administra- <br />tion, wherever possible previously generated hydrologic <br />calculations were used by your consultant. The CWCB does not <br />object to that practice. We do, however, have a concern with <br />regard to consistency within the El Paso County area. We have <br />included a plot of C.S.M. ratios (cubic feet per second/square <br />mile) versus drainage area. Despite the variety of hydrologic <br />methodologies used, there is a reasonable consistency in the <br />calculated 100-year discharges within the study area. There are, <br />however, two stream basins, where discharges appear significantly <br />low in comparison to the majority of discharges: the Sand Creek <br />basin and the Spring Creek basin. <br /> <br />Discharges in both of these basins were calculated using the <br />SCS method. We spoke with your consultant regarding the possi- <br />bility that revised soils data might change the flows. His <br />cesearch indicated that such revisions would not increase the <br />flows. One possible revision which would cause an increase in <br />flows would be to use 6-hour storm data rather than 24-hour storm <br />data. We would recommend that such a revision and other revi- <br />sions be examined and that the hydrology for the two basins be <br />reviewed carefully. In particular the Sand Creek basin has <br />experienced several instances of flooding in recent years with <br />some culverts and bridges being unable to convey flood flows <br /> <br />600 <br />(djl/l) <br /> <br />John R. Felcher, Chairman - Herberl H. Vandemoer, Vice Chairman <br />C.M. furneaulC, Floyd l. Gell,Uale F. Gran!, Robert ^. Jackson, Richilld W. Johnston. Jr..Frederick V. krocger, David W. Robbins <br />