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COl.OIlADO WAnA CONSERVATION aoARD Deponmenl of N_Iural Resources m Scata CefttenniAJ luildi"l lnJ Shennan SI'eet 0...-. CoIo<adoilIlm ........: CJlllI_~' STATE OF COLORADO O.~. , . .. . ., , conditions overbank flooding will occur in Adams County at streamflows in excess of 4000 cfs. In addition a 5000 cfs flow at Chatfield Dam will greatly attenuate by the time it reaches the Denver gage. When considering the District's basin assumptions and selected methodology, we can endorse the 100-year discharge values as shown on Figure 6 of subject report for the USDCM procedure. To assist in the administration of the State's Floodplain Management Program, I would request that your final study report be supplemented by a technical addendum. As a minimum, th7 tech- nical addendum should contain all the basic data, hydrolog~c and hydraulic computations, hydrographs, table of discharges and dis- charge profiles, computer printouts for input and summary output - all included printouts should be reduced to 8 1/2" x 11" sheet size and labelled final, the methodology and assumptions used for each study phase, hydraulic parameters and a listing of Manning's roughness coefficiencies with pictorial illustrations, and other applicable data which were developed during the different analy- ses and used in the study. The technical addendum should be a bound and paged document which can be a three ring notebook. I look forward to working with the District on this project. Mr. Ben Urbonas, P. E. Chief, Master Planning Program 2480 W. 26th Avenue Suite 156B Denver, CO 802~1 o ~ra~1Wry~~~ \' "'-'~"'903 II , ',..\...." b U. D. & F. C. D. ..chard O. ymm eo..,_ I. Willa.m McDon.IkI Director OavKfW w..lh, Depury Chrector October 24, 1983 Dear Mr. Urbonas: I welcome the opportunity to participate in the District's Major Drainageway Planning Study for the South Platte River through the Denver-Metro Area. This type of investigation is long overdue. As you are aware, many governmental agencies have performed a certain level of flood-related analysis on the sub- ject stream reach. :fSincerelY ~i '1 r C'"7 r',\ arry . La g, P~., Chief Flood Control and Floodplain Management Section We have reviewed and evaluated the District's South Platte River Hydrologic Study for the study reach from Chatfield Dam to Sand Creek dated May, 1983. We find that the methodology which was used to be acceptable for the drainage area understudy. When comparing the 100-year flood hydrologic values to those of the Corps of Engineers, we find a favorable comparison for similar hydrological conditions. At specific locations, we find: 100-year discharge Location Corps UD&FCD Bowles Ave. 8,500 cfs 8,000 cfs U/S Bear Creek 15,000 cfs 13, 500 cfs U/S Cherry Creek 19,000 cfs 19,500 cfs U/S Sand Creek 21,000 cfs 21,600 cfs LFL/bm In reviewing the Corps hydrology analysis, we further find that for the South Platte Channel project reach they are using 100-year flood discharge values which are approximately 35 per cent greater because of the assumed releases from Chatfield Dam during periods of flood. However, a release rate of 5000 cfs during times of flood is quite unlikely because under dry 704 Urbonas MEMORANDUM -2- October 24, 1983 'oiI" &. fetcher. ChunNIn . Heriaen H. Vandernoft. Vice Chai'l'NIn c.M.. .ume.....Io,dL Getz.OaIe,. Crane, RODen A. McUon.atchard W.IOhnllon. Jr..FrltGenck v. Kroept'.Oavtd W. Robbin.