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<br />OfH583 <br /> <br />PROJECT TITLE: <br /> <br />Flow Resistance in Steep-Mountain and Ephemeral <br />Streams in Colorado (fig. 12) <br /> <br />COOPERATI NG AGENCY: Co.lorado Department of Natural Resources, Colorado <br />Water Conservation Board <br /> <br />PROJECT CHIEF: Robert D. Jarrett, District ,Dffice, Lakewood <br /> <br />PERIOD OF PROJECT: May 1979 to September 19B2 <br /> <br />Problem.--Recent legislative actions involving land-use planning and flood- <br />insurance programs have greatly increased the demand for detailed flood studies. <br />Hydraulic computations for these studies are largely based on the Manning <br />equations, which require the selection of a roughness coefficient (n value) to <br />represent flow resistance along a stream channel. Although guidel ines are <br />available to aid in the selection of roughness coefficients for many types of <br />channels, insufficient data exist for the range of hydraul ic conditions found in <br />steep-mountain and ephemeral streams in Colorado. <br /> <br />Objeotives.--Verify Manning's roughness coefficients for selected reaches on <br />steep mountain and ephemeral streams to provide guidel ines for the selection of <br />n for flow regimes in Colorado. Evaluate existing guidel ines for the selection of <br />roughness coefficients and compile a manual presenting the selection criteria and <br />photographs of representative-channel types. Use study results as a basis for <br />design of a more comprehensive project if evidence indicates selection guidel ines <br />for roughness coefficients are not valid for the types of streams being studied. <br /> <br />Approaoh.--Collect data at selected sites to compute roughness coefficients. <br />Data wi 11 include discharge measurements at high stages, color stereo-sl ides <br />during each measurement and at low stream stage, channel cross-sections, water- <br />surface elevations, and selected roughness coefficients. Use the data to verify <br />the roughness coefficients. Compare the roughness values selected and the veri- <br />fied values to evaluate the present roughness-coefficient guidelines. <br /> <br />Progress.--The collection of streamflow data for various flow depths has been <br />completed at 20 sites typical of steep-gradient streams in Colorado. Analysis of <br />the data indicates that the roughness coefficient, n, changes significantly with <br />depth during flood flows on steep gradient streams that have very coarse bed mate- <br />rial. A standard step-backwater analysis generally shows that the flow is super- <br />critical in streams in which the slope exceeds about 1.5 percent; however, the <br />data indicate that flow is subcritical in streams in which the slope exceeds <br />3 percent. <br /> <br />Plans.--Prepare a technical manual in cooperation with the Colorado Water <br />Conservation Board that presents the criteria used in selecting representative <br />channel types and photographs of these types. <br /> <br />33 <br />