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<br />developed and transposed to locations on the White River Plateau and <br />the Park Range. This produced a 40-hour storm with a 3.35-inch <br />rainfall over the Yampa River drainage above Agate Creek and a 4.13- <br />inch average rainfall on the Park Range, <br /> <br />Loss rates for the basin were estimated due to lack of reproducible <br />rainfloods in the area. Study of runoff in the Grand Mesa area <br />during the 1970 Four Corners storm indicated high (greater than 1.5 <br />inches) initial losses. A study of streamflow and precipitation data <br />published by the U.S. Forest Service (Reference 4), for subalpine <br />watersheds (including West Walton, North Fish, and Soda Creeks) <br />indicates that approximately 2.0 inches of rain must fall in the <br />August to October period for appreciable runoff to occur. Loss rates <br />selected for the 100- and SOQ-year general rain floods were 1.60 <br />inches initial loss and 0.15 inch per hour constant loss on the Yampa <br />River and 2.00 inches initial loss and 0.15 inch constant loss on the <br />tributary creeks. A lower initial loss was used on the river because <br />the density of vegetation is lower than on the tributary streams. <br /> <br />Little definitive data on cloudburst flood events are available for <br />the Steamboat Springs area. The June 1921 flood may have been the <br />result of a cloudburst that augmented abnormal snowmelt runoff. <br />Contributing rainfall totaled more than 2.5 inches, but the period of <br />time in which the rain fell is not definitely known. A newspaper <br />account indicated that it may have been between 12 and 20 hours. <br />However, 2.04 inches of rain in 3 hours and a 24-hour rainfall of <br />3.24 inches have been recorded in the region. Thus, it appears that <br />cloudburst floods are possible in the Steamboat Springs area. <br />Cloudburst storm criteria were developed from National Weather Service <br />data (Reference 5). A 40 percent value of the probable maximum <br />precipitation point rainfall for Steamboat Springs, plotted at about <br />the SOO-year frequency on a 3-hour rainfall frequency curve for <br />Craig, Colorado (the nearest recorded), was selected for use as a <br />cloudburst standard project storm. Loss rates were identical to <br />those used for general rainfall. The ratio of lOa-year cloudburst <br />precipitation to the standard project storm precipitation was developed <br />from this 3-hour frequency curve. <br /> <br />The 10- and 50-year floods were determined to be basically snowmelt <br />events and, for the Yampa River, were modeled after major snowmelt <br />flows that occurred in June 19S2, adjusting the 50- and la-day <br />volumes for their corresponding frequencies. The 1-, 5-, and la-day <br />flows for the tributary creeks were taken from their corresponding <br />frequency curves. This was considered an adequate procedure in that <br />the snowmelt frequency curves appear well defined below 50-year <br />events and the results that would be obtained by more complex pro- <br />cedures would not appreciably differ. Flows for streams without <br />frequency curves were derived from frequency curves from streams with <br />characteristics similar to those under study. <br /> <br />18 <br />