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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />the inflow hydrograph volume matched the volume of the outflow hydrograph plus the <br />estimated storage volume available in the reservoir at the beginning of the runoff season. The <br />final inflow volumes were compared to the estimated water content of the snowpack for the <br />corresponding runoff season, The resulting inflow hydrograph volume for 1997 was <br />approximately 75 percent of the estimated water content, and for 1998 the inflow hydrograph <br />volume was approximately 90 percent of the estimated water content. Figures 2.2 and 2.3 <br />illustrate the resulting hydro graphs determined for the 1997 and 1998 runoff seasons. <br /> <br />A very cursory analysis of flood control potential for Steamboat Lake was performed, Based <br />on the current operation level of the lake, very little storage volume is available before the <br />emergency spillway would begin to overflow. ICON looked at the amount of storage required <br />for 5 days of storage for flow rates from 300 to 500 cfs. To store an estimated 300 cfs for 5 <br />days would require close to 3,000 ac-ft in the reservoir. To store an estimated 500 cfs for 5 <br />days would require close to 5,000 ac-ft in the reservoir. Based on the elevation versus volume <br />curve from the original plans, the reservoir would have about 800 ac-ft of volume per foot of <br />elevation at the upper lake levels, This would mean that the lake would need to be drawn down <br />between 4 to 6 ft to provide the necessary volume to offset these release rates (Appendix D), <br />The entire watershed above Steamboat Lake only accounts for about eight percent of the Elk <br />River drainage area. Benefits to the Elk River from providing any flood storage at the lake <br />would have to consider impacts to the operation of the marina and the quality of the fishery, <br /> <br />A rating curve for the outlet works of Steamboat Lake was prepared as part of this study. The <br />rating curve accounts for repairs and modifications that were made to the structure in 1993, <br />The resulting rating curve for partially opened outlet works correlates well with information on <br />Steamboat Lake releases for May and June, 1997, Copies of the supporting information is <br />included in Appendix D. <br /> <br />2.2.3 Travel Time Issues <br /> <br />Gage information along the Elk River and Willow Creek was used to perform a travel time <br />analysis for determining a relationship between Steamboat Lake releases during the June 1997 <br />flood event and the Elk River discharge at the Milner gage, As previously mentioned, several <br />stream gages exist along the Elk River. At least two independent storm events were used to <br />correlate peak discharges between the Milner and Clark gages and the Clark and Above Clark <br />gages to determine a reasonable estimate of the travel times for peak discharges between the <br />gage sites, The estimated travel time between the Milner gage and the Clark gage is just under <br />3,8 hours, The estimated travel time between the Clark gage and the Above Clark gage is 0,7 <br />hours (Appendix E), Travel time was also estimated for the Steamboat Lake releases along <br />Willow Creek. The streamflow velocity was estimated from copies of meter reading notes <br />received from the Colorado Division of Water Resources that were taken at the Willow Creek <br />Below Steamboat Lake gage section just downstream of the Steamboat Lake outlet works, <br />Using this velocity, the estimated travel time between Steamboat Lake and the Elk River is <br />around 2,4 hours, From the Willow Creek confluence with the Elk River to the Clark gage is <br />around 0,5 hours, Therefore, the overall travel time between Steamboat Lake and the Milner <br />gage is about 7 hours. <br /> <br />9 <br />