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<br />'. . <br /> <br />.' <br /> <br />.' <br />.. <br /> <br />1725 <br /> <br />With the recommended channel. maintenance flow hydrograph a . maximum- <br />monthly release of 3384 acre-feet would 'be required. However,..if the <br />average discharge released for other purposes in a given month were (for- <br />example) 60 cfs, the total volume released would be about 3570 acre-feet, <br />or about 186 acre-feet more than channel maintenance flow requirements. <br />This release would be relatively constant through the ,month (Fig. 7), in <br />contrast to the vl\riable releases required by the channel maintenance <br />hydrograph. Assuming that the 60 ds generated by these "other" releases <br />satisfied the discharge equal to or less than 60 ds required ,in the <br />channel maintenance hydrograph, the channel maintenance flow demand on the <br />te~1:Voir would be that volume of the channel maintenance hydrograph above <br />60 cfs, or 1527 acre-feet (see Fig. 7). This volume vould have to be <br />released in the prescribed manner in order to satisfy channel maintenance <br />flow requirements. <br /> <br />Based on this procedure, the channel maintenace flov requirements vere <br />integrated into the operational: model. Results indicated that over a <br />30-year period for which historical data are available, .the average annual <br />channel maintenance flow requirement vould be' 995 acre-feet under a West <br />Slope water marketing scenario, or 686 acre-feet under an East Slope 'water <br />marketing scenario (a potential short-term condition), compared to 'an <br />average annual yield of 23,B90 acre-feet. <br /> <br />.-;:- <br /> <br />CONCLUSIONS <br /> <br />Development of the channel maintenance hydrograph. as outlined above, <br />provides a reasonable solution considering site, characteristics and the <br />site specific sediment transport conditions of the Rock Creek Proj ect. <br />Both the existing supply limited condition of Rock Creek, particularly for <br />finer particle sizes. and reduced downstream sediment supply for post- <br />project conditions have been accounted for in the analysis. The analysis <br />is considered conservative since only gravel sized particles were assumed <br />to be trapped in the reservoir; consequently, the post-project downstream <br />flows were required to transport the pre-project supply of medium and <br />coarse sands, a large portion of which will most likely also be trapped- in <br />the reservoir. The conservative nature of this assumption is further com- <br />pounded by lack of a significant sediment supply to reaches downstream of <br />the project. From the proj ect downstream to the confluence of Egeria <br />Creek, which is nearly to the confluence of Rock Creek vith the Colorado <br />River, no major sediment producing tributaries enter Rock Creek. Conse- <br />quently, pre-project sediment supply is not achieved at any point down- <br />stream of the project. <br /> <br />Vegetation encroachment concerns have also been addressed by the <br />analysis procedure. The peak discharge recommended to meet sediment <br />transport requirements was evaluated considering maximum velocity and <br />inundation depth. It should be noted that the maximum daily drawdown rate <br />calculated from a serict application of the Chapter - 30 procedure was <br />adopted in developing the recommended channel maintenance hydrograph; <br />therefore, the resulting procedure capitalizes on certain elements of the <br />basic USFS procedure and modifies other elements to more accurately address <br />site specific conditions. <br />