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~", . " ~. 001557 . . POST-PROJECT.CHANNEL MAINTENAN~E HYDROGRAPH ; Establishment of the 'other' flows of the . post-project chaimel main- tenance hydrograph (rise and recession flows) could not be accomplished with the USFS procedure since the procedure is empirically based and assumes no change in sediment supply and equilibrium conditions. However, the USFS procedure does provide an estimate of maximum daily drawdown rate and, as discussed above in relation to encroachment, duration of the required bypass flow. Using a 20 cfs drawdown rate (from strict applica- tion of the procedure) and a 16-day bypass flow duration, the total annual bypass volume for Site A was 5858 acre-feet and for Site B, 4220 acre-feet , (see Fig. 5). .' . Preliminary ,results from other disciplines . of the EIS study team indicated that Site B. .is ,the preferred Rock Creek site. Under the sug- gested 50 cfs bankfull discharge, both velocity (5.5 fps) and submergence may contribute toward minimizing:~egetation encroachment. Consequently, at Site B it may be more effective to provide higher discharges for. shorter periods of time, so that a greater portion of the wetted perimeter.' can be: inundated. Under this assumption, a modified channel maintenance.' hydro- . . graph was developed and recommended (see Fig. 5) that achieved. a 190 cfs discharge for 2 days, providing 100 percent coverage of the pre-project wetted perimeter. The total annual bypass volume for this hydrograph was 5610 acre-feet, compared to the 4220 acre-feet volume for 50 cfs for 16 days. In other words,. it takes a 33 percent greater bypass volume to prevent vegetation encroachment on 11 percent of the wetted perimeter. ".--::: Fig. 6 illustrates the relationship of the water year 1980 hydrograph, the channel maintenance hydro graph from a strict Chapter 30 interpretation and the recommended post-project, Site B, channel maintenance hydrograph. In order to eliminate need for mechanical maintenance, this 5610 acre-feet hydro graph is suggested as the preferred alternative and is considered as effective as the pre-project channel maintenance hydrograph which required a bypass volume of 13,200 acre-feet. RESERVOIR OPERATIONAL MODEL The Colorado' River Water Conservation District has developed an: operational model for the Rock Creek Reservoir. Assuming Site B will be the preferred site on Rock Creek, the recommended Site B channel mainte- nance hydrograph (Fig. 6) developed above was incorporated into the opera- tional model. To integrate channel maintenance flows into the operational model, it was assumed that other controlled releases could be used to meet a .part of channel maintenance flow requirements. Since the channel main- tenance release is a single event per year, the release was timed to occur in that month, from April to September, when "other" controlled releases were greatest. Any additional water required to meet the channel mainte- nance flow requirement would represent a demand on the reservoir, over and above the normal monthly controlled release. It is important to realize that the "other" releases are reasonably constant through a given month, while the channel maintenance flow releas.e requires a hydrograph of well defined, variable releases (Fig. 5). Therefore, determination of the channel maintenance flow requirement must be based on the discharges released that month, not the total volume released.