PROJ00492 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I 4. Proposed Reservoir Operation I I I I The majority of the annual sediment load of the Colorado River is transported during the spring runoff months of May and June. For example, 67% of the annual sediment load as measured at De Beque for water year 1975 was carried in May and June alone. This was a fairly average year and in a wetter year the effect would be even more pronounced. The fact that flood flows and sediment transport capacities are so markedly seasonal and are coincident with the period of excess water and reservoir refilling, points to a reservoir operation procedure to limit the fOrnlation of a delta which could aggravate flood stages, to a point located approximately one mile or more downstream of the Parachute Bridge. The proposed mode of operation would consist of lower- ing the reservoir in average to wet water years to El 5058 at the beginning of the spring runoff and holding it at this elevation until the peak of the spring hydrograph had passed. As the hydrograph falls the reservoir would be refilled. Sediment that deposits in the reach near Parachute after reservoir refilling and the sedimentation that occurs during the low-flow months would be scoured downstream deeper into the reservoir during drawdown the following spring. A typical inflow-outflow spring hydrograph and reservoir drawdown curve are shown in Fig. II-11, assuming a full reservoir prior to the start of the spring runoff hydrograph. When dry years are forecast, the reservoir would not be drawn down. Any sediment deposition would be scoured downstream in a subsequent wet year. This mode of reservoir operation combined with a runoff forecasting program should prevent significant deltaic deposition in the Parachute area and flood stage aggravation by the Project without any decrease in the potential water yield. I I I I I II-23