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. 2. Available Head When a dam or diversion is constructed, it blocks the normal river flow that is carrying sediment. The flow of sediment downstream is slowed, and sediment settles out upstream within the reservoir area. As river flows are reestablished downstream from the dam, new sediment is picked up from the river bed. Over a period of time, natural degradation or lowering of the river bed through erosion will occur. For the Webster Hill Dam and power plant site, the amount of degradation is estimated to be between 10 and 15 ft. To obtain this benefit earlier, and to increase the available head at the site, the river bed downstream from the dam would be excavated or dredged at depths varying from IS ft at the dam to 0 ft at a point approximately 6,000 ft downstream. . Lowering the downstream riverbed when the dam is constructed would allow the tailwater elevation to be lowered to an elevation of 5,180 ft compared to a present tail water elevation of 5,190 ft, thereby i ncreas i ng the avail ab 1 e head from 60 to 70 ft. The avai lable head of 70 ft was used for the design considerations and preliminary general arrangement of the power plant. The available head was determined by assuming that the reservoir pool is maintained at the normal elevation of 5,250 ft as much as possible. Tailwater elevations were derived by lowering the existing tailwater curve by about 15 ft through downstream channel excavation. Head losses in the penstock intakes, penstocks, valves, and exit flow were calculated and deducted from the available head for each monthly average flow. . 3. Plant Configuration The number, size, and type of hydroturbine units selected is based on many factors, including available flow duration, effective head, and the expected mode of operation. It is often found that the capac ity factor of a power plant can be 3686-a/9 V-2