Author(s): J. S. Wang; Frederick A. Locher
Linked Author(s): Frederick A. Locher
Keywords: Reservoir operation; Sediment deposition; Sediment pass-through
Abstract: Ralston Afterbay Dam is located on the Middle Fork of the American River in Placer County, California, about 80 km North-east of Sacramento, California, and about 52km upstream from Folsom Dam and reservoir. Ralston Afterbay serves as a reregulating reservoir for Ralston Powerhouse, an 86 MW facility which is located on the Rubicon River at the head of Ralston Afterbay. The reservoir is a long, narrow, run of the river reservoir with a volume of about 3. 4 million cubic meters. Since Ralston Afterbay Dam was completed in 1966, the owner and operator, Placer County Water Agency, has experienced problems with sediment that have curtailed operation of the two powerhouses, and have required periodic dredging to keep the backwater from the reservoir from interfering with the operation of Ralston Powerhouse. A sediment by-pass or Sediment Pass-Through (SPT) concept was developed for Ralston Afterbay Reservoir that will prolong the life of the reservoir and reduce the necessity for dredging the reservoir. Basically, the Sediment Pass-Through concept is a change in the way that the spillway gates and low-level outlet at Ralston Afterbay Dam are operated during floods, which will permit more sediment to pass through the reservoir than would be possible with the present operating procedures for the gates and low-level outlet. Thus, the concept is to change operating procedures so that more sediment passes through the reservoir, and less is deposited in the reservoir. This concept was developed using HEC-6 for calculation of the deposition in the reservoir. Use of HEC-6 was complicated by the fact that the bed material consists of sands, gravels and cobbles, with a d50 in the upper reservoir deposits of 20 to 80mm and a significant fraction of cobbles. The application of HEC-6 to analysis of the sediment deposition problem, the potential environmental impacts, and the recommendations for the changes in reservoir operating procedures are summarized in this paper.
Year: 1999