Abstract

  Excessive sediment accumulation at hydroelectric facilities reduces impoundment capacity, escalates turbine wear, and increases the potential for accidental sediment releases. Habitat could be adversely affected by an increase in how embedded they are if a sediment release exceeds downstream transport capacity. As a response to an inadvertent sediment release during maintenance, and to provide a basis for future sediment management decisions, an evaluation of the Salmon River, New York, and two run-off river impoundments was performed. River morphology, substrate composition, hydraulics and sediment transport capacity data were collected at 23 river and 24 impoundment cross-sections.
   Potential sediment management strategies included reservoir routing, sediment flushing, dredging, and erosion control in upland contributing basins. Because the transport capacity of the river portions exceeds that of the impoundments by a significant margin during high flows, a sediment-pass-through operation (hybrid of high-flow routing with limited reservoir flushing) would provide a cost-effective long-term sediment management solution. To assess the feasibility of a pass-through operation, the magnitude, timing, and duration of the required high-flow event were examined. Using empirical techniques based on hydrologic or geomorphic characterization and various critical shear criteria for sediment entrainment, a range of flows for maintaining sediment suspended in the downstream water column without causing excessive deposition of fines in the gravel and cobble bed river, was estimated. Flows sufficient to scour fines from gravel and 15-cm cobble were computed for each cross section, and used to define a minimum threshold (750 cfs) for pass-through operations. The pass-through would be most effectively applied during early spring, while snow cover reduces downstream tributary sediment loading. A 24 to 36 hour duration provides sufficient travel time and substrate flushing time.   Carefully designed sediment pass through reservoir operations may provide a satisfactory balance of long-term hydraulic, ecological and operational considerations.