Generation and fate of fine sediment from dam flushing

Abstract

Freshwater is stored in reservoirs, obtained by constructing dams across rivers, for domestic and industrial uses, irrigation and hydropower generation. Dams not only regulate the water flow downstream but also trap the incoming sediment, which results in a gradual reduction in the storage capacity of their reservoirs. Flushing is a measure for re-establishing at least a part of the lost storage capacity and consists in releasing sediment to the downstream river by opening the bottom gates of the dam. Most studies dealing with flushing operations focus on the ability to restore the water storage capacity of the reservoir. Fewer studies focus on the impact of flushed sediment on downstream rivers, and these studies do not concentrate on fine sediment. Fine sediment is responsible for supplying nutrients to the downstream region, but high concentrations of suspended solids might alter the water characteristics and create stress conditions to fish and other aquatic biotas. Fine sediment deposition upon or within gravel might damage the spawning habitats of several fish species. Hence the study of the fate of flushed suspended sediment is key to achieve sustainability of flushing operations. This research is conducted on the Sandy River, Oregon, USA. The study is carried out by using a numerical model to generate and follow the fate of suspended sediment from a virtual dam in the downstream river and study its impact on salmons. The study site is characterized by an exceptional availability of data related to Marmot Dam removal (2007), allowing for model calibration and validation. Reservoir flushing is reproduced in by assuming the presence of the dam and applying virtual gate opening procedures. The results show that sediment type and availability in the reservoir govern suspended sediment concentration and processes in the downstream river. Deposition of fine sediment mostly depends on fall velocity and is guided by the presence of morphological forms (channel expansions and restrictions). Release of large amounts of fine sediment threat the eggs and larvae stages of Chinook salmon, in two ways: by increasing the stress due to exposure to high concentrations of suspended solids; by reducing the surface area of suitable spawning habitats. The stress condition increases in the downstream direction because the duration of exposure to high concentration becomes longer. The suitable area for spawning shows a 62% reduction after 9 days of flushing, due to fine sediment deposition. This means that flushing operations have a negative long-term impact on salmon spawning habitats. Planning of flushing operation should take into account the recovery of spawning habitat, which is dependent on the incoming discharge and reservoir sediment, whereas further study about the recovery time due to suspended sediment doze needs to be done to optimize the flushing and minimize the impact on salmon habitat