Drawdown reservoir flushing
Introduction
Dams act as a barrier for sediment transport in river systems. Sediments-laden inflows bring sediments from upstream catchment that will be trapped when reaching the reservoir. Sediments deposit in the bottom of the reservoir and reduce its storage capacity. In geographical areas with very high sediment yields, reservoirs can be filled with sediments after some years of operation, and the reservoirs not fulfill their water storage function anymore.
Reservoir flushing consists in removing deposited sediments out of the reservoir to retrieve the reservoir storage capacity. As a result of flushing, deposited sediments will be mobilised and released downstream the dam inducing a supply of sediments in the downstream river at the time of operation. Reservoir flushing is mainly a reservoir management measure. However, the supply of sediments induced by flushing could help to re-balance the deficit in the downstream river if the timing of the operation and the amount of sediments flushed match the transport capacity of the river (Morris et al. 1998, Tigrek et al. 2011, Kondolf et al. 2014).
Reservoir flushing necessities a complete emptying of the reservoir when planned outside flood periods, thus interferes with hydropower operations. The reservoir is drawn down to establish flow conditions through the reservoir that are similar to river flow conditions. Low-level gates at the dam are opened to let water flows out of the reservoir. The free water surface should be at or lower than the gate level. However, in large rivers with runoff-rivers hydropower plants, sediment flushing does not necessarily require a complete stop of power production
Methods, tools, and devices
During planning
Future needs for flushing operations should be anticipated already in the planning phase of dam and reservoir construction. Appropriate location and level of gate will depend on the characteristics of the dam and should be incorporated in the design of the dam. In some cases, low-level gates can still be implemented after the construction of the dam, but the cost of implementation will be higher.
Long-term time-series of hydrologic and sediment data are necessary to design the dam and anticipate flushing operations. Sediments data consists in suspended sediments sampling and monitoring upstream and downstream the dam, as well as surveys of reservoir sediments. Sediment sampling in river can be monitored using acoustic sampling, optical sampling, or trap samplers.
Hydraulic and sediment transport model can be used to estimate yield of sediments to the reservoir, make projections of future sedimentation load, estimate sediment trapping rate in the reservoir and evaluate sediment transport capacity of the river.
