The favourable conditions for flushing are: reservoirs located in narrow valleys with steep slides, steep longitudinal slopes, presence of low-level gate at the dam, river discharge sufficient to mobilize and transport sediments (Morris and Fan 1998), conjointly to an impoundment ratio (reservoir storage divided by mean annual flow) not exceeding 4 % (Sumi 2008).įrom an ecological point of view, the capacity of the downstream river to transport the flushed sediments further downstream, as well as the potential change in water quality during the flushing need to be considered. Methods, tools, and devices During planningīoth technical and ecological challenges should be considered prior to flushing reservoirs and included in a comprehensive analysis about the occurrence and the extent of sedimentation processes in the reservoir.įrom a technical point of view, the succeed of flushing will depend on the amount of deposited sediments which have been scoured and if a significant part of the capacity storage has been recovered. However, in large rivers with run-of-the-river hydropower plants, sediment flushing does not necessarily require a complete stop of power production. The free water surface should be at or lower than the gate level. Low-level outlet gates at the dam are opened to release water flows out of the reservoir. The reservoir is drawn down to establish flow conditions through the reservoir that are similar to the river flow conditions prior to dam construction. Reservoir flushing necessitates a complete emptying of the reservoir when planned outside of flood periods, thus interfering with hydropower operations. 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. Reservoir flushing is mainly a reservoir management measure. As a result of flushing, deposited sediments will be remobilised and released downstream of the dam inducing a supply of sediments in the downstream river at the time of operation. Reservoir flushing consists in removing deposited sediments out of the reservoir to retrieve the reservoir 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. Sediments deposit in the bottom of the reservoir and reduce its storage capacity. Sediments-laden inflows transport sediments from upstream catchment that will be trapped when reaching the reservoir. Figure 2: River Aare in Haslital (CH) with highly increased suspended load after the flushing at reservoir Räterichsbodensee.ĭams act as a barrier for sediment transport in river systems.
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