Drawdown reservoir flushing

Introduction

Figure 1: Scheme of reservoir flushing (Kondolf et al.2014).

Figure 2: River Aare in Haslital (CH) with highly increased suspended load after the flushing at reservoir Räterichsbodensee (Photo: Markus Zeh).

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

The approach to assess the suitability of this measure would be to investigate if shelter and morphological variation seem to be limiting the development of the fish populations. If this is the case, the introduction of dead wood should be considered introduced, on equal basis with other measures mitigating the same problem. If the conveyance capacity is critical, the effect of the introduction dead wood should be considered. Standard tools for hydraulic calculations can potentially be used, but according the authors' knowledge, this has to a very little extent been done.

During implementation

Access to local wood would make to construction of this measure very simple, by simply bringing a chain saw and dragging the timber to a proper place in the river where the wood would not drift away as soon as the water level increases.

During operation

There is basically no maintenance needed for this measure, except that the process of adding dead trees might be needed to be repeated if the wood is gradually degraded or drifts away in larger parts. Monitoring of the effect of the measure can be done by for instance electro-fishing in order to assess the density of juvenile fish and compare this to densities in sections where shelter is limited.