Impact of hydroelectric power releases on the morphology and sedimentology of the bed of the Warche River (Belgium)

The Butgenbach dam on the Warche River was built in 1932 in order to maintain a sufficient supply of water to the Robertville reservoir situated 7 km downstream, for the production of hydroelectricity. During winter months, releases are made almost every day from the Butgenbach dam. From a hydrological point of view, this has resulted in significantly reducing the number of discharges that are higher than bankfull. Despite the reduction in peak discharge, there is a significant increase in the number of efficient discharges (0·6 bankfull). The impacts of these hydrological modifications on the bed morphology and sedimentology below the Butgenbach dam have been studied and the following geomorphological modifications have been identified: a doubling of the width of the channel in 45 years, a reduction in the number of riffles and pools, an increase in the number of gravel bars and islets and an increase in bedrock outcrops in the channel. Moreover, the finest bed particles are mobilized by the almost daily releases, inducing a significant increase in bed‐material size sorting. The reduction of sinuosity and the disappearance of bed differentiation and riffle/pool sequences have produced a diminution of bed roughness and an increase of the competence of the river. Thus relatively small floods can remove the armoured layer.