Page - 375 - in Book of Full Papers - Symposium Hydro Engineering

1. INTRODUCTION Integrated management of sediment and flows downstream of dams is still relatively rare. In France, we have developed a "morphogenic water release concept” which aims at optimally design the necessary flows/duration of these releases on the basis of "expected results" clearly defined by the stakeholder. The most common "expected results" range from simple surface cleaning of coarse alluvial substrates (low flow/duration releases) to active lateral erosion of meanders (high flow/duration releases) and removal of alluvial vegetation from the medium bed. One of these objectives may concern reducing sand accumulation in downstream reaches that occurs, as a result of reduced bed- mobilizing flows and high sediment load from tributaries. This is the case of the Selves River (164 km² watershed in the Massif Central - France) downstream of the Maury dam (Fig.1), which creates a reservoir with a 35 million m3 storage capacity. Flow diversion has had a major influence on both average flows and floods. Dam spillovers are infrequent: a single event has occurred since the impoundment of the dam in 1947. The bypassed reach downstream of the dam (approximately 11 km long) is located in a narrow valley with numerous tributaries, which transport large quantities of sand during thunderstorms. Because of flow reduction downstream of the Maury dam, most of this sand accumulates in the riverbed of the bypassed section. This sand accumulation has been noted for several years by both stakeholders and the local hydroelectric company, Electricity of France (EDF). Therefore, it has been decided jointly to implement targeted water releases (i.e., flushing flows) to remove the excess of sand in order to improve fish habitat (refuges and reproduction zones). Fig. 1 The Selves River in France and the bypassed reach downstream of the Maury dam. 375