Seite - 381 - in Book of Full Papers - Symposium Hydro Engineering

SUMMARY Integrated management of sediment and flows downstream of dams is still relatively rare, including in France. Flow experiments are often necessary to determine appropriate management actions and to verify that environmental objectives (often various) are being met. One of these objectives may concern reducing sand accumulations in downstream reaches that occur 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, which creates a reservoir with a 35 million m3 storage capacity. Flow diversion has 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 section downstream of the dam is approximately 11 km long. This section is located in a narrow valley with numerous tributaries, which load 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 documented by both stakeholders and the local hydroelectric company, Electricity of France (EDF), for several years. It has therefore been decided jointly to implement targeted water releases to remove the excess sand in order to improve fish habitat (refuges and reproduction zones). The first empirical calculations estimated that the displacement rate of the sand would be about 10 m3/s. Implementing a water release at a discharge below this value would risk using water with no significant effect on riverbed morphology. Therefore, 3 operational water release tests (10, 15 and 20 m3/s) were carried out in September 2016 to define the discharge necessary to transport a maximum of sands without mobilizing the coarser elements (gravels and pebbles) necessary to aquatic organisms. The release duration was limited to 5h for each test to minimize environmental impacts and potential problems associated with dam operations. Suspended sediment monitoring carried out during the releases show a recovery of the stock of fine sediments (clays - silts) present in the bypassed section. At one of the stations, fluorescent particles (2-4 mm) were added prior to releases to observe the distance traveled by the particles as a function of the released discharge. In addition, biological monitoring is underway to elucidate the long-term ecological effects of the releases. The latter two approaches will only be partially discussed here. The results indicate that the flow rates were selected in the appropriate range for the stated objectives. Furthermore, it was possible to determine the optimum discharge for future releases: a flow rate equal to 10 m3/s allows a significant mobilization of suspended sediments and a flow rate of 15 m3/s allows for substantial mobilization of the sandy elements. Above 15 m3/s, although the risk of mobilizing coarse elements remains relatively low, the cost-benefit ratio of this discharge level does not appear to be advantageous. From an environmental perspective, the different objectives were achieved. 381