Experimental investigation on turbidity current venting under restrained outflow discharges

Reservoir sedimentation is a worldwide problem affecting sustainable storage use as well as sediment transport downstream of dams. Various techniques are applied for sediment mitigation, among which, venting of turbidity currents. This paper reviews, discusses and evaluates venting of turbidity currents based on an experimental approach. Data acquisition during the tests is accomplished using five different measuring instruments. The efficiency of venting is analyzed by varying the relative outflow discharge. The study reveals the potential of optimizing venting operations in terms of water losses by employing adequate outlet discharges during venting. One of the main transportation processes of suspended sediments in long and deep reservoirs is through turbidity currents. The latter are formed during yearly floods when sediment-laden flows arriving from the watershed reach a reservoir and plunge beneath the clear water surface due to their high density. After plunging, a turbidity current is formed and flows on the bed of the reservoir, entraining the suspended sediments near the dam. In case no outlets/intakes operate, suspended sediments of the turbidity currents will settle and fill the reservoir on the long-term. Therefore, in reservoirs where turbidity currents frequently occur or present the main source of inflowing sediments, venting represents the most adequate means to evacuate the entrained sediments. This technique is more and more appreciated by dam operators mainly for economic but also for environmental reasons (Morris & Fan 1997). In fact, unlike other operations such as flushing or dredging, venting of turbidity currents deals with a direct transiting of the sediments while they reach the dam. Therefore, if applied in optimized conditions, the main advantage of such an operation is that sediments are evacuated before they settle. Additionally, drawdown of the reservoir is not needed, which makes this technique even more appealing particularly in arid regions where water is in shortage (Brandt 2001). However, venting of turbidity currents in optimized conditions can be complex. In fact, a successful venting operation is directly linked to a good knowledge of the dynamics of the turbidity currents reaching the dam. Since such currents occur in flood conditions, field measurements are