Victoria Lake
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VENTING TURBIDITY CURRENTS IN RESERVOIRS Josef Schneider1, Gabriele Harb1, Hannes Badura2 1 Institute of Hydraulic Engineering and Water Resources Management, Graz University of Technology, Graz, Austria 2 Verbund Hydro Power AG, Vienna, Austria
Description Turbidity currents are generally understood to stand for stratified flows in lakes or reservoirs caused by the differences in density of suspended sediment concentrations between the inflowing and the standing water. Turbidity currents occur mainly during floods since high discharges and suspended sediment concentrations are often a consequence. What is significant about turbidity currents is the sudden submersion of the clear stagnant water by the turbid water, the plunge point. Subsequently, the turbidity currents layer at the bottom of a reservoir, continue moving toward the dam within the former streambed, and come to a halt when reaching the dam and transform in a so-called submerged muddy lake. The barrier of the dam causes the suspended material to settle, which is in many cases undesirably. The probability of turbidity current events is a matter of correlation between hydrological and sedimentological predispositions, as well of the topography of a reservoir, e.g., its geometry, which is decisive for the tolerance of turbidity currents; since the reservoir has to be relatively short and straight with a substantial gradient, the valley should be V-shaped with steep slopes with the bottom outlet situated at a low level in the vicinity of the dam. Fundamentally, such turbidity currents could be vented through a reservoir by opening low-lying gates like bottom outlets at precisely adjusted moments. The advantage of venting turbidity currents is in many cases the
desired reduction of sedimentation, the less frequent necessary variant of flushing, and the more ecological justifiable way to release sediments out of a reservoir. According to respective literature, venting turbidity currents through reservoirs has been dealt with so far mainly in Asia, America, and Africa. Efficiencies between 50% and 80% have been documented concerning the prevention of inflowing sediment deposition, and that is very representative. Not all reservoirs are suitable for supporting the venting of turbidity currents. This is predominantly a matter of whether turbidity currents occur at all along the reservoir bottom, and most importantly, whether they do not dissolve on their own after a certain distance. It is conceivable that turbidity currents mix with clear lake water along their way and reduce, therewith, their concentration differential, on one hand or on the other hand, it is also imaginable that they remobilize already-deposited fine material along their path and become, thus, consequentially stronger, heavier, and more accelerated. Knowledge about the flow situation, travel time between inlet and bottom outlet, as well as sediment concentrations, and temperature distributions in the reservoir are essential to assess the feasibility of venting. Measurements of turbidity pe
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