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Floating Debris Retention Racks at Dam Spillways

Author(s): Lukas Schmocker

Linked Author(s): Lukas Schmocker

Keywords: Dam safety, floating woody debris, large wood blocking, retention racks, spillway

Abstract: Flood events in mountainous areas may entrain and transport large amounts of floating woody debris or large wood (LW). LW may endanger the save operation of dam spillways, as it can result in blocking of the spillway cross section. Already partial blocking of the spillway can decrease the discharge capacity considerably. Due to the resulting backwater rise, the freeboard requirement may not be guaranteed and in an extreme case, uncontrolled overtopping of the dam may occur. The blocking of the Palagnedra spillway during the flood event in 1978 is a prime example demonstrating the hazard potential of transported LW. Two main measures handle LW at spillways and prevent blocking: (1) safe passage of the LW over the spillway; or (2) retention measures in the reservoir. If a spillway is prone to blocking and cannot be modified to guarantee safe passage, the floating debris must be retained in the reservoir. Given both the flow velocities and the reservoir water level variations are small, log or debris barriers may be suitable to retain LW in the reservoir. Otherwise, a rack placed in front of the spillway may prevent blocking of the spillway itself. The effect of such a rack on the reservoir level increase is systematically investigated using hydraulic model tests. According to the results, the rack considerably reduces the backwater rise, as the LW is retained upstream of the weir, where flow velocities are small. The rack must extend considerably below the open weir fields of the spillway to provide enough clear area below the rack for the flow to pass, even when the rack gets blocked. The backwater rise may consequently be reduced and the freeboard requirements can be met


Year: 2017

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