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Backwater Rise Due to Large Wood Accumulation at Protruding Piers of Dam Spillways

Author(s): Barbara Stocker; Adriano Lais; Isabella Schalko; Robert M. Boes

Linked Author(s): Isabella Schalko

Keywords: Dam spillway; Large wood accumulation; Backwater rise; Bridge piers; Dam safety

Abstract: During flood events in mountainous areas, huge amounts of large wood (LW) may be transported. When reaching a dam overflow spillway in operation, LW can result in blocking the spillway cross section and therefor decrease the discharge capacity considerably. Due to the resulting backwater rise, freeboard requirements may not be guaranteed and even uncontrolled dam overtopping may occur. As a prime example serves the blocking of the Palagnedra spillway (Switzerland) during a flood event in 1978. To prevent spillways from blocking with LW, either safe passage over the spillway or retention measures in the reservoir are possible. If a safe passage of LW over the spillway cannot be guaranteed and blocking may occur due to e.g. bridge piers, LW must be retained in the reservoir. By placing a retention rack upstream of a spillway, LW accumulates at the rack instead of directly at the spillway crest, allowing the water to flow below the LW accumulation and therefor leading to a lower backwater rise. Piers protruding in upstream direction have a similar function as a LW retention rack. A few existing studies investigated LW blocking at spillways using flume experiments with channelized approach flow and a weir height W up to 3 times the design head of the spillway weir HD. In this study, the effect of the pier overhang pc (distance from pier nose to weir crest) on the resulting backwater rise in case of LW accumulation has been investigated, using a physical model with a model scale factor of λ = 30. The model consists of a curved arch dam with a height W of 30 m (prototype dimensions), a WES-shaped spillway weir with a design head HD = 2 m and five similar spillway openings with a width B = 6 m each, separated by four protruding piers. The model allows close-to-prototype approach flow conditions with radial flow and very low velocities in the reservoir. A formula is provided to estimate backwater rise for LW blockage at piers of high overflow structures in dependence of the pier overhang pc. The results show backwater rise of less than 5% of the overflow height Ho, if pc ≥ Ho. Additionally, the characteristic wood volume, i.e. the volume that leads to the main part of the backwater rise, is described and can be estimated. In general, the article helps understanding LW accumulation formation at piers of high overflow structures, provides information on the required overhang and enables backwater rise estimations for blocked spillways.


Year: 2022

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