Author(s): Elisabetta Persi; Gabriella Petaccia; Elena Pibia; Stefano Sibilla
Keywords: Spillway; Ogee crest; Dam; Large wood
Abstract: Trunks, branches, and wood debris transported by a river can reach dams and accumulate at spillways, as revealed by past events like the disaster of the Palangedra dam (Switzerland, 1978) or the most recent accumulations at the Comelico dam after the Vaia storm (Italy, 2018). The reduced flow velocity and the presence of sluice gates may increase the probability of wood accumulation, while the interaction with the gates can lead to malfunctioning, with additional risks both for the structure and for the population living downstream of the dam. Measures to prevent wood accumulation include installing rakes or floating booms that keep wood far from the crest, thus requiring the removal of the accumulated material and interrupting floating sediment continuity. Alternative solutions are gate maneuvers that can facilitate wooden debris to pass over the dam, although their efficacy is uncertain, or the re-design of the dam crest, like the removal of walls and bridges, that implies additional costs and is not always feasible. A deeper knowledge of the hydrodynamics of wooden debris close to the spillway crest can help chose the most preferable solutions. The experimental campaign here presented, performed at the Hydraulic Laboratory of the University of Pavia, aims at analyzing the hydrodynamic behavior of cylindrical woods upstream of an ogee crested spillway. The measure of the drag coefficient is performed for different cylinder dimensions, roughness, submergence, and distance from the crest. The water level is set equal to or higher than the reference value that should facilitate wood passage. Top view recordings are also performed. The experimental results show that the drag coefficient at the spillway increases with respect to the free-water value, mainly due to the blockage ratio and to the flow velocity distribution. Upstream values are lower, showing the reduction of the hydrodynamic force. If the water level at the crest is higher than the value suggested by literature, woods tend to float freely and to pass over the dam, despite a reduction of the drag force due to the lower velocity.