Measurement and Analysis of the Drag Coefficient of Wood Accumulations at an Ogee Crested Spillway

Trunks, branches, and wooden pieces transported by rivers during flood events can reach dams and accumulate at spillways. The reduced flow velocity and the presence of sluice gates may increase the probability of wood accumulation and gate malfunctioning. To prevent these effects, rakes or floating booms are installed upstream of the crest, thus requiring the removal of the accumulated material, and interrupting floating sediment continuity. An experimental campaign is carried out to increase the knowledge of the hydrodynamic behavior of wood jams upstream of an ogee-crested spillway. The drag coefficient for samples with different dimensions, roughness, submergence, and distance from the crest is measured under a hydraulic condition just below the one that, according to literature suggestions, should facilitate wood passage. The number of identical trials required to obtain statistically sound results is also analyzed. Experimental results show that the drag coefficient of wooden samples upstream of a spillway increases with the submergence of the samples and with the proximity to the spillway. The drag coefficient increases with submergence more than measurements in open-flow conditions, due to flow-wood-spillway dynamics. The distance from the spillway strongly affects the drag coefficient, followed by the shape and surface roughness. The blockage ratio for a spillway is defined, and the relation between the drag coefficient, the blockage ratio, and the non-dimensional distance is derived to compute the hydrodynamic force on a wood accumulation upstream of an ogee crested spillway.