Experimental Measurement of the Resistance to a Dam-Break Flow on a Sloping Channel Covered by Dense Vegetation

The dam-break problem is associated with flow resulting from a sudden release of water behind a vertical wall or dam [Whitham, 1955]. Dam-break flows are of considerable practical and academic interest due to both flow characteristics (unsteadiness and inertia being balanced by hydrostatic pressure gradients and resistive forces) and the enormous impact on the downstream territory. Here the focus is on small dams, meaning reservoirs having height and crest length comparable with the size of vegetation downstream (e.g. woodsland, orchard).

The effect of the distributed drag force on the evolution of the water surface profile following an instantaneous dam-break is studied by mean of a physical model. The experimental setup consists in a channel (12m long, 0.5m wide, 0.6m deep) where a gate is set at 1/3 of channel length. Downstream of the gate a rod canopy is set. The lateral walls of the channel are transparent and the water is added with rhodamine. After the instantaneous lift of the gate water flow along the channel and 3 cameras image the profile made easy detectable by 2 green lasers. Experiments investigated different slopes (0%,1%,2%,3%) and different water height behind the gate (0.15m, 0.20m, 0.25m,0.30m) both in case of smooth and vegetated (0.10m height rods array) downstream channel. In Melis et al. (2018) the authors show that, when employing Manning`s formula with constant roughness, it was possible to match the measured wave front speed but not the precise shape of the water surface profile. The agreement between measured and modelled water surface both in space and time is achieved adopting a distributed drag force with a computed constant C_d ≈0.4 which is smaller than the constant computed for uniform flow experiments.

In this study new experiments are presented. The force against a single rod is measured. Water is seeded by small neutro-buoyant spheres (0.0006m diameter) to study the velocity field both in case of smooth channel and in presence of rods. The goal is to experimental understand the reasons behind the reduction of the drag coefficient aforementioned.

Whitham,G.B.(1955),The effects of hydraulic resistance in the dam-break problem, Proceedings of the Royal Society of London: A, 227 (1170), 399-407.

Melis M., Poggi D., Fasanella G.O.D., Cordero S. and Katul G.G., Resistance to flow on a sloping channel covered by dense vegetation following a dam-break, submitted in August to Water Resource Research (under review)