Abstract. Open check dams are strategic structures to control sediment and large wood transport during extreme flood events in steep streams and piedmont rivers. Large wood (LW) tends to accumulate against such structures, to obstruct their openings and to increase energy dissipation and thus, flow levels. To which extent open check dams' stage-discharge relationships are consequently modified by LW presence was not clear so far. This question is key (i) to estimate how much bedload transport might be trapped in the related backwater areas and (ii) to estimate how high is the overflowing depth atop the structure. These flows, when sufficiently high, might trigger a sudden release of the previously trapped LW with eventual dramatic consequences downstream. This paper provides experimental quantification of LW-related energy dissipation and simple ways to compute the related increase in water depth at dams of various shapes: trapezoidal, slit, slot and SABO (i.e., made of piles), including flow capacity through their open body and atop the spillway. It was additionally observed that LW is often released over the structure when the overflowing depth, i.e., depth above the spillway, is about 3–5 the mean log diameter. Two regimes of LW accumulations were observed: dams with low permeability generate low velocity upstream and LW then accumulates as floating carpets, i.e., as a floating single layer. Conversely, dams with high permeability maintain high velocities close to the dams and LW tends to jam them in dense complex 3D patterns because drag forces are stronger than buoyancy and logs are sucked below the flow surface. In such cases, LW releases occur for higher overflowing depth and LW-related head losses are higher. A new dimensionless number, namely the ratio buoyancy to drag force, enables to compute whether or not flows stay in the floating carpet domain where buoyancy prevails.
The effects of large wood elements during an extreme flood in a small tropical basin of Costa Rica
