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Natural Hazards and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/nhess-2020-158
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/nhess-2020-158
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  02 Jun 2020

02 Jun 2020

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This preprint is currently under review for the journal NHESS.

Open check dams and large wood: head losses and release conditions

Guillaume Piton1, Toshiyuki Horiguchi2, Lise Marchal1,3, and Stéphane Lambert1 Guillaume Piton et al.
  • 1Univ. Grenoble Alpes, INRAE, ETNA, 3800 Grenoble, France
  • 2National Defense Academy, Yokosuka, 239-8686, Kanagawa, Japan
  • 3AgroParisTech, Paris Institute of Technology for Life, Food and Environmental Sciences, 75231 Paris, France

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.

Guillaume Piton et al.

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Latest update: 14 Aug 2020
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Short summary
Open check dams are flood protection structures trapping sediment and large wood. Large wood obstructs openings of dams, thus increasing flow level. If flow level become higher than the dam crest, the trapped large wood may overtop the structure and be suddenly released downstream, which may eventually obstruct downstream bridges. This paper is based on experiments on small scale models. It shows how to compute the increase of flow level and the conditions leading to sudden overtopping.
Open check dams are flood protection structures trapping sediment and large wood. Large wood...
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