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

Abstract

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 at such structures, obstruct their openings and increase energy head losses, thus increasing flow levels. The extent and variability to which the stage–discharge relationship of a check dam is modified by LW presence has so far not been clear. In addition, sufficiently high flows may trigger a sudden release of the trapped LW with eventual dramatic consequences downstream. This paper provides experimental quantification of LW-related energy head loss 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), with consideration of the flow capacity through their open bodies and atop their spillways. In addition, it was observed that LW is often released over the structure when the overflowing depth, i.e. total depth minus spillway elevation, is about 3–5 times 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 single floating layer. Conversely, dams with high permeability maintain high velocities immediately upstream of the dams and LW tends to accumulate in dense complex 3D patterns. This is because the drag forces are stronger than the buoyancy, allowing the logs to be sucked below the flow surface. In such cases, LW releases occur for higher overflowing depth and the LW-related head losses are higher. A new dimensionless number, namely the buoyancy-to-drag-force ratio, can be used to compute whether (or not) flows stay in the floating-carpet domain where buoyancy prevails over drag force.

Debris Flows, Boulders and Constrictions: A Simple Framework for Modeling Jamming, and Its Consequences on Outflow

“…Relevant literature on wood‐jams includes NILIM (2007); Piton and Recking (2016c); Piton, Horiguchi, et al. (2020); Tateishi et al. (2020); Chen et al.…”

“…Nonetheless, debris flow materials are often shear‐thinning, and the shear rate as a flow passes through the constriction is likely to be very high, suggesting that applying the value of 0.65 directly to non‐Newtonian fluids may not be unreasonable. Previously, μ has been reduced to account for diminished outflow caused by partial obstructions (e.g., Piton, Horiguchi, et al., 2020). However, in reality, the obstruction develops with time during the events, and there is no basis for determining a single representative μ value that approximates the outflow reduction.…”

Modeling Large Wood Transport in Semi-Congested Regime with Multiple Entry Points

“…This is the case of congested and semi-congested transport [27], characterized by a stronger interaction between wooden materials. The wood mass mainly floats on the water surface and is transported as a wood carpet, similar to that observed in the early-stage formation of wood barriers at check dams [28], so that the detailed behavior of a single wood piece does not need to be specifically modeled. Such an observation leads to a different perspective for numerical models, abandoning the deterministic approach, typical of Lagrangian models, in favor of an Eulerian approach that focuses on the transport of overall wooden mass.…”

Blockage Probability Modeling of Large Wood at Reservoir Spillways With Piers

“…The classification of LW includes stems longer than 1 m and with a diameter larger than 0.10 m (Braudrick et al., 1997; Ruiz‐Villanueva, Piégay, Gurnell, et al., 2016; Wohl et al., 2016). The transport of LW can induce dangerous obstructions at hydraulic infrastructures like bridges, weirs and spillways during floods (Allen et al., 2014; Bénet et al., 2021; De Cicco et al., 2016, 2020; Gschnitzer et al., 2017; Hartford et al., 2016; Hartlieb, 2012; Iroumé et al., 2015; Panici & de Almeida, 2018; Pfister, Capobianco, et al., 2013; Piton et al., 2020; Schalko et al., 2020; Schmocker & Hager, 2011).…”