Hazards due to large wood accumulations: Local scour and backwater rise

Abstract

Large wood (LW) in rivers increases the flow variability and provides habitats for various species. During flood events, transported logs can accumulate at river infrastructures and increase the flood hazard. LW accumulations result in an upstream backwater rise and may increase local scour, for instance at bridge piers. Consequently, estimates of the resulting backwater rise and local scour are necessary to improve the flood hazard assessment. This study presents the findings of flume experiments with a movable bed on local scour and backwater rise due to LW accumulations. The approach flow conditions and the bed material were varied systematically for a specific LW accumulation volume. For all experiments, the initial condition for the bed material was defined as weak transport, since the bed shear stress was slightly below the critical bed shear stress for incipient motion. The inflow Froude number was identified as the governing parameter for backwater rise due to LW accumulations. The present study confirms the hypothesis that the resulting local scour reduces backwater rise. For the local scour, the unit discharge and the grain size diameter are the decisive parameters.

Numerical Modeling of Potential Large Wood Entrainment in Rivers: Application of Hybrid Modeling in the Inter‐Dam Reach of the Dyje River, Czechia

“…(2014b) refer that LW pieces oriented parallel and oblique to the flow direction are transported in the line of maximum velocity in their original orientation. Schalko (2018) demonstrated that the deposition probability was six times lower for those pieces transported in parallel orientation with the flow direction. In the studied river reaches, the LW dimensions suggest the fast onset of LW transport; however, their prevailing orientation and log‐log and log‐living tree interactions in the channel and inundated floodplain may be limiting factors for long transport trajectories.…”

Flume Experiments Evaluating the Efficacy of a Large Wood Trap Featuring Horizontal Rods

“…Despite such benefits, trees that fall from forest slopes after heavy rainfall become LW that accumulates in rivers, posing hazards to people and property, e.g., [6]. LW disasters caused by heavy rainfall, occasionally accompanied by sediment-related disasters, have been reported in many countries [7][8][9]. In Japan, LW disasters occurred in Asakura City (Fukuoka Prefecture) and Hita City (Oita Prefecture) during heavy rainfall in 2017 in northern Kyushu [10], and in Hiroshima City and Kure City (both in Hiroshima Prefecture) during heavy rainfall in 2018 [11] (Figure 1).…”