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Transport of driftwood during heavy rainfall events can intensify the flood hazard. Driftwood accumulates at a bridge and backwater rises. Accumulated logs sink against the buoyancy force and the whole depth region is blocked. However, the mechanism governing the sink motion of accumulated logs at bridges is not fully understood. In this paper, we conducted the driftwood accumulation experiment at a bridge with a single pier to examine the formation and growth of log jam. The observations revealed that the accumulation process could be divided into four phases. Backwater rise was compared with the porous board tests to evaluate the blockage ratio of log jam. Next, we examined the effect of the velocity field on the accumulation process after a large number of logs accumulated at bridges. Simultaneous measurements of the instantaneous flow velocity below the log jam and the instantaneous motion of the incoming logs was conducted by PIV. Then, the effects of the approach flow velocity, channel width and model log length on the log jam formation were examined. These results revealed the governing parameter of the accumulation process (sink motion of logs) and the effect of the driftwood carpet shape.

The paper addresses the influence of intense sediment transport (bed load and debris flow) on the efficiency of the structures aimed at the interception of wood logs. In the literature different devices designed to intercept woods are proposed: steel barriers, net barriers, check dam with steel bars positioned in the opening. In this paper we firstly define some fundamental dimensionless parameter governing the phenomenon, in order to determine a rational criterion to evaluate the efficiency of the different kind of devices. In particular, we deepen the interaction between slit check dams and driftwood, in both bed load and debris flow conditions. Starting from the results of this first analysis, we propose some arrangements of steel bars be installed in the check dam. Through a laboratory experimental investigation, by now conduced in simplified conditions (i.e. spherical mono-dispersed sediments), we define some criteria to obtain the best design parameters for the bars, that is their optimal disposition and spacing, in function of the logs characteristics (mainly the lengths). We investigate also the influence of different lengths of the transported woods, finding a general criterion to evaluate an overall length representing the logs ensemble.

Predicting the motion of driftwood around hydraulic structures such as bridge piers and spur dikes is important. We propose a numerical model for simulating driftwood motion that is based on coupling an Eulerian-type three-dimensional flow model and a Lagrangian-type two-dimensional driftwood model. Laboratory tests were carried out on the driftwood motion in a curved channel and around obstacles to obtain reference data. The computational results showed that three-dimensional flow features considerably affect the motion of driftwood in a curved channel. We defined the driftwood Richardson number (DRI) to classify the three-dimensional behavior of driftwood around obstacles. The experimental results showed that an increasing DRI indicates more three-dimensional behavior by driftwood and a decreased capture ratio by obstacles. We also developed a two-way model in which the drag force from driftwood on the flow is modeled to simulate the backwater elevation and the flow deceleration behind the stacked driftwood. The computational results showed that the two-way model could reproduce the increase of water level and the decrease of velocity at the upstream region of the obstacles. However, such effects caused by the driftwood jamming were underestimated if the jamming happened in a three-dimensional manner.