Effects of driftwood on erosion of debris flows

Debris-flow Flume experiments of deposition and fan formation have been mostly conducted over hard non-porous plane. However, the surface a debris-flow travels on is influencing the dynamics of the flow and the deposition process as it has been shown recently. We have argued that (1) water exchange occurs with the surface and (2) material exchange (erosion and deposition) is also occurring in the deposition area. Continuing from this set of experiments, we are now attempting to clarify the role of debris in the debris-flow, one of them which are often neglected is wood debris.

In the present contribution, the authors attempted to quantify the role of wood debris on deposition and deposits in a controlled environment. For the present experiment, we used a flume with a reception pan that is 185.6 cm length x 95.6 cm width, with walls 26.2 cm high. The channel is 400 cm long, 25.0 cm high, and 18.5 cm wide. Repeating earlier experiments without wood debris, the receiving pan was spread with a 1-2 cm thick sediment layer. In the channel, a set of wood debris were erected. From this setup, the authors ran multiple experiment setups with 5 repeats of the same experiment each time. Using video-camera recording of the flow, photogrammetry of the deposits. The authors also collected sediment samples over the surface to compare porosity and density of the deposit.

The results have shown that the wood debris increased the variability in density and porosity in the deposit, regardless of the starting setting with woods, in comparison to “no-wood” patterns. The authors argue that this is due to mixing with the wood debris during deposition, modifying the semi-linear energy loss on a typical deposition fan. The mixing is further complicated as wood debris collided with each other based on the video analysis, so that the variability is not only due to the characteristics of the debris, but how it interacted with other debris and with the sediments. The authors further argue that the difference in erosion of the substratum, with wood-debris rich experiments tended to erode less, may be due to reduced momentum due to the energy consumed to transport the wood debris.