Large wood remobilization in Asakura (North Kyushu, Japan): Adapting strategies to climate change and rural population depletion

Abstract In the aftermath of the heavy rainfall events of 2017 in North Kyushu, Asakura (Japan), driftwood-related disaster risk has proven to be as real as it was in 1933, when wood-laden debris-flow invaded the city centre of Kobe City near Sannomiya station. Despite a temporary truce obtained thanks to extensive Sabo dam constructions, climate change, the return of forest on agricultural land due to rural exodus and an ageing population are once again increasing driftwood hazard and disaster risk. In the face of these novel challenges, new management strategies may need to be considered, and the present contribution aims to propose a new approach to driftwood hazards and disaster risk. For this purpose, the present contribution is analysing the 2D hydrodynamic of scenario-based floods on driftwood deposited after the 2017 heavy-rainfall event in Asakura. The boundary conditions for the simulation were generated using UAV photogrammetry calibrated against existing DEM data to generate a DEM at 25 cm horizontal resolution and an orthophotographs at 5 cm resolution. The results show that with the new river configuration (post-2017), an instantaneous peak flow of 90 m3/s is necessary to flood the areas where the driftwood has stopped with a water depth of >40 cm. This is 9 times the mode discharge calculated from the inlet geometry. The majority of the driftwood is thus not an immediate hazard, and leaving the wood instead of removing it should be considered as a management strategy whenever it is feasible. Recent research in Western Europe and Northern America has already shown the importance of driftwood for biodiversity and wildlife, and combining both environmental and hazards and disaster risk objectives may be a solution. For this purpose, the authors propose that medium-size ditch and artificial “abandoned channels” could be created in order to trap the wood in the floodplain, so that the cost from removing the wood can be alleviated, horizontal trapping could complement vertical trapping at slit dams, and the local wildlife could benefit from the wood decomposition in the floodplain.