DEVELOPMENT OF AN UNSTRUCTURED-GRID DRIFTWOOD SIMULATION MODEL CONSIDERING THE DRAG FROM WIND

Abstract

 In the Ariake Sea, driftwood drifting due to heavy rains and typhoons adversely affects the environment, fisheries and facilities. The deposited driftwood negatively impacts the rich ecosystem, and fishing boats may not be able to go fishing for several months due to concerns about collisions with the driftwood. The driftwood on the coast hinders the function of wave-dissipating blocks and causes water to flow into the land due to storm surges. The Kyushu Regional Development Bureau (KRDB) collects driftwood using vessels relying on sightings, however, in the Ariake Sea where the current is fast, the driftwood often can not be found. Therefore, in this study, I developed an unstructured-grid drift prediction simulation model considering the influence that the drag from wind has on driftwood for the purpose of efficient recovery of driftwood. The particle tracking module of FVCOM, which is the unstructured-grid model, was improved to consider the buoyancy of driftwood, the drag from wind, and changes in the projected area ratio depending on the rotation of driftwood. The simulation for the period of heavy rains in July 2018 and Typhoon Kong-rey in 2018 was conducted. We estimated the times and locations of the driftwood outflow from the rivers by comparing the simulated driftwood drifting locations with the actual driftwood recovery data provided by the KRDB. During that typhoon period, the difference of 3.2 km occurred 48 hours after the release of driftwood by the drag from wind. Thus, considering the drag from wind against driftwood is an indispensable factor in developing a model for accurately predicting positions of driftwood.