Submerged wood detection in a dam reservoir with a narrow multi-beam echo sounder

In 2017, the Susobana Dam in Nagano prefecture became uncontrollable because sediments and submerged driftwood clogged the bottom outlet during flood control. This incident was induced by a combination of large amounts of sedimentation residue which reached the bottom of the gate and driftwood clogged the gate travelling through the flushing water. Such incidents will occur more frequently as now more and more reservoirs are suffering from sedimentation and climate changes, which are a result of these severe flood events nowadays. However, there are few studies on the process of how driftwood is generated, submerged and transported by a flood, thus the driftwood movement during the incident is still a matter of speculation. In this study, as a first step for understanding the dynamics of driftwood movement in reservoirs, we applied the Narrow Multi-Beam Echo Sounder (MBES) system to detect the current distribution of driftwood in the Susobana reservoir.