X-Band Radar Observations of Sand Dune Erosion and Flow Velocity Patterns Due to Floods at the Mouth of Tenryu River

The Enshu Coast is experiencing erosion and authorities plan to increase the sediment supply from the Tenryu River to mitigate it. They made a straight cut in a river dune near the mouth, and the floods in July 2020 eroded it, supplying sediment to the coast. We observed these changes using a land-based X-band radar. X-band radar provides imaging of the river channel, dune locations, and river bank position. It was used to monitor erosion during the two flood events, which had a significant impact on the river dune's morphology. We averaged the raw radar images hourly to understand the erosion process from changing position of the sand dune tip. During the two floods, the sand dune tip was submerged to varying degrees and eroded significantly. This erosion continued even after the flood receded, persisting into the post-flood period. Further, we applied PIV analyses to estimate the flow velocity pattern at the tip of the dune. The first flood, despite reaching 2 m/s, causes minor erosion due to its short duration. Moreover, the second flood with its elevated velocity and extended duration leads to significant erosion. Erosion processes are influenced by high velocity and flow duration. Dune erosion can potentially occur when a velocity range of about 2 m/s is present, with the extent of erosion depending on the duration of the velocity. This study illustrates the capability of X-band radar to monitor changes in sand dune erosion and patterns of flow velocity during severe weather conditions.