Topographic change and sediment transport on Maeshima Tombolo tidal flat

Abstract

For the management of the tidal flat environment and the preservation of several important functions of the tidal flat, the accurate topographic monitoring is indispensable. However, the appropriate measurement of the tidal flat topography is very difficult because water level changes every moment due to tidal oscillation, and the accountable time for the measurement is limited within several hours under low tide condition. In this study, topographic measurement of tidal flat using Unmanned Aerial Vehicle (UAV) was conducted 5 times over a year, and Digital Elevation Model (DEM) obtained by UAV photogrammetry (UAV-DEM) of the tidal flat was constructed in each measurement. The area of 400m in the land-island (L-I) direction and 100m in the west-east (W-E) direction was set on the developed UAV-DEM to analyze the short-term topographical changes (2 weeks including typhoon events, half year and year). By the influence of typhoon passing, sand on the tidal flat was transported from east to west obviously within typhoon event. However, after the topographic change due to the typhoon event, the sand moved opposite direction. By the estimation of the sand volume on the tidal flat using UAV-DEM, it was found that the change of the volume was quite small in one year. These results indicate that the sand volume on this tidal flat is mostly preserved although active sand transport occurs on the tidal flat.