Ryota TANIGUCHI

Application of LiDAR in river surveying

Minjiao LU

It is known that measuring water surface gradient is highly significant in understanding flow conditions. In recent years, the use of point cloud data and 3D topographic data in river surveying has been attracting attention from the viewpoints of data acquisition efficiency and safety, and many studies have been conducted on surveying methods using LiDAR. Many studies have been conducted on LiDAR-based surveying methods. However, there are still some problems with this method, such as the fact that the water's edge area is a small area where both water depth and flow velocity are small, and the environment is different from that of the flow center, and a suitable straight section is required to obtain a stable water surface gradient. Therefore, this study attempted to obtain the water-surface gradient at the center of the stream by using a rope with floats installed at intervals of approximately 5 m. The rope was then floated downstream in the river channel. This method can acquire the water-surface gradient at the center of the channel regardless of the shape of the channel, by using LiDAR to acquire a point cloud of floats floating in the channel. The acquired water-surface gradient was verified by creating a river channel model of the measured section and conducting a river channel flow analysis using iRIC. The roughness coefficient calculated by Manning's equation from the water surface gradient was used as a calculation condition for the analysis. Three roughness coefficients were used in the analysis: stream center roughness calculated from the water surface gradient obtained from the stream center, waterline roughness calculated from the water surface gradient obtained from the waterline point cloud, and general roughness coefficient set as a commonly used coefficient. The results of the analysis showed that the stream center roughness tended to be closer to the observed low water flow rate than the waterline roughness and general roughness. This indicates that the stream center roughness may be suitable for reproducing the channel conditions.