Based on field surveys and terrain analysis using open data of aerial LiDAR point cloud (VIRTUAL SHIZUOKA), we analyzed the characteristics and causes of flooding in the lower basin of the Tomoe River caused by the heavy rains associated with Typhoon 15 in 2022. The water level of the main river was diverted into the Oya drainage channel and dropped once, but the level rose again and overflowed where the tributaries merged. The levee itself was not damaged or breached. Tributaries that had not undergone sufficient river improvement, and thus still had many meanders and shallow channels, flooded significantly, while tributaries that had undergone advanced river improvement also flooded due to increased water levels before joining the main river. Tributaries often overflowed in front of the insufficient cross-sectional area of the river channel where they passed under roads and railroads. Comparison of the flood depth distribution with existing hazard maps showed that the main channel flooding was generally on the planned scale. Some of the tributary flooding corresponded to the assumed maximum magnitude, while others fell far short of it. The high resolution of the topographical map based on point cloud data is considered effective for understanding flood risk and raising public awareness because it realistically reconstructs the flooded urban landscape.

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