Wide-Area Extraction Method for Sediment Runoff Range After Mountain Disasters in Japan

This study is focused on the region surrounding Murakami City, Niigata Prefecture, which experienced sediment runoff due to heavy rainfall in August 2022. Four types of optical satellite data with different spatial resolutions were used to calculate the NDVI difference values (ΔNDVI) over a wide area. Specifically, we extracted NDVI values over an extensive area and then investigated the effects of differences in spatial resolution on the applicability of the extraction method and its extraction rate. The relationship between precision and recall can be approximated by the quadratic equation y = ax2 + bx + c, and there was a trade-off relationship between the two metrics; as the threshold value rose, precision increased while recall decreased. The optimal NDVI threshold for maximizing the F-measure ranged from 0.20 to 0.25. The medium-resolution satellite platforms Planet and Sentinel-2 had higher F-measure values, and the efficacy of NDVI extraction was not proportional to the fineness of the spatial resolution. The reason for this was that the area distribution of the sediment runoff range in the target area was dominated by relatively small areas with a mode of 42 m2 and a median of 253 m2, which were considered to increase precision and recall. Consequently, selecting a spatial resolution that matches the area of the sediment runoff range in the target area is considered to be effective.