Spatial-Temporal Variations and Severity of the 2020 Catastrophic Floods in the Yangtze River Basin from Sentinel-1 SAR Data

Flood is one of the most frequent natural disasters in the Yangtze River Basin. Flood risk evaluation is of great social significance, especially for large hydrological systems. Rainfall is both temporal and spatial, influencing surface hydrological activities. The water body range is the final outcome of a flood and can be observed from synthetic aperture radar (SAR) images under any weather condition. A flood severity evaluation model is proposed to quantitatively evaluate the flood based on water body range from area disparity and flood duration. Large hydrological objects usually span a wide range and have significant differences. This results in different initial water areas in each region. This approach addresses the issue through normalization processing. In this paper, Sentinel-1 data are used to extract the temporal water body using the adaptive bimodal method, and the water level data were also incorporated to improve the observation frequency for water area. The flood severity evaluation approach can be used to assess flood risk between any region of large hydrological systems or any flood event, regardless of their regional spatial differences and rainfall duration differences. The results show that: (1) In general, the average water body area in 2020 was 20.40% larger than it was in 2019, and the daily water body areas in 2020 were all greater than the average of 2019 with 71.36% of the days in 2020 having an area greater than the maximum in 2019. The flood severity in 2020 was 1.75 times as much as that of 2019; (2) Reach performance indexes in 2020 were in order of Yueyang (2.21) > Jiujiang (2.04) > Hankou (1.44) > Chizhou (1.32), which were inconsistent with the spatial site; (3) Flood event impact indexes in 2020 were in order as No.2 (1.64) > No.3 (1.61) > No.1 (1.44) > No.4 (1.17) > No.5 (1.15); (4) The flood was more likely the result of cumulative rainfall for 30 days.