Detecting and mapping flood inundation areas in Fogera-Dera Floodplain, Ethiopia during an extreme wet season using Sentinel-1 data

Abstract

The study area, Fogera-Dera Floodplain, has experienced frequent flooding events in the past several decades related to the topography of the region, mainly during the rainy season. In this study, Four Sentinel-1A Synthetic Aperture Radar (SAR) data was utilized to detect, map, and analyze flood water inundations in the 2020 extreme wet season. Besides this, HEC-RAS 2D (Hydrologic Engineering Center-River Analysis System) hydrodynamic model was utilized for result validation and comparison. Doing so, Sentinel- 1A VV polarized SAR data sets were selected, and histogram-based thresholding method was applied to extract flood and non-flood inundated areas. The result showed that flood inundated area was about 32.02 km2 on July 04, 2020, 79.96 km2 on July 28, 2020, 127.48 km2 on August 09, 2020, and 83.89 km2 on August 21, 2020. Flood water expansion was linearly increased from 04 July up to 09 August, which was about 95.46 km2and reached its peak stage on August 09, 2020. Afterward, from the period of 09–August 21, 2020, the flood was receded and decreased by 43.59 km2. In contrast, the Hydrodynamic model was used to simulate flood inundations based on Ribe and the Gumara River time-series discharge data. The model simulation determines the flooding extent of the study area on August 09, 2020, and the result shows an inundation area of 135 km2, which is similar to flood inundations extracted by SAR imagery. Generally, floodwater was expanded towards Ribe and Gumara river courses, on wetlands, depression areas, and along Lake Tana shoreline, where the elevation was extremely low. Therefore, a Time-based flood inundation map extracted from SAR images can give valuable information for the analysis of flood propagation and flood risk management.