Assessment of Incoming Sediment with Flash Flood: A Case Study of the 2020 Flood in the Northeastern Part of Bangladesh using SAR Interferometry

Bangladesh, a riverine South Asian country with many Haor areas, is extremely vulnerable to flash flooding, which occurs primarily between the months of April and May (pre-monsoon). A Haor is a type of wetland ecosystem found in Bangladesh's northeastern region that is essentially a tectonically active shallow depression with a bowl or saucer shape where water flows from upstream basins. Floods and the resulting sediment have both positive and negative impacts on the affected Haor region, with broader implications for agricultural, water, fisheries, and other resource planning and management. However, till now there is no measurement or literature on the amount of sediment deposition caused by these flash flooding events. Threfore, the primary goal of this study was to determine the amount of incoming sediments associated with flash floods in Haor regions using remote sensing and to validate it in the field. The amount of incoming sediment associated with the flash flood that occurred in June 2020 was estimated for a selected region in the affected northeastern part of Bangladesh for this purpose. Using Sentinel-1 satellite images, interferometric techniques were used to create Digital Elevation Models (DEMs) of the pre and post-flood period of 2020. A total of eight Sentinel 1 A and Sentinel 1 B images covering the study area were collected from 22 July to 26 July 2019 to assess pre flood land conditions and from 21 July to 27 July 2020 to assess post flood land conditions. Our study revealed that the overall sediment deposition was found to be about 2.8 cm on average for the selected entire region. Furthermore, it has been observed that relatively less flashy areas gained sediment increase of about 7.3 cm on average within this one year interval, and relatively upstream areas with steep gradient gained 4.5 cm increase. Any anthropogenic interventions in this area should take into account the natural sediment distribution pattern and avoid impeding sediment spreading pathways, as sediment acts as a natural countermeasure to tectonic-subsidence of this area.