Elevation change of Bhasan Char measured by persistent scatterer interferometry using Sentinel-1 data in a humanitarian context

This study investigates the elevation changes for the island of Bhasan Char, located in the Bay of Bengal, which was selected for the relocation of around 100,000 refugees of the Rohingya minority which were forced to leave their homes in Myanmar. Eighty-nine Sentinel-1 products were analysed using persistent scatterer interferometry (PSI) beginning August 2016 through September 2019, divided into three periods of one year to reduce the impact of temporal decorrelation. The findings indicate that the island is a recent landform which underlies naturally induced surface changes with velocities of up to ±20 mm per year. Additional displacement is probably caused by heavy construction loads since early 2018, although we found no statistical evidence for this. The main built-up area shows stable behaviour during the analysed period, but there are significant changes along the coasts and artificial embankments of the island, and within one separate settlement in the north. The moist surface conditions and strong monsoonal rains complicated the proper retrieval of stable trends, but the sum of findings supports the assumption that the island underlies strong morphologic dynamics which put the people to be relocated at additional risk. Its suitability for construction has to be investigated in further studies.