Flood inundation mapping and monitoring using SAR data and its impact on Ramganga River in Ganga basin.

Abstract

Remote sensing-based flood inundation mapping and monitoring is very crucial input before, during, and after floods. Ganga-Ramganga doab is one of the prolonged flood-affected area in middle Ganga plain due to seasonal monsoon which leads to rise in water levels of Ganga and Ramganga rivers. The focus of the present study is to map severe flood condition captured through synthetic aperture radar (SAR) data during August-September 2018, and to explain the impact on Ramganga river morphology. SAR data is preferred for flood mapping and real-time monitoring in all weather conditions. In this study, dual-polarized (VV and VH) Sentinel-1 SAR images coupled with hydrological data (river water level) were used to produce flood inundation maps. Thresholding technique has been applied to determine the flood mapping through Sentinel-1 data. VH and VV polarisation methods have been applied for a comparison of their respective accuracies in delineating surface water. Results have been validated against a Sentinel-2 optical image, and both polarisations produced a total accuracy of more than 93%. VV polarisation has high accuracies than VH polarisation as similar results are observed in previous studies as well. The finding reveals that severe bank erosion took place in the Ramganga channel which significantly affected the channel morphology, mainly the massive mobilisation of channel sediments. The results show that the average channel width increased from 46 to 336 m. The proposed approach demonstrates that the microwave remote sensing data along with GIS can be used efficiently for flood inundation mapping, monitoring, and analysing its effect on channel morphology. Therefore, the results of this study will help to take the initiative to reduce the flood hazard impact in the doab area and increase the flexibility in the process of flood management.