Flood Inundation and Hazard Mapping of 2017 Floods in the Rapti River Basin Using Sentinel-1A Synthetic Aperture Radar Images

Abstract

Globally, the flood magnitude and flood-induced damage are increasing. Hence, the geospatial technology has been used to minimise the adverse effects of floods and to plan the floodplain for the betterment of floodplain dwellers. One of the major causes of floods in the Rapti River basin is heavy rainfall induced by the break-in-monsoon condition. These days, geoscientists and planners use Sentinel-1A IW GRD synthetic-aperture radar (SAR) image for flood extent mapping. Gauge level and flood duration data recorded at Bhinga, Balrampur, Bansi, Regauli, Birdghat, Kakarahi, Uska Bazar and Trimohinighat sites provide the basis for the selection of SAR images. Extensive floods occurred in the Rapti River basin during August 13–September 01, 2017. The flood duration in the Rapti River basin varied from 3 (Bhinga) to 18 days (Birdghat) in 2017. The flood duration, normally, increases from the upstream to downstream along the Rapti River due to decreasing slope and discharges contributed by the tributaries. In this study, Sentinel-1A GRD SAR images of August 21 and 25, 2017, have been selected for flood mapping in the Indian part of the Rapti River basin. The water level of rivers was above the danger level (DL) at Bansi, Regauli, Birdghat, Kakarahi, Uska Bazar and Trimohinighat gauge and discharge (G/D) sites on August 21 and 25, 2017. The propagation of flood peaks and affected areas has been analysed using water level data and SAR images for the mentioned periods. The actual flooded areas covered 2046.7 km2 area of the Indian part of the Rapti River basin during August 21–25, 2017. The validation of flooded areas has been done using GPS way points collected during field survey (November 2017) and Landsat 7 ETM+ images (August 24, 2017). Breach sites in flood-prone areas have been mapped using Sentinel-2A and B MSI images. The z-score method has been used for the standardisation of development block-wise flooded areas (km2) and number of flood-affected villages. After standardisation, these two parameters have been added to formulate development block-wise flood hazard index (FHI). High to very high FHI values have been observed in Siddharthnagar and Gorakhpur districts.