Demarcation of Flood-Prone Zones in the Indian Part of the Ganga Delta Based on the Highest Floods between 1995 and 2020

The Ganga–Brahmaputra–Meghna Delta (GBMD) at the northern apex of the Bay of Bengal, is the world’s largest in respect of area (c. 120 × 10³ km²) as well as annual discharge of sediments (c. 10⁹ t). Contributed by the Ganga River and its numerous distributaries, the southwestern part of the GBMD is known as the Ganga Delta which spans over a number of districts in Bangladesh and West Bengal (India). The Indian state of West Bengal occupies the western portion of the GBMD, where the delta plains drained by the Ganga and its distributaries measure 42,371 km². The estimated population residing in this region in 2021 is about 76 million. The Ganga Delta is completely enveloped by the alluviums deposited by the floods and channel deposits in the last 10,000 years. Nearly every major river in the region is embanked on their both flanks to prevent overspilling during the high stages in the monsoon season (June–September). Floods occur nonetheless in years of exceptionally high rainfalls, often brought about by tropical cyclones, when these embankments are breached or overtopped by the river water.

This study aims to delineate the flood-prone zones of the Gangetic West Bengal (GWB) based on the highest floods that occurred between 1995 and 2020, and to extract data on extension, land use and resident population of the flood-susceptible area on different administrative levels.

Using five highest-magnitude flood events for five overlapping zones, it is found that 33% of the GWB is susceptible to inundation by floodwater. Overlying the inundation area over 226 administrative blocks of 14 districts of the region reveals that 51 highly populated blocks located close to the principal rivers are susceptible to flooding. The deepest flood localities of the east–central GWB noticeably coincides with the blocks with highest percentage of inundated area (>50%) and also with the blocks having fairly large population size. 77 out of 226 blocks are susceptible to inundation of 50% or more of their total area. On a higher level, if the distribution of flood inundation across the districts constituting the GWB is considered, the districts of Nadia and Murshidabad are found to have relatively more inundation area, with almost 16% of the total flood-susceptible area of the GWB lying within each of these districts, followed by Purba Medinipur (13%) and Malda (11%). The study connotes that floods and the region’s cultural landscape—consisting of farmlands, habitations, and lines of communications—are closely related. Floods occur despite all human endeavours to prevent them, affecting approximately 14,500 km² area and 18 million people.