The average annual rainfall and the summer monsoon rainfall (South West Monsoon) have been showing a decreasing trend across India (Mondal et al., 2015; Radhakrishnan et al., 2017). The annual and seasonal rainfall trends in the state of Bihar are also showing a decreasing trend for the last century (Kumar et al., 2010). However, the flood frequency, magnitude and duration of flooding in the state of Bihar have shown an increasing trend, especially in the northern Kosi river catchment. The state of Bihar spends substantial resources every year on flood relief and recovery programs. Yet, the issue of floods has not been adequately addressed and it remains a persistent problem (Finance Department, 2021; Pushyamitra, 2021). The main objective of this paper is to investigate the underlying causes of increasing flood events both biophysical and socio-institutional and assess its implications on the livelihoods of communities residing in this region- particularly their exposure to increased vulnerability due to flood risk in the last few decades.

The literature on precipitation patterns in India suggests that there are clear disparities and heterogeneity in terms of the spatial distribution of annual rainfall and summer monsoon rainfall in India (Saha et al., 2018). The spatial heterogeneity of the rainfall pattern in the flood plains of the river Ganga is an example of such disparity and heterogeneity due to the vastness of the landscape (Bisht et al., 2018) and the terrain. The foothills of the Nepal Himalaya in the state of Uttar Pradesh, Bihar, and Sikkim in India receive some of the heaviest rainfall in the summer monsoon from June to September (June, July, August and September or JJAS) because of orographic terrain (Singh et al., 2019). The flood plains of Kosi in North Bihar, located in the southern part of the Kosi sub-basin, are one of the regions expected to receive more rainfall in the coming decades as compared to the middle and upper Himalayan part of the Kosi sub-basin (Singh and Sontakke, 2002; Rajbhandari et al., 2018).

The Kosi river, a major tributary of the river Ganga forms one of the largest inland deltas in the Gangetic river basin in India due to its constantly shifting river channels (Gole and Chitale, 1966). The flood plains of Kosi are the most permanently flooded areas in the state and are under-developed and socio-economically marginalized, which makes it a very hazard-prone and flood-vulnerable region both in the state of Bihar and India (Pandey et al., 2010; Singh et al., 2011). At the same time, there has been a gradual increase in the population located in the high-risk and very high-risk land categories in the North Bihar region over the past three decades (Modak and Kapuria, 2020). The Coupled Model Intercomparison Project Phase 6 (CMIP6) for the 2070–2099 period projects that the Ganges river basin will be experiencing the largest flood risk because higher flood peaks would become common by mid-2030 (Eyring et al., 2016). This makes the region highly disaster-prone giving policy makers ad communities relatively short window of time for adaptation (Gädeke et al., 2022).