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The Ganges, Brahmaputra and Meghna rivers, which constitute the Ganges-Brahmaputra-Meghna (GBM) mega delta, originate from the Himalayas-Lushai mountain and hill ranges and fall into the Bay of Bengal flowing through an extensive alluvial floodplain in India and Bangladesh (Jain & Sinha, 2003). The low gradients of the rivers in the plains, high rainfall over their catchments during the south-west monsoon, huge catchment areas of the rivers and their common drainage route in the downstream bring forth heavy flood events to lower riparian countries as a common yet devastating natural phenomenon (Tockner et al., 2008). The enormous discharge generated at the upstream catchment areas flows through Bangladesh and inundates its vast floodplains. During the catastrophic floods of 1987, 1988, 1998, 2004 and 2007, severe damages occurred on the banks of the Brahmaputra and Ganges rivers (Yang et al., 2015). Even during the recent floods of 2014 and 2016, many crop fields and dwell areas adjacent to the Brahmaputra River in the north-western part of Bangladesh were inundated causing untold sufferings to the local inhabitants. In 2017 as well, Kurigram, Gaibandha and Lalmonirhat administrative districts in the Brahmaputra and Teesta floodplains were inundated by flood water above the danger level which caused loss of lives, damaged household assets, standing crops and rural infrastructures, and brought unbearable miseries to the local people.
The GBM river floodplains are potentially high-risk zones from riverine flood damages. Being a braided river, the Brahmaputra continuously creates new char lands where people start to live in soon after their formation. These unprotected char lands are particularly vulnerable to flood hazard because of the fragile geophysical setting of the living and livelihood environment for the local communities, lack of effective flood forecasting and warning system, poor flood protection measures, etc. Though the natural phenomenon like flood cannot be controlled, it can be managed to some extent by taking necessary precautionary as well as post-event measures.
Flood hazards have long been managed mostly by infrastructural interventions like embankments, polders, dams, barrages, etc. However, such infrastructural interventions are not suitable for flood management in char environment. Chars are islands, attached lands, or mainland, which experience flooding and erosion regularly. They form within or alongside large alluvial rivers and estuaries and have unstable landforms. The GBM rivers of south Asia, the Mekong River of east Asia, the Amazon River of South America, the Fraser River of Canada and the Lower Yellow River of China are among the alluvial rivers having chars. In char environment, flood shelters are commonly used as a flood damage mitigation strategy. A flood shelter is basically a flood proofing measure provided in the alluvial floodplain areas, particularly in island chars, to protect the local vulnerable communities during the flood events. It is considered as a vital flood damage mitigation strategy to protect lives, livestock and properties during a flood disaster and to manage relief and rehabilitation activities during the post-disaster period (Alam & Ali, 2002). In Bangladesh, about 4% of its total population of 160 million is estimated to live in chars. There are 56 large and 226 small chars in the country (Asiatic Society of Bangladesh, 2007).