Hydrological Responses of the Brahmaputra River Basin using CMIP6 GCM Projections for Supporting Climate Resilient Infrastructure Design

Abstract Bangladesh is one of South Asia's leading countries sensitive to climate change because of its unique geography and the dominant hydro-meteorological influence of torrential monsoon rainfall. The central north of Bangladesh, which possesses low density and quality of sustainable rural infrastructures, is affected by seasonal flooding and frequent river erosions. Districts under this region are among the country's poorest and most vulnerable due to environmental stress and socioeconomic circumstances. Flooding has become a regular phenomenon in the Teesta-Brahmaputra river basin during monsoon, which causes severe damage to rural infrastructures every year. As mentioned in the latest Intergovernmental Panel on Climate Change (IPCC)’s Sixth Assessment Report (AR6), burgeoning climate variability will aggravate the situation, leaving millions more vulnerable to these extreme natural events. This study aims to assess the future impact of climate change on the discharge and water level of the Teesta-Brahmaputra river system through rigorous data processing, detailed modeling, and frequency analysis, as well as to develop a guideline for designing climate-resilient infrastructures in flood-prone areas. The Soil Water Assessment Tool (SWAT) hydrologic model is used to simulate future discharges using 13 bias-corrected General Circulation Models (GCMs) under moderate SSP2-4.5 and extreme SSP5-8.5 scenarios for near (2025–2054) and far (2071–2100) future periods with respect to baseline (1985–2014). Future water level fluctuations of six major rivers in the study region have been assessed using these flow outputs by simulating the HEC-RAS model and frequency analysis. Results indicate that the dry season will experience a more significant flow increment than the wet season under both SSP2-4.5 and SSP5-8.5 scenarios due to the impact of climate change in the Brahmaputra basin. The consequent rise in water level in the Jamuna river will vary from 38 cm in the near future (2025–2054) to 83 cm in the far future (2071–2100) for a return period of 100 years. The Old Brahmaputra, Dharala, and the Brahmaputra will also experience a sharp increase of 39 cm to 90 cm, 33 cm to 74 cm, and 25 cm to 63 cm, respectively. Based on these future projected water level values, required updates to existing design principles have been recommended for building sustainable and climate-resilient infrastructures in the study area.