A Study on Fluvial Flood Hazard and Risk Assessment of Arial Khan River Floodplain under Future Climate Change Scenarios

Abstract

Bangladesh is highly susceptible to flood due to its location at the confluence of the world’s three major basins– Ganges-Brahmaputra-Meghna (GBM) and hydro-meteorological and topographical characteristics of the basins. Increase in temperature and precipitation due to climate change will significantly increase the monsoon flow of the GBM rivers which may lead to more intense and frequent floods in Bangladesh in upcoming decades. In this study, the fluvial flood hazard and risk of Arial Khan River floodplain have been assessed for the predicted climate change scenarios of RCP 2.6 and RCP 8.5. A calibrated and validated SWAT model of GBM basins has been used to project the future flow magnitudes at Bahadurabad Transit (Brahmaputra River) and Hardinge Bridge (Ganges River) for RCP 2.6 and 8.5 scenarios. Using the flow magnitude of these stations as the upstream boundaries, a 1D HEC-RAS model has been set up for the Brahmaputra-Ganges-Padma River for generating future flow magnitude at Mawa of Padma River. Later, the discharge at the offtake of Arial Khan has been calculated establishing a linear regression equation between the Arial Khan and Padma River. Finally, a 1D-2D coupled model of Arial Khan River floodplain has been set up in HEC-RAS. This coupled model is calibrated and validated for Manning's roughness coefficient ‘n’ = 0.015-0.02 for the year of 2015 and 2017 respectively. After calibration and validation, the model is simulated for different periods of RCP 2.6 and RCP 8.5 scenarios: baseline (1976-2005), 2020s (2006-2035), 2050s (2036-2065) and 2080s (2066-2095). The model result shows that the total flood affected area is nearly 15% at the base condition which is increased to 30% and 47% approximately for the 2080s of RCP 2.6 and RCP 8.5 respectively. The hazard assessment reveals that at the base condition, 48%, 34% and 18% area are in the very low, low and high hazard zone respectively. It is increased to manifolds such as 21%, 27%, 34% and 18% in the very low, low, medium and very hazard zone respectively for the 2080s of RCP 2.6 and it became 22%, 26%, 34% and 18% in the low, medium, high and very high hazard zone respectively for the 2080s of RCP 8.5. Similar to hazard, risk assessment also shows that the very low and low risk zone will decrease and high and very high risk zone will increase significantly by the end of 21st century compared to the base period. Incorporation of vulnerability and exposure gives a complete different horizon to the overall assessment. It is found that some medium hazard zones have a high risk of flood damage due to its high exposure and vulnerability to flood while some high hazard zone falls into the low risk zone because of its low exposure and vulnerability. The overall flood assessment for different projections of RCP 2.6 and RCP 8.5 show that there is an increasing trend of the flood from baseline to 2080s, both for RCP 2.6 and RCP 8.5. From baseline to 2050s, the difference between RCP 2.6 and RCP 8.5 is slight. However, this variation becomes very drastic after the 2050s. The inundation pattern, hazard and risk extent increase manifolds in the 2080s of RCP 8.5 than those of RCP 2.6. So future climate change is going to have a terrible effect on the flood situation of Arial Khan River floodplain. The results found in this study provide useful information on projected changes of flood hazard and risk in the Arial Khan River floodplain over the next decades to a century. Additionally, this study will act as a guideline regarding how to incorporate the globalize climate change scenarios of a large basin to a small local river and its floodplain and find out localized flood hazard and risk maps under climate change scenarios.