Study on monsoon flood hazard and vulnerability assessment of old-Brahmaputra river floodplain under climate change scenario

Abstract

This study is formulated to assess the monsoon flood hazard, vulnerability and risk of Old-Brahmaputra River and its surrounding floodplain for future climate change scenario of RCP 8.5 using numerical modeling approach. Firstly, a calibrated and validated hydrologic model of Brahmaputra river basin in HEC-HMS has been used to obtain the future flow magnitude for early (2020s), mid (2050s) and late century (2080s) for predicted climate data of RCP 8.5 scenario. Then the future flow at the Mymensingh of Old-Brahmaputra River has been obtained by conducting linear regression analysis considering the historical flow at Bahadurabad transit of Brahmaputra-Jamuna River as independent variable and the flow at Mymensingh of Old-Brahmaputra River as dependent variable. Then the study presents the simulation of flood flow of old Brahmaputra River and surrounding floodplain using HEC-RAS 1D-2D coupled hydrodynamic model. The 1D flow simulation was applied for the Old-Brahmaputra river channel and then integrated into 2D flow simulation incorporating the contribution of major tributaries and distributaries in the floodplain area using RAS Mapper. The developed hydrodynamic model is then calibrated and validated for Manning's roughness coefficient, n=0.014-0.017 for the year 2017 and 2016 respectively. To calibrate the inundation extent, flood inundation map obtained from simulation has been compared with the Sentinel-1 satellite image and flood map produced by FFWC on 16th August of 2017. Mean flood depth obtained in each upazilla under the study area has also been compared with the mean flood depth for the upazilla within the study area recommended by FFWC for 16th August of 2017 that provided satisfactory matching. Calibrated model was then simulated to obtain flood depth, flood flow velocity and inundation area of the years 1988, 1998, 2004, 2007, 2010, 2013, 2016 and 2017. Analysis results depicted that among the historical flood years, flood event of 1988 and 1998 were of devastating consequences comparing with others. Then the synthetic inflow hydrographs of Old Brahmaputra river generated for baseline, 2020s, 2050s and 2080s of RCP 8.5 scenario were fed into validated HEC-RAS model of Old Brahmaputra to generate the flood inundation depth, velocity and inundation area maps in the study area. The analysis results show an increasing trend of flood variables from baseline to 2080s. From baseline to 2080s, the total inundation area extended from 1975 km2 to 3923 km2 which is almost half of the study extent. Additionally, the mean flood depth and mean flood flow velocity are found to be increased as well. Thereafter, flood depth, velocity and inundation area have been incorporated with weightage obtained from PCA to calculate the flood hazard. The percentage area under different hazard zones for RCP 8.5 Scenario has been analyzed which shows that from baseline to 2080s the very low, low, medium and high and very high hazard zone changed from 25%, 36%, 36%, 3% and 0% to 5%, 36%, 40%, 14% and 5% highlighting significant increase of high to very high hazard zone. Thereafter, sensitivity, adaptive capacity, vulnerability and exposure have been estimated for the present and future socio-economic condition. Future population of each Upazilla was estimated using the logistic growth method. Selected indicators have been predicted for 2020s, 2050s and 2080s by trend analysis of the indicators of previous decades obtained from Population and Agricultural Census. Analysis on vulnerability maps highlights significant increase of moderate and high vulnerability zone from present to 2080s. Similarly, assessment of exposure for present and future highlights significant shift of few of the upazillas from high to very high exposure zones from present to 2080s. Thereafter, the flood risk is calculated for baseline and future multiplying flood hazard of each of the upazilla with the exposure and vulnerability of that particular unit. Risk assessment shows that all the upazillas are in the low to medium risk zone except Gaffargaon. Major part of the total study area may exist under the low to medium risk zone throughout the century. Thus, it can be interpreted that future climate change is going to have moderate impact on the flood situation of major portion of the old Brahmaputra River floodplain even if the wettest climate change scenario of RCP 8.5 is considered but there is an increasing trend of the flood from baseline to 2080s and the increment is significant after 2020s. It is hoped that the results would help floodplain management authorities in minimizing flood damage and loss of lives through technical approach.