Modelling the increased frequency of extreme sea levels in the Ganges–Brahmaputra–Meghna delta due to sea level rise and other effects of climate change

Abstract

Coastal flooding due to storm surge and high tides is a serious risk for inhabitants of the Ganges-Brahmaputra-Meghna (GBM) delta, as much of the land is close to sea level. Climate change could lead to large areas of land being subject to increased flooding, salinization and ultimate abandonment in both West Bengal, India, and Bangladesh. IPCC 5 th assessment modelling of sea level rise and estimates of subsidence rates from the EU IMPACT2C project suggest that sea level in the GBM delta region may rise by 0.63 to 0.88 m by 2090, with some studies suggesting this could be up to 0.5 m higher if potential substantial melting of the West Antarctic ice sheet is included. These sea level rise scenarios lead to increased frequency of high water coastal events. Any effect of climate change on the frequency and severity of storms can also have an effect on extreme sea levels. A shelf-sea model of the Bay of Bengal has been used to investigate how the combined effect of sea level rise and changes in other environmental conditions under climate change may alter the frequency of extreme sea level events for the period 1971 to 2099. The model was forced using atmospheric and oceanic boundary conditions derived from climate model projections and the future scenario increase in sea level was applied at its ocean boundary. The model results show an increased likelihood of extreme sea level events through the 21st century, with the frequency of events increasing greatly in the second half of the century: water levels that occurred at decadal time intervals under present-day model conditions occurred in most years by the middle of the 21 st century and 3-15 times per year by 2100. The heights of the most extreme events tend to increase more in the first half of the century than the second. The modelled scenarios provide a case study of how sea level rise and other effects of climate change may combine to produce a greatly increased threat to life and property in the GBM delta by the end of this century.

Projected changes in area of the Sundarban mangrove forest in Bangladesh due to SLR by 2100

“…The GDTR is also assumed uniform and constant for the entire study area and simulated period. While the projections of Kay et al (2015) support the assumption of relative small changes in GTDR over time, the observed amplification along the tributaries is not captured by SLAMM simulations. Any effect of changing coastline on the tidal range is also neglected.…”

“…Observations of the spring tides on the Indian side of the Sundarbans Estuarine System, by Chatterjee et al (2013), suggest that the degree and rate of amplification are not uniform and follow a complex pattern along the different distributaries, such that the observed spring tidal range for all coastal stations is always larger than 4.5 m and can be as large as 6.7 m. Results from Kay et al (2015), using a regional ocean model, suggest that the tidal range has little variation along the Sundarbans coastline and over three different epochs in the future up to 2100. SLAMM was developed primarily for the micro-tidal ranges and predominantly diurnal tides in the US and implicitly assumes that mangroves persist from Mean Lower Low Water (mean of the lower low water height each day, hereinafter referred to as MLLW) as the lower elevation boundary for this category up to an elevation equivalent to the Mean High Higher Water (mean of the higher high water height each day, hereinafter referred to as MHHW).…”

Climatic Impacts on the Fluvial and Tidal Inundation Patterns in the Ganges-Brahmaputra-Meghna Delta

“…Time series of discharge is specified as the upstream boundary condition, with a time series of water level for the downstream boundary condition. Discharge boundaries are generated from the output of a hydrological (INCA) model [11] and for downstream water level boundaries, tides are generated by using GCOMS [12].…”

Assessing the impact of socio-economic development and climate change on faecal indicator bacteria in the Betna River, Bangladesh

“…For RCP8.5, global mean SLR was estimated to be 0.3m and 0.63m for the time horizons 2040s and 2090s respectively (Elshemy and Khadr, 2015). Recent projections from the IPCC AR5 suggest that sea level in the northern Bay of Bengal (close to the study area) may rise between 0.1 and 0.3m by 2050 and 0.3 and 0.6m by 2100 without including local effects, such as land subsidence (Kay et al, 2015). The annual land subsidence rate of the study area is likely to be around 2.5mm (Kay et al, 2015, Dasgupta et al, 2014.…”

“…Recent projections from the IPCC AR5 suggest that sea level in the northern Bay of Bengal (close to the study area) may rise between 0.1 and 0.3m by 2050 and 0.3 and 0.6m by 2100 without including local effects, such as land subsidence (Kay et al, 2015). The annual land subsidence rate of the study area is likely to be around 2.5mm (Kay et al, 2015, Dasgupta et al, 2014. In this study, based on the above data and including annual land subsidence of 2.5mm, the relative mean SLR of 0.26m and 0.42m (for RCP4.5); and 0.44m and 0.76m (for RCP8.5) were used for the 2040s and 2090s, respectively.…”