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The Dominant Climate Change Event for Salinity Intrusion in the Gbm Delta

Rabeya Akter
Department of Geography, Ohio University, Athens, OH 45701, USA

Tansir Zaman Asik
Corresponding author
Institute of Water and Flood Management, Bangladesh University of Engineering and Technology (BUET), Dhaka 1000, Bangladesh

Mohiuddin Sakib
Department of Earth and Environmental Sciences, University of New Orleans, 2000 Lakeshore Dr, New Orleans, LA 70148, USA

Marin Akter
Institute of Water and Flood Management, Bangladesh University of Engineering and Technology (BUET), Dhaka 1000, Bangladesh

Mostofa Najmus Sakib
Department of Civil Engineering, Boise State University, 1910 W University Dr, Boise, ID 83725, USA

A. S. M. Alauddin Al Azad
Institute of Water and Flood Management, Bangladesh University of Engineering and Technology (BUET), Dhaka 1000, Bangladesh

Montasir Maruf
Department of Civil and Environmental Engineering, Northern Arizona University, San Francisco St, Flagstaff, AZ 86011, USA

Anisul Haque
Institute of Water and Flood Management, Bangladesh University of Engineering and Technology (BUET), Dhaka 1000, Bangladesh

Md. Munsur Rahman
Institute of Water and Flood Management, Bangladesh University of Engineering and Technology (BUET), Dhaka 1000, Bangladesh

Abstract/ Executive Summary:

Salinity intrusion through the estuaries in low-lying tide-dominated deltas is a serious threat that is expected to worsen in changing climatic conditions. This research makes a comparative analysis on the impact of salinity intrusion due to a reduced upstream discharge, a sea-level rise, and cyclonic conditions to find which one of these events dominates the salinity intrusion. A calibrated and validated salinity model (Delft3D) and storm surge model (Delft Dashboard) are used to simulate the surface water salinity for different climatic conditions. Results show that the effects of the reduced upstream discharge, a sea-level rise, and cyclones cause different levels of impacts in the Ganges-Brahmaputra-Meghna (GBM) delta along the Bangladesh coast. The reduced upstream discharge causes an increased saltwater intrusion in the entire region. A rising sea level causes increased salinity in the shallower coast. The cyclonic impact on a saltwater intrusion is confined within the landfall zone. These outcomes suggest that, for a tide-dominated delta, if a sea-level rise (SLR) or cyclone occurred, the impact would be conditional and local. However, if the upstream discharge reduces, the impact would be gradual and along the entire coast. The measured time series’ salinity magnitude data were discontinuous, and observed salinity stations did not cover the whole coast uniformly. Hence, a comparison of time series of salinity magnitude between model simulation and measurement was not possible. Measured tidal water level data were available in specific locations from the Bangladesh Inland Water Transport Authority (BIWTA). From this study perspective, three water level measurement locations representing three estuarine systems along the coast were initially selected, and data were collected from the BIWTA. However, due to poor data quality and unclear data, only one station was selected to compare the model simulated water level with measurements. To quantify the qualitative visual performance of the model, 8 statistical indicators—the mean, standard deviation (STDEV), mean absolute error (MAE), root mean square error (RMSE), percent bias (PBIAS), Nash-Sutcliffe efficiency indicator (NSE), ratio of the RMSE between simulated and observed values to the standard deviation of the observations (RSR), and coefficient of determination (R2 )—were calculated, and their “acceptable” ranges (which were reported in different literatures). These indicators are generally used as model evaluation statistics. Out of these 8 statistical indicators, the mean and STDEV represent descriptive statistics that quantitatively express the main feature of a dataset [60]. When the difference between these two indicators is “small” between the model and measured values, the main features of the model data and the measured data are similar. However, this does not guarantee that the spatial distribution of the model and measured data is also similar Climate scenarios were developed for three main events that affect SI in the GBM Delta. Studies suggested that salinity ingress is likely to be more severe in the future since (a) reduced upstream discharge, as flows from rivers in the Himalayas are predicted to decrease, (b) sea levels are predicted to rise gradually, and (c) extreme events (i.e., cyclones and storm surges) are expected to be intensified in the changing climate. As this research aimed to represent the extremes of salinity intrusion, scenarios were developed in such order. Upstream discharge reduction scenarios considered the dry season in the driest condition (reducing both the monsoon and dry season flow).

Keywords: Salinity intrusion, GBM delta, Climatic impact, Reduced discharge, Sea level rise, Tropical Cyclone

Location: In Bangladesh

Start Date:

End Date:

Program Area: Risk Management in Agriculture

Commodity: Climate change, Salinity intrusion

Objectives:

To determined by the combined action of tides and freshwater flow.

Methodology:

The Delft 3D open-source hydrodynamic and salt-transport model was selected to simulate salinity transport in estuaries, as it is a widely-used open source numerical modeling system. The Delft3D depth average model solves the continuity and momentum equations for an incompressible fluid, adopts a structured grid, and performs the spatial discretization of the equations using a cell-centered finite difference method. For this study, variable grid sizes were chosen in estuaries and oceans in such a way so that a finer resolution (186–243 m) lied in the inland and in the estuarine system that covers the study area satisfactorily, whereas a coarser resolution (1100–1700 m) was used in the ocean. Measured discharges upstream in the Ganges, Brahmaputra, and Meghna rivers and tidal water levels in the ocean were specified as upstream and downstream boundaries. Measured discharge data were collected from the Bangladesh Water Development Board (BWDB) for the year of 2011 (a dry year which is considered a base year for this study). The downstream tidal water level boundaries for 2011 were specified by using the global ocean tide model ‘NAO-tide’ that used the Nao 99b tidal prediction system considering 16 major tidal constituents. The bathymetry of the Bay of Bengal was generated using the open-access bathymetry data produced by the General Bathymetric Chart of the Oceans (GEBCO). The inland ground elevation data were collected from the Water Resources Planning Organization of Bangladesh (WARPO). The measured cross-sectional data for each of the estuary in the estuarine systems of the GBM delta at selected locations were surveyed by the ESPA delta (http://www.espa.ac.uk) and DECCMA (www.deccma.com) projects. For the salinity model, a constant sea salinity of 35 ppt for the Bay of Bengal [38] was specified as a sea boundary condition, and a constant sea salinity of 0 ppt for freshwater was specified for the upstream condition. For the cyclone-generated storm surge model, SIDR (made landfall in the central coast, 2007) was selected, as this cyclone is representative of a high-intensity cyclone, as well as landfall time-synchronized with a typical maximum salinity period. A SIDR track was collected from the Indian Meteorological Department (www.imd.gov.in). This research used a ‘generated’ SIDR scenario by changing the landfall time (the actual SIDR landfall was on 15 November; the usual cyclone season in the Bangladesh coast is April–May and October–November) of SIDR to be in the maximum salinity period, i.e., April 15. The wind and pressure field of cyclone SIDR was generated by using the Delft Dashboard, which was coupled with the salinity model (through the Delft3D interface). The simulation period for salinity intrusion coupled with the storm surge model was two years, from which the one year’s simulation result was used to attain a stable initial condition for the second year’s simulation. The time step used in the model simulation was 10 minutes.

Source of Information: Climate 2019, 7, 69;

Article Link (Online Journal): doi:10.3390/cli7050069

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Considering three dominant climatic events of salinity intrusion, this study found that the reduced discharge causes an increase of salinity in all types of the coast. The impact of SLR is more prominent in a relatively shallower coast. Salinity intrusion due to cyclonic scenarios is confined close to the cyclone landfall location. Cyclonic events increase the tidal range for a short duration during the passage of a cyclone along the coast. The relationship between flood velocity and an increase in salinity is non-linear for all types of coast Scientific studies in the low lying deltaic region (for example, in the Bangladesh coast) had mentioned with great care that SLR (or the rising tide level) is going to affect the coastal area severely by pushing more tidal water inland, inundating more coastal areas; in another way, this effect can be interpreted as bringing more salinity intrusion along the coast. Such emphasized scientific research of SLR impact on salinity intrusion results in more focus on governmental or non-governmental projects for the mitigation or adaptation of SLR. While there is no doubt of the increased salinity intrusion due to SLR, this research emphasis downplays the effects of reduced discharge conditions. This raises an important question: Whether the policymakers should only focus on SLR (which is a global phenomenon) or should focus on ensuring the optimal flow of an estuary (which is a regional phenomenon) to push saline water back to the ocean. In this era of globalization and anthropogenic development, barrages and dams are parts of river development and control projects, and, keeping this in mind, it is necessary to check whether such projects are interfering with the upstream flow that contributes to reducing downstream salinity of a delta. By investigating this idea, we might end up having much more devastating results than a 1.0 m SLR and increased cyclone frequency and intensity. Salinity intrusion is a lengthy process, and it is uncertain whether intruded salinity will reduce completely. Even if it reduces, it will be another very long-term process. As such, with increasing salinity intrusintrusion, it would be difficult to minimize the adverse effects. Keeping this fact in mind, regional cooperation in the basin-scale is necessary so that the present upstream discharge will not reduce.

Knowledge Management: Journal