Impact Analysis of Sand Dredging from Alluvial Tidal River

The Government of Bangladesh has planned to make a large build up area (BUA) in a newly developed sedimentary point bar (locally known as ‘Char’) at Lebukhali along the northern bank of Payra river in the southern part of the country. From a study it was revealed that, this sedimentary bar has to be raised by 3.45 meters from MSL to keep it free from any extreme events (like tidal and storm surge effects) for 100 years return period. Accordingly, it was planned to collect the required filling material (estimated 57.6 million m3 of sand/soil) by dredging the same river bed. In Bangladesh dredging of river is very popular and widely undertaken to collect filling and construction material, but its adverse impact on river bed, bank and adjacent areas are hardly probed. This study aimed at analyzing the adverse impacts of dredging from Payra River bed. Four alternative sand bars have been identified as dredging area at varying distances located both upstream and downstream of the proposed site. The dredging impacts were analyzed for different boundary conditions. The study is based on physical survey (topographic and bathymetric surveys), historical data and numerical model analysis. Numerical model is applied through Delf 3D to investigate the impact on flow parameters: flood depth and velocity magnitude. Continuous field visit and observations were made through last one year to observe real impact. The analysis revealed that dredging of any of the bars have resulted changes in flow field. The distance of dredging point does not control the flow field rather the dredging volume, depth and upstream conditions determine the effect on bank erosion. As such, the dredging locations have been prioritized considering the least possible effects on river bank erosion and founded that little erosion is taking place for both upstream and downstream region. However, the long term impacts in other areas of the river need to be investigated as the time pass

Impact Analysis of Sand Dredging from Alluvial Tidal River

The Government of Bangladesh has planned to make a large build up area (BUA) in a newly developed sedimentary point bar (locally known as ‘Char’) at Lebukhali along the northern bank of Payra river in the southern part of the country. From a study it was revealed that, this sedimentary bar has to be raised by 3.45 meters from MSL to keep it free from any extreme events (like tidal and storm surge effects) for 100 years return period. Accordingly, it was planned to collect the required filling material (estimated 57.6 million m3 of sand/soil) by dredging the same river bed. In Bangladesh dredging of river is very popular and widely undertaken to collect filling and construction material, but its adverse impact on river bed, bank and adjacent areas are hardly probed. This study aimed at analyzing the adverse impacts of dredging from Payra River bed. Four alternative sand bars have been identified as dredging area at varying distances located both upstream and downstream of the proposed site. The dredging impacts were analyzed for different boundary conditions. The study is based on physical survey (topographic and bathymetric surveys), historical data and numerical model analysis. Numerical model is applied through Delf 3D to investigate the impact on flow parameters: flood depth and velocity magnitude. Continuous field visit and observations were made through last one year to observe real impact. The analysis revealed that dredging of any of the bars have resulted changes in flow field. The distance of dredging point does not control the flow field rather the dredging volume, depth and upstream conditions determine the effect on bank erosion. As such, the dredging locations have been prioritized considering the least possible effects on river bank erosion and founded that little erosion is taking place for both upstream and downstream region. However, the long term impacts in other areas of the river need to be investigated as the time pass

Recent sediment flux to the Ganges-Brahmaputra-Meghna delta system

The physical sustainability of deltaic environments is very much dependent on the volume of water and sediment coming from upstream and the way these fluxes recirculate within the delta system. Based on several past studies, the combined mean annual sediment load of the Ganges-Brahmaputra-Meghna (GBM) systems has previously been estimated to vary from 1.0 to 2.4 BT/year which can be separated into components flowing from the Ganges (260 to 680 MT/year) and Brahmaputra (390 to 1160 MT/year). Due to very limited data and small contribution of the Meghna system (6–12 MT/year) to the total sediment flux of the GBM system, the data of the Meghna is not considered in the analysis assuming the sediment flux from GB system as the sediment flux of GBM. However, in this paper our analysis of sediment concentration data (1960–2008) collected by Bangladesh Water Development Board shows that the sediment flux is much lower: 150 to 590 MT/year for the Ganges versus 135 to 615 MT/year for the Brahmaputra, with an average total flux around 500 MT/year. Moreover, the new analysis provides a clear indication that the combined sediment flux delivered through these two major river systems is following a declining trend. In most of the planning documents in Bangladesh, the total sediment flux is assumed as a constant value of around 1 billion tons, while the present study indicates that the true value may be around 50% lower than this (with an average decreasing trend of around 10 MT/year)

Sustainability of the coastal zone of the Ganges-Brahmaputra-Meghna delta under climatic and anthropogenic stresses

The Ganges-Brahmaputra-Meghna (GBM) delta is one of the world's largest deltas. It is currently experiencing high rates of relative sea-level rise of about 5 mm/year, reflecting anthropogenic climate change and land subsidence. This is expected to accelerate further through the 21st Century, so there are concerns that the GBM delta will be progressively submerged. In this context, a core question is: can sedimentation on the delta surface maintain its elevation relative to sea level? This research seeks to answer this question by applying a two-dimensional flow and morphological model which is capable of handling dynamic interactions between the river and floodplain systems and simulating floodplain sedimentation under different flow-sediment regimes and anthropogenic interventions. We find that across a range of flood frequencies and adaptation scenarios (including the natural polder-free state), the retained volume of sediment varies between 22% and 50% of the corresponding sediment input. This translates to average rates of sedimentation on the delta surface of 5.5 mm/yr to 7.5 mm/yr. Hence, under present conditions, sedimentation associated with quasi-natural conditions can exceed current rates of relative sea-level rise and potentially create new land mass. These findings highlight that encouraging quasi-natural conditions through the widespread application of active sediment management measures has the potential to promote more sustainable outcomes for the GBM delta. Practical measures to promote include tidal river management, and appropriate combinations of cross-dams, bandal-like structures, and dredging

Recent sediment flux to the Ganges-Brahmaputra-Meghna delta system

The physical sustainability of deltaic environments is very much dependent on the volume of water and sediment coming from upstream and the way these fluxes recirculate within the delta system. Based on several past studies, the combined mean annual sediment load of the Ganges-Brahmaputra-Meghna (GBM) systems has previously been estimated to vary from 1.0 to 2.4 BT/year which can be separated into components flowing from the Ganges (260 to 680 MT/year) and Brahmaputra (390 to 1160 MT/year). Due to very limited data and small contribution of the Meghna system (6–12 MT/year) to the total sediment flux of the GBM system, the data of the Meghna is not considered in the analysis assuming the sediment flux from GB system as the sediment flux of GBM. However, in this paper our analysis of sediment concentration data (1960–2008) collected by Bangladesh Water Development Board shows that the sediment flux is much lower: 150 to 590 MT/year for the Ganges versus 135 to 615 MT/year for the Brahmaputra, with an average total flux around 500 MT/year. Moreover, the new analysis provides a clear indication that the combined sediment flux delivered through these two major river systems is following a declining trend. In most of the planning documents in Bangladesh, the total sediment flux is assumed as a constant value of around 1 billion tons, while the present study indicates that the true value may be around 50% lower than this (with an average decreasing trend of around 10 MT/year).</p

Sustainability of the coastal zone of the Ganges-Brahmaputra-Meghna delta under climatic and anthropogenic stresses

The Ganges-Brahmaputra-Meghna (GBM) delta is one of the world's largest deltas. It is currently experiencing high rates of relative sea-level rise of about 5 mm/year, reflecting anthropogenic climate change and land subsidence. This is expected to accelerate further through the 21st Century, so there are concerns that the GBM delta will be progressively submerged. In this context, a core question is: can sedimentation on the delta surface maintain its elevation relative to sea level? This research seeks to answer this question by applying a two-dimensional flow and morphological model which is capable of handling dynamic interactions between the river and floodplain systems and simulating floodplain sedimentation under different flow-sediment regimes and anthropogenic interventions. We find that across a range of flood frequencies and adaptation scenarios (including the natural polder-free state), the retained volume of sediment varies between 22% and 50% of the corresponding sediment input. This translates to average rates of sedimentation on the delta surface of 5.5 mm/yr to 7.5 mm/yr. Hence, under present conditions, sedimentation associated with quasi-natural conditions can exceed current rates of relative sea-level rise and potentially create new land mass. These findings highlight that encouraging quasi-natural conditions through the widespread application of active sediment management measures has the potential to promote more sustainable outcomes for the GBM delta. Practical measures to promote include tidal river management, and appropriate combinations of cross-dams, bandal-like structures, and dredging