Abstract
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.
Highlights
Bangladesh has a large network of rivers, streams and canals with a total length of the river up to 50 m width at least 24,000 km and an area of 4,600 km2 [7]
The relative flood depth of the study area and estimated velocity for various sections are presented in table 3 and table-4 respectively
Impacts on flow field due to scenario-2 show that minimum flood depth in the study area is increased to 460%, where minimum right bank velocity will increase 114% and 179% for main channel
Summary
Bangladesh has a large network of rivers, streams and canals with a total length of the river up to 50 m width at least 24,000 km and an area of 4,600 km2 [7]. The nature of any river channel is a result of three main variables: the material properties of the bed and banks, the flow hydraulics and the sediment transport within the river All of these variables interact to produce the channel form, and an alteration in any one of them will cause a knock-on effect upon the others. Unusual depression of river bed caused due to dredging, alters the equilibrium profile of the streambed, creating a locally steeper gradient at upstream as shown in figure 1 [3] This over-steepened nick point (with its increased stream power) commonly erodes upstream in a process known as head cutting which may propagate upstream for kilometres on the main river [4,5] and up tributaries. Created gravel pits trap much of the incoming sediment, passing hungry water downstream, which typically erodes the channel bed and banks to regain at least part of its sediment load [6]
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