Abstract:
Dredging in a river is certainly a human intervention need to be analyzed for better planning and decision processes. Intensive assessment of river response due to dredging is needed for theses analysis though it is quite esoteric task as many variables are involved. In this study, an attempt has been made to analyze the river responses due to dredging on a selected reach of the Jamuna River by preparing a morphological model of this river. The study reaches covers from 30 km upstream of Bangabandhu Bridge to 20 km downstream of this bridge. Various important hydraulic structures are situated around this area like East Guide Bund and West Guide Bund of Bangabandhu Bridge, Sirajganj Hard Point and Bhuapur Hard Point. Among all, Sirajganj Hard Point area has been focused here as a prime concern. Prior to this, the two Idealized test channels i.e. straight and meandering channels are also modeled and the results are explored for better understanding the theoretical response of river due to dredging. Both the idealized test channels are 15 km long and 1 km wide. The conveyance areas of these channels are kept similar to the channel in front of Sirajganj Hard Point. To setup these morphological models, MIKE21C, an advanced two-dimensional mathematical modelling software developed by DHI, has been applied and numbers of simulations have been conducted for different dredging conditions to fulfill the study objectives.
From analyses, it has been found in idealized test channels that with the increasing of dredging depth as well as with the dredging width, dimensionless velocity increases along the dredged channel and decreases along the bank. Similar incidences have been observed also in Jamuna model that along the dredged channel the dimensionless velocity increases with the increase of relative dredging depth. It is also observed comparing the model results with Shield diagram that the bed materials along bank remain at the boundary of erosive and non erosive zone during average velocity of Jamuna. With higher dredging depth condition, the velocity along bank decreases in such an amount so that it becomes lower than the critical velocity and the bank becomes non erosive zone. In Jamuna River bed scour near Sirajganj Hardpoint decreases maximum 33.4% for 7 m of dredging depth and average rate of decreasing of bed scour is 5.5 % per meter of dredging depth. However, if dredging executed near Sirajganj Hardpoint, the channel became wider as the dredging depth increases. Finally, it has been found that the bank erosion is decreases with the increasing of dredging depth. In some point the bank erosion decreases about 50m due to dredging.
