Asymmetrical tides cause an uneven sediment redistribution within a system. The faster water level rise during the flood tide, compared to the slower decline during ebb tide, activates a tidal pumping process that transports additional sediment into the waterway. When the duration of the ebb tide exceeds that of the flood tide, there is less resuspension during ebb. Sediment primarily settles during slack tide, gradually filling the riverbed. One of the most significant consequences of this process is the decline in navigability of the waterway. An example of such an asymmetrically tide-dominated delta can be found in the southwest of Bangladesh, specifically in the Mongla-Ghasiakhali waterway. This area experiences high sediment concentrations and substantial sedimentation volumes. To address the sedimentation, a conceptual design has been developed, introducing locks at Mongla and Ghasiakhali.
The amount of sedimentation within the lock chamber and channel is strongly influenced by the frequency of lock gate operations. The primary water exchange processes during lock operation include the filling and emptying of the chamber, density currents, and boat movements. The schematized box method is employed to model the water and sediment movements resulting from these water exchange processes.
A tidal cycle consists of multiple operational cycles and, depending on the operational scheme that is used, possibly a resting period. Two operational schemes were simulated: a two way traffic scheme with 12 out of 24 operating hours and a fully operational scheme. Sedimentation in the chamber and channel can be minimised by adopting the two way traffic scheme with a flushing cycle after every 12-hour operating period. Under these conditions, the sediment layer thickness is 2.65∗10^-1 m in the chamber and 2.19∗10^-3 m in the channel after one month. When the growth of the sediment layer thickness is extrapolated to represent one year of operation, the annual sediment layer thickness growth is 3.22 m in the chamber and 2.67 ∗ 10^-2 m in the channel.
After analysing the behaviour of sediment, the optimum choices for the structural and maintenance design of the lock system are evaluated. These choices are based on the functional requirements, with a focus on sediment characteristics. The most suitable lock gates, considering sediment dynamics, are mitre gates, vertical lift gates, and radial (or tainter) gates. These gates offer low to moderate opening/closing times and are effective at pushing sediment away from the lock system. The preferred filling and emptying system for locks dealing with high sediment concentrations, is openings in lock gates.
The total required dredging volume is approximately 35 times smaller than what would be required in the absence of the lock construction. The minimum frequency of dredging the chamber is once per year, whereas the channel can be dredged once every 37 years. The dredging of the chamber must be carried out during the lock’s non-operating hours, while the channel dredging can take place throughout the day. The introduction of two navigation locks will reduce the total maintenance costs for the Mongla-Ghasiakhali waterway.