Laboratory study on the efficiency of water injection dredging

Abstract

The Port of Rotterdam experiences large amounts of siltation from the upstream rivers in its basins and canals every year. Furthermore, container vessels increase in size every decade and therefore require a larger navigational depth. These two factors cause the yearly amount of dredged material to increase. Currently, mainly trailing suction hopper dredgers are used to remove the large amounts of sediment, but these vessels come with high costs and operational time. A more time and cost-efficient dredging method is therefore desired.

A potential solution to the increasing problems of costs and operational time by dredgers is the technique of water injection dredging. To make this technique as efficient and effective as possible, this thesis has the objective to measure and analyze flow and sediment properties for different parameter settings of water injection dredging and find the optimal parameter settings of this dredging technique on mud from the Port of Rotterdam. Large-scale experiments have been conducted in the water-soil flume of Deltares, where a jetbar was trailed (or run) over a bed 27 meters long and 0.5 meters deep of port mud while injecting it multiple times with water.
A positive correlation is observed between the production rate and jet momentum. The production is related linearly with the jet momentum using the Vlasblom equation combined with a non-dimensionless empirical fitting parameter per run per traverse velocity.
What is remarkable is that the intrusion depth by the jets increases with ascending runs while the jet parameter settings stay the same. The data shows that the difference between the mass flux by the density current and the mass flux stirred up by the jets when the sediment concentration of an undisturbed bed is assumed, increases between runs when an increased intrusion depth between those runs is observed as well. This indicates that the volume penetrated by the jets contains a smaller amount of sediment than was initially assumed, thus is disturbed by the previous run and is therefore decreased in strength. This decrease in strength results in a larger intrusion depth during the run itself in comparison to the previous run.

The influence of the SOD of the jets is analysed by comparing runs with a SOD to runs without a SOD but with similar remaining jetting parameter settings. When a SOD is applied, the jet pressure applied to the bed is outside the flow development region and therefore the jet pressure decreases with distance from the jet nozzle. The mass of sediment stirred up by the jets, for a SOD of 300 mm, is lower in comparison to a SOD of 0 mm. The density current transports relatively more sediment, however, when a SOD of 300 mm is applied. So, if a large amount of sediment needs to be stirred up, and therefore a large intrusion depth is required, no SOD should be applied and when large horizontal transport by the density current is desired, a SOD outside the flow development region of the jets should be used.