On the behaviour of fine sediment in settling basins

Improving the understanding and prediction of return water quality assessments by a comparative modelling study

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In this study, the behaviour of fine sediment in settlings basins has been investigated. The understanding and prediction of return water quality assessments is improved by a comparative modelling study. Settling basins are required to decrease the exhaust of fine sediment particles when dredging and reclaiming land. Fine sediment particles can cause turbidity, which damages underwater life and the environment. Turbidity should therefore be prevented.
The study focusses on the assessment on settling basins during a tender phase. Often, limited information is available and time pressure is high. Therefore, it is required to have a straight-forward but robust method to evaluate the effectiveness of a settling basin design in meeting outflow criteria, based on a solid process understanding.
With a comparative modelling study, the main assumptions of the current assessment by Van Oord (width-uniform conditions, neglectable density effects) are researched. The models compared are an inhouse tool of Van Oord (RWQ model) and Delft3D. Next to the main assumptions, different types of settling behaviour throughout the basin are researched.
The research shows the importance of density driven currents in distributing the sediment concentration over the settling basin. This leads to width-uniform sediment concentration. This explains why the Delft3D model and the RWQ model predict the same outflow concentration (order of magnitude) while having different assumptions. Wind causes additional mixing and therefore higher outflow concentrations. The settling velocity of particles has a very high influence on the outflow concentration.
For future assessments the RWQ model can be used. For better process understanding, a Delft3D model could be added. When outflow concentrations are near critical, mitigating measures should be implemented. Mitigating measures are often a fraction of the costs compared to cancelling dredging vessels.