Modelling of Sediment Clump Dynamics in Open Water Disposal

With the rapid growth of population and economy in coastal cities worldwide, dredging and land reclamation have become vital activities to facilitate maritime industry navigation and infrastructure expansion. However, open water disposal of sediment during these processes presents significant environmental concerns, particularly the accuracy of sediment placement as well as high turbidities induced by suspended sediment. To address these issues, a comprehensive modelling of the near-field sediment dynamics is crucial for engineering assessment. In current practices, the open water disposal of sediment is often modelled as a group of dense particles released instantaneously into water, which induces a circulating vortex sediment cloud that holds the particles together while descending. However, this may not accurately represent the actual scenario, especially when finer sediments are involved which can lead to the formation of sediment clumps. These clumps feature a fast- descending sediment core but also with sediments shedding along its path. They pose a greater threat to the environment as larger amounts of sediments become suspended in the water column. Despite their importance, a comprehensive understanding of the sediment clump dynamics is still lacking. This study proposes a new mathematical model based on force balance to assess the sediment clump behavior. The model accounts for the two key aspects of sediment clumps: the fast settling of the front core, and sediment shredding due to the lee-side vortices. The sediment shedding is a critical aspect of the model, determining the sediment mass in the front core. It is modelled as a function of the Cloud Number which is a dimensionless parameter governing the disposed sediment formation. Experimental measurements were obtained in the laboratory to validate the accuracy and reliability of the model. The results demonstrate that the proposed model is able to predict the gross sediment clump dynamics, and thus it is an useful tool in the assessment of open water sediment disposal strategies to mitigate environmental impacts.