Estimation of loss ratio of fine sediments for dredging and land reclamation

Dredged sediments from the sea bottom are being used as filling material in the Saemangeum reclamation area located on the west coast of Korea. The dredged sediments of Saemangeum contain sediments such as silt or mud from which the coarse sediments can be immediately deposited, whereas the fine sediments can remain in suspension in the spillway. Consequently, when the fine-sediment content in the dredged sediments is extremely high, the resulting loss ratio of sediments may become significantly larger than expected, thereby resulting in increased construction costs. In addition, the suspended sediments released through the outflow can increase the turbidity of seawater, which can form one of the major causes of seawater pollution. Therefore, for economical dredging and reclamation in the case of high content of fine sediments, estimation of the loss ratio of sediments is essential. In this study, we develop a novel approach based on the theory of cohesive fine-sediment transport to measure the loss ratio of sediments at the Saemangeum reclamation site. We perform a series of deposition experiments with an annular flume to determine the equilibrium suspended-sediment concentration for a given flow velocity or shear stress, which is used to estimate the loss ratio of sediments. In addition, we perform settling experiments for the purpose of reducing the loss ratio near the spillway. Unlike previous methods of assessing the loss ratio, our method has the great advantage of providing detailed information that can be used to reduce the loss ratio. Deposition experiments show that the loss ratio for dredged sediment is ~14.9-20.5%, which is very reasonable when compared with field data value of 15.3%. Further, the loss ratio can be greatly reduced if the travel time (at least 6 h) of the dredged sediment is increased to reach the equilibrium concentration and sufficient settling time (at least 1.4 h) is provided through the construction of a sediment basin near the outflow.