Probabilistic design of settling basins for environmental compliance

Abstract

The environmental impacts caused by suspended sediments are an important issue in the environmental impact assessment (EIA) of dredging and reclamation projects. This thesis is restricted to provide insight into the emission of suspended sediment particles due to the release of return water. This water is used to pump dredged material to the disposal area after which the excess water is released. Settling basins can be used to remove fines particles from the return water. The remaining concentration of suspended particles (outflow concentration) is hard to predict due to varying circumstances (e.g. wind, discharge and inflow concentration) and uncertainties in the settling process (e.g. agglomeration of clay particles). A probabilistic approach is a powerful method to incorporate these uncertainties. A probabilistic approach requires an efficient model that takes into account the relevant physical processes in a simplified way. A model is developed to simulated the transport of suspended sediment in the two dimensional vertical plane. Besides the turbulent mixing, processes as flocculation (agglomeration of clay particles) and secondary flow are included. This enables the model to provide realistic predictions of the concentration of suspended sediment in the vertical plane. The model is compared with measurement data and existing solutions which gives promising results. A probabilistic analysis of a case study provides insight into the main sources of uncertainty in the outflow concentration. Wind has a significant impact on the outflow concentration due to turbulent mixing and secondary flows. Furthermore, processes related to the clay particles (minimal settling velocity and flocculation) are very decisive. These parameters are proposed as calibration parameters. There appears to be an optimal basin depth. When the basin is deeper than this optimal basin depth, the positive effect of the longer residence time is eliminated by the larger turbulent mixing that is caused by the increased depth. By expressing the environmental risk of the contractor in a financial risk (fine or downtime), it is possible to determine an economic optimal design of a settling basin. This economic optimum is determined for both the basin depth and the discharge (the latter can be considered as the choice of equipment). The optimal choice of equipment is between the minimal production costs at an acceptable risk. For this risk, time effects and the time period over which the risk can be spread, play an important role. Finally, the profitability of wind protection for settling basins is investigated. This appears to be beneficial for the case study. The availability of a probabilistic model for determining the outflow concentration of settling basins offers interesting possibilities for a probabilistic analysis of environmental impacts of dredging and reclamation projects. This is because not only emissions are quantified but also insight is provided into the uncertainties and the sources of these uncertainties. This also enables the determination of an economically optimal design of a settling basin and provides insight in the associated financial risks.