Modeling study of flocculation effects on sediment transport in estuaries.

摘要

Our current understanding of sediment transport in rivers and estuaries is insufficient to permit quantitative predictions of the fate of fine-grained sediment particles. In order to improve our ability to model sediment transport and depositional patterns, flow models need to be coupled with models that allow for the creation and destruction of flocs and subsequent changes in their settling velocity. A size-resolved flocculation model has been developed and tested to fulfill this goal. The flocculation model can predict the temporal evolution of the floc size distribution undergoing aggregation and breakup. In addition to flocculation, a one-dimensional (1-D) model has been developed where we consider particle settling, deposition and erosion and calculate the floc size distribution depending on friction velocity. The 1-D simulation is verified by comparing with observed size distributions over tidal cycles by Bale et al. [2002]. The flocculation scheme has been successfully incorporated with the sediment transport component in a 3-D hydrodynamic circulation model (Princeton Ocean Modeling (POM)). An idealized study to simulation of ETM variations over tidal cycles has been carried out. The continuous variations in floc size and settling velocity from the model study indicate that a fixed settling velocity does not well represent particle settling. The combination of gravitational circulation convergence and tidal asymmetry associated with settling flocs are primarily responsible for an ETM formation. Lateral circulation in estuaries which results in cross-channel transport of water mass and suspended sediments is important for lateral trapping of particles and formation of axial asymmetrical channel profile. An idealized three-dimensional simulation is done to investigate the effects of lateral circulation on lateral trapping of sediments associated with flocculation processes.