Modelling particulate transport of sediment disposal in marine environment – a near-far field coupling approach

Shu Ning Chan, Adrian C.H. Lai, Adrian W.K. Law

Wednesday 1 july 2015

12:00 - 12:15h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 9A. Sediment - Transport

In marine dumping of dredged materials, sediment is usually disposed as instantaneous sources from barges and hoppers. It is essential to quantify and assess the transport and fate of sediment particles, in order to minimize the impact to the marine environment, and to maximize the cost effectiveness of operation. The “near field” dynamics of an instantaneous disposal of sediment in the open ocean is characterized by the formation of a dense thermal due to the downward buoyancy force by sediment particles. Particles are initially transported by the thermal induced vortex circulation downward, in a vertical velocity greater than the settling velocity of individual sediment particles. As the thermal descent velocity is comparable to settling velocity of individual particles, sediment starts to settle out from the fluid phase under its inertia and is transported by the ambient flow and settle in their individual terminal velocities, subjected to mixing and dispersion by turbulence (the “far field”). This paper presents the development of a modeling methodology for predicting the transport of coarse and fine sediment in an instantaneous disposal operation in a moving ambient from the near field to the far field. The near field transport/descent of sediment cloud can be predicted by an integral model accounting for the conservation of fluid volume, momentum and buoyancy induced by sediment particles. Particle motion is solved using the Lagrangian governing equation of motion. The far field transport and dispersion of suspended sediment in ambient current is predicted using a shallow water hydrodynamic model, solving the Eulerian advection-diffusion equation for sediment. The two regimes are seamless coupled by releasing mass sources of sediment in corresponding grid cells, when the thermal descent velocity is equal to the particle settling velocity. Model prediction of bottom deposition profiles compares well with laboratory data of instantaneous sediment disposal in a current. An example of model application for suspended solid impact assessment in a tidal environment is demonstrated.