Upscaling of bottom-generated turbulence in large-scale 3D models for sediment transport in estuaries and coastal zones

Currently used 3D numerical sediment transport models still fail to make good quantitative predictions. To a great extent, this can be attributed to the inadequate description of physical processes which occur at the subgrid scale level. From flume experiments it is known that particle-turbulence interactions near the bed significantly change the effective roughness experienced by the overlying water column. This results in different transport rates if not accounted for.From a theoretical perspective, bed load transport, sheet flow and fluid mud flow are all occurrences of supersaturated suspension flow in the inner near-bed layer comprising the viscous sublayer and the transient layer. Its thickness increases with sediment load, since particle-particle interactions (four-way coupling effects) consume considerable amounts of the available stream power. In order to know how much energy is left over to compute the transport capacity of the outer, fully-developed layer, it is necessary to quantify the energy budget in the inner layer.This is a difficult task. Every modelling approach has its draw-backs and limitations. Lagrangean particle tracking is hopeless, since the required number of particles to approach field conditions is much too high, and the volumes occupied by the particles cannot be neglected. Grain sizes are non-uniform in nature and concentrations near the bed very high, making it very difficult to give an accurate description of the momentum exchange between fluid and solid phase, which accounts for particle collisions. Therefore, in view of large-scale applications, a one-fluid approach is adopted. This implies that the momentum equation is solved for the suspension, together with a turbulence closure model and the sediment mass balance.Since the thickness of the supersaturated inner layer mostly is very small relative to the water depth and the vertical discretization in large scale applications, it is not possible to resolve this layer with a traditional low-Reynolds model approach, which requires a very fine grid. A new approach is proposed, where a modified Prandtl-mixing length (PML) model is used for the bed layer, and a new low-Reynolds model is applied in the outer layers. In this way it is possible to obtain a correct behaviour for tidal oscillating flow in estuaries, where low-Re effects enter high in the water column during slack water.The correction factor for the PML eddy viscosity and the damping functions for the low-Re k-epsilon turbulence model are constructed based on theoretical constraints, DNS and LES generated data, as well as experimental flume data. In parallel, LES and improved two-layer low-Re models are developed to simulate flow over rough bottoms without and with sediment, in order to generate data very close to the bed surface, where no measurements can be made. These additional data are used to help interpret experimental flume data, which always show relatively high experimental errors, and to extend the new damping functions for the cases with bottom roughness and suspended sediment.Preliminary results of the new coarse grid RANS model for open-channel flow with various roughness conditions without and with suspended sediment will be shown, compared to LES results for flow over a wavy bottom, low-Reynolds RANS results over rough bottom and experimental flume data

Impact assessment for the improved four boundary conditions (at bed, free-surface, land-boundary and offshore-boundary) on coastal hydrodynamics and particulate transport

The FIELD_AC project aims at providing an improved operational service for coastal areas and at generating added value for shelf and regional scale predictions. Coastal-zone oceanographic predictions seldom appraise the land discharge as a boundary condition. River fluxes are sometimes considered, but neglecting their 3D character, while the "distributed" continental run-off is not taken into consideration. Moreover, many coastal scale processes, particularly those relevant in geographically restricted domains (coast with harbors or river mouth areas), are not well parametrized in present simulations.Work package 3 dedicated to Boundary Fluxes aims to establish and use the best possible boundary conditions for coastal water quality modelling. On this scale, all boundaries become important. For the land boundary side the needed products are distributed and point wise run-off both quantitatively and qualitatively. For the offshore boundary condition, 3D current, water quality field, and wave spectra will be used. For the atmospheric boundary, products from local scale meteorological models (wind, atmospheric pressure and rainfall) are needed. For the seabed, boundary information on sediment composition, bedforms and bathymetry and bio-geo-chemical parameters is essential.This report addresses the impact assessment for improvements in the four boundary conditions (boundary fluxes from land, free-surface boundary condition, seabed boundary condition and open boundary fluxes) on coastal hydrodynamics and particulate transport. The description of the improved four boundary conditions is followed by examples of concrete impact assessment of the theory into the Catalan coast, Liverpool Bay, German Bight and Gulf of Venice