Transport of cohesive sediments: Classification and requirements for turbulence modelling

Transport of cohesive sediments: Classification and requirements for turbulence modelling

Bruens, A.W.

Civil Engineering and Geosciences

Hydraulic Engineering

1999-01-01

This report describes a classification of sediment-laden flows, which gives an overview of the different transport forms of fine sediment and the interactions of the different processes as acting in an estuary. At the outs et of the proposed classification a distinction in physical states of water-sediment mixtures is made. A water-sediment mixture can exist in four physical states: 1) dilute suspension, in which the mutual interference between sediment particles and the turbulent flow field is negligible, 2) concentrated benthic suspension, in which sediment induced stratification and buoyancy become important, 3) fluid mud, in which the flocs start to form a network (a gel), the motion of fluid mud is laminar by definition, 4) consolidating bed, in which a strength, large enough to withstand the driving forces, has developed. The dynamics of a water-sediment mixture due to (hydrodynamic) forcing depend upon the different physical states in the vertical. On the other hand, the dynamical response of the mixture affects this vertical structure due to exchange processes at the interfaces between the different classes. The acting processes can be: (hindered) settling, deposition, entrainment, erosion, consolidation and liquefaction. One of the objectives of the MAST3-cOSINUS project is to extend existing turbulence models in order to simulate sediment-laden flows for a wide range of Reynolds numbers. The molecular viscosity of sediment-water mixtures ean be substantially larger than the viscosity of clear water. Due to buoyancy effects near interfaces or a decrease in driving forces, the turbulent viscosity can be substantially decreased, resulting in low Reynolds numbers. If the standard turbulence models do not predict these viscous effects, a low-Reynolds number turbulence model has to be implemented. The 1DV POINT MODEL of Delft Hydraulics, with a constant molecular viscosity, is used for simulating the entrainment of a high-concentrated near bed layer. The agreement between experimental results and simulations is reasonable. Next a molecular viscosity depending on sediment concentration is implemented in the mean flow equation of the model. Preliminary results from this extended model show that in this model the fluid mud is dragged along by the overlying flowing fluid as is observed in experiments.

mud
mudflow
fine sediment

http://resolver.tudelft.nl/uuid:6b0d3003-1fba-4198-9549-140dd88d37e9

TU Delft, Department of Hydraulic Engineering

Report 2-99 - Prepared for the European Commission, DG XII MAST3 - COSINUS Project, Contract No. MAS3-CT97-0082

Institutional Repository

report

(c) TU Delft, Department of Hydraulic Engineering