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Sediment transport modelling

Method indicator
Bottom-Up Hybrid Top-Down
YES    

Summary of key issues:

Issue Description
Description Process based modelling of bed load and suspended sand and/or mud movement, with relationships to determine the rates of erosion and deposition.
Temporal applicability Typically applied to the short to medium-term (single tide up to several months)
Spatial applicability Typically estuary wide
Links with other tools Most often dynamically linked to hydrodynamic models thus allowing for a change in the hydrodynamic taking account of the change in bed elevations over the simulation time period. Can be linked to sediment budget analysis, expert geomorphological analysis, sediment quality and water quality, for example.
Data sources Model set up
Sediment characteristics:
  • Grain type and size;
  • Settling velocity;
  • Sediment density;
  • Critical shear stress;
  • Shear strength.
Sediment distributions:
  • Initial sediment thickness;
  • Mixing coefficients.
Boundary conditions:
  • Seaward and riverine suspended sediment concentrations;
  • Location of sediment inputs.
Calibration and verification data:
  • Suspended sediment concentrations;
  • Historic bathymetric data.
Necessary software tools / skills Hydrodynamic modelling system linked with a sediment transport module.
Typical analyses Mass balance equation uses predictions of current velocity and various sediment characteristics to output calculations of sediment concentrations. The effect of turbulence is included by applying a well-known Reynolds procedure.
Momentum balance for the fluid-sediment mixture is solved to represent the modification of transport of particles resulting from the presence of other particles in suspension (Brownian motion).
Limitations Correct sediment distributions are only produced if salinity is well reproduced as well as correct river discharges, tidal flow, topography and, intertidally, the occurrence of vegetation. Different equations are needed to deal with the differences between transports of cohesive or non-cohesive material,  Hence different models are often used when modelling sand and mud sediment.

Introduction

Sediment transport modelling methods are aimed at providing information regarding the movement of sediment within hydrodynamic systems, such as estuaries. Outputs from a sediment transport model include estimates of suspended sediment concentrations, rates of sediment erosion and deposition and sediment transport pathways.

The majority of sediment transport models are based on either outputs from hydrodynamic models or solve both hydrodynamic and sediment related equations at each time step of the model simulation. In both cases predictions of water levels and currents, driven by tidal, discharge, wave and meteorological forcing are used in the numerical solving of the equations describing sediment movements.

Because of the variety of sediment types, sizes and transport mechanisms occurring within an estuary a range of mathematical equations are required to simulate the sedimentary processes occurring in estuarine systems (Fredsøe & Deigaard, 1992; van Rijn, 1993; Soulsby, 1994; Whitehouse et al. 2000). In different models, varying formats and types of sediment transport equations are solved. Different equations are needed to deal with the differences between transports of cohesive or non-cohesive material. Hence different models are often used when modelling sand and mud sediment.

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Analysis and modelling

Last Modified on: 19 June 2011
Printed from the Estuary Guide on 17/09/2019 15:26:16