Cohesive Sediments

Sand and coarser sediments are non-cohesive, while finer sediments can cohere to each other. Cohesive material is usually defined as material with diameter less than 0.063mm and consists of silt and clay sizes.

The transporting capacities for the non cohesive sediment sizes are calculated using the Engelund or Ackers & White sediment transport functions and are multiplied by their proportions in the bed material, as discussed above. The concept of a continuous interchange between suspended and bed material, which underlies sediment transport computations for non cohesive sizes, does not apply for finer sediments since cohesion prevents material present on the bed from being available for transport. Also large concentrations can be transported when there is no cohesive material in the bed. Thus a method for calculating silt transport is needed which does not depend on the bed material composition. Westrich and Jurashek's (1985) method, which was originally developed for fixed boundary channels, can be used. It has been adapted, as described in Atkinson (1992), to enable it to apply to cohesive sediment mixtures covering a wide range of settling velocities.

Erosion of cohesive material is often inhibited by its cohesive properties. This can be described by a threshold shear stress, below which no erosion can occur. The sediment properties, the rate at which it was deposited and the degree of consolidation that has occurred since deposition determines this threshold. These processes are not included in the model and so a threshold value is input for each cohesive sediment fraction, advice on determining the threshold is provided in the last section of this manual.

The settling velocities for the cohesive sizes are also given in the cohesive sediment input. They should usually be derived directly from field data.

When erosion of cohesive material is being modelled using the SORTED algorithm, which is recommended, then erosion of a fraction is prevented when there is none of that fraction in the active layer. (For non-cohesive sediments this feature is incorporated automatically by the direct use of bed material composition in the sediment transport computations).