Although computational sediment transport models are routinely used to support these projects, they often simply assume that the sediment is predominantly muddy (cohesive) or sandy (non-cohesive). This is clearly an over-simplification, and although models for the transport of mud/sand mixtures do exist, their applicability is limited by the lack of knowledge about how cohesive and non-cohesive sediments interact when mixed together. 

To advance present knowledge, HR Wallingford has funded measurements of the settling characteristics of a range of sediments in a laboratory flume. Mass settling fluxes of various mud/sand mixtures were measured by Dr Andrew Manning from the University of Plymouth, a Visiting Research Fellow at HR Wallingford.

'The tests covered a wide range of suspended sediment concentrations, mud/sand proportions and turbulence levels, explained Dr Manning. 'The LabSFLOC measurements have improved our understanding of how mud/sand mixtures settle under various combinations of shear stress, concentration and mud/sand proportions. We found that mud/sand suspensions behaved differently to mud only or sand only suspensions. The results highlight the need for wider investigations on erosion, transport and deposition of mixed sediments.'

The next step for HR Wallingford will be to implement the lab results in computational

models and identify how it can extend accurate simulation of mixed sediments.