Combined effects of hydrodynamics and cohesive clay on bedform morphology and migration on sandy tidal flats
Mobile sediments form part of the morphology of the coastline, are habitats for diverse ecosystems and can help defend the coast from storm waves. A better understanding of sediment dynamical processes could lead to improved regional models for coastal management and defence.
Natural sediments can be made up of a varied mixture of mineral and organic material, reworked and stabilised by physical and biological processes. Many coastal and estuarine environments are dominated by theses mixtures of biologically active sand and mud. While good sediment predictors are available for clean sands, there is limited knowledge of the behaviour of mixed sediments composed of cohesive mud and non-cohesive sand. This could limit the applicability of models in such muddy-sand environments.
Recent laboratory experiments on mixed cohesive and non-cohesive sediments, influenced by biological stabilisation, have shown that bedform development time increases and dimensions decrease with increasing bed cohesive content. In this talk, the effect of cohesive material on sediment dynamics in a natural mixed sediment environment is explored, using measurements from fieldwork carried out on tidal flats in the Dee estuary. Over a two week period covering a spring-neap cycle, data were collected for hydrodynamics, bed morphology and seabed properties. Overall, sediment cohesion was found to make bedforms lower in height and shorter in length, and slows the bedform migration and material transport. As mixed cohesive sediment is found in many coastal areas worldwide, these results may have global relevance, if applied to regional models for sediment transport and coastal morphology.