Predicting sediment erosion, and the interactions of a sediment bed with oscillatory flows in the nearshore, is of crucial importance to understand the morphological behaviour of the coastal system. Computations of sediment erosion typically employ simplistic models for incipient motion and resultant bedload and suspended transport. These models predict that incipient motion occurs when the force of a wave-induced flow is in excess of some average critical value, such as the Shield’s (e.g. Nino et al., 2003) or Sleath’s (e.g. Sleath, 1995) parameter, or a combination of the two (Frank et al., 2015). However these simplistic sand transport models fail when predicting wave-induced sand transport over ripples.
Ripples, typically 0.01–0.1 m high and 0.1–1.0 m long, are common bed form features across the coastal nearshore region (e.g. Williams et al., 2003). Above rippled beds, water movement and the associated sand dynamics in the near-bed layer are dominated by coherent motions, specifically by the process of vortex formation in the stoss- and lee-side of ripples (van der Werf et al., 2007). Therefore peak sediment concentrations do not correspond with the peak force, as often assumed in sand transport models. One consequence is these models are poor at replicating large-scale sediment transport patterns because they do not account for the net transport in the offshore direction caused by the stronger contribution of the lee-side vortex (Hurther and Thorne, 2011). This poor predictive capability has important implications for modelling the morphological evolution of the coastal system.
This evidence reveals that predictive models accounting for the interactions between near-bed coherent flow structures, ripple morphology and particle dynamics in wave-induced flows are needed. As a first step towards a fundamental understanding of the complex nature of ripple regime sand transport, a detailed study of the role of coherent flow structures on erosion is required. Without this information it will remain impossible to predict accurately sediment erosion rates, nor understand the role that coherent flow structures have on other processes such as seabed morphological evolution and the transport and dispersion of pollutants and nutrients. This information is crucial if we wish to understand how the increased threat of climate change and the resulting rise in sea level may accelerate erosion.
Aims and objectives
The studentship aims to understand the role of coherent flow structures in sand transport in the nearshore zone of coastal environments. The project will focus on the following objectives:
Characterizing the coherent flow structures present in the near-bed region over ripples in oscillatory flow
Understanding the effects of ripple morphology, flow depth and wave period and amplitude (as per rise and fall of the tide) on the size and dynamics of these flow structures
Linking the properties of the flow structures to particle entrainment and transport so that more suitable parameterisations of wave-induced sand transport can be developed