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A research project conducted within the Coastal Processes Research Group within the School of Marine Sciences and Engineering at the University of Plymouth, funded by the Natural Environment Research Council.

Introduction.

Reliable models of bathymetric evolution in the nearshore region are of fundamental importance for studies on the coastal impacts of global warming and relative sea level rise as well as for appropriate evaluation and execution of coastal engineering projects. Recent improvements in conceptual data fed models such as Marino-Tapia et al [2007] have not been matched by equivalent improvements in process based models which continue to require ad-hoc adjustments of model free parameters in order to begin to simulate the complex onshore and offshore migration of bars and troughs which are the most common features of nearshore morphology change. The goals of this project are oriented towards both understanding what critical processes may be missing from such models and then working towards developing methods by which these processes can be utilized in such models. While the motivator for the investigators of this project is the evolution of nearshore bathymetry, the processes identified and the knowledge gained, clearly have relevance in all areas of fluid granular transport.

Sediment-turbulence interaction.

Sediment transport models have evolved from single-phase hydrodynamic models with little or no corrections for sediment effects, assuming they are unimportant. Such assumptions are being challenged for high sediment concentrations, typical of conditions during storms. Important sediment-turbulence interactions and near-bottom density stratification have been observed, which suggests that discrepancies between measured and modelled flows and sediment concentrations can be explained by flow-sediment interaction effects. At very high sediment concentrations above the bottom destroy turbulence, thereby reducing Reynolds stress and bed shear, and increasing the thickness of the laminar sublayer. These concentrations may be closely related to the maximum suspension capacity of the fluid.

In a recent example, Conley et al. [2008] utilized comparisons between a column turbulence model and field observations to investigate the two way feedback between the instantaneous flow stratification which is caused by suspended sediment and the turbulence which mobilizes and suspends the sediment. They demonstrated that fluid turbulence is modified by the presence of suspended particles. Accounting for this process resulted in higher near-bed sediment concentrations and reduced concentrations higher in the water column which means that near bed onshore directed transport is increased and offshore directed transport higher in the water column is reduced. For changes in nearshore morphology, where transport divergence represents small difference between large gross quantities, the neglect of a physical process such as this may well explain the inability to simulate basic bathymetric changes such as onshore and offshore bar and trough migration.

Hypotheses and Research Approach.

The leading hypothesis for this work is that the two way feedback between instantaneous flow stratification by suspended sediment and the turbulence which suspends that sediment is a key factor in the generation of nearshore morphology. A subsidiary hypothesis is that the nature of this feedback is fundamentally dependant on the distribution of grain sizes in the sediment involved in the feedback.

These hypotheses will be tested by carefully developing a framework in which to study this process, validating and testing the framework, and then applying that framework to recreate carefully controlled field observations of nearshore morphological development in which the recreations are successively performed accounting for and ignoring the feedback.

TSSAR Waves involves a collection of approaches that will be utilized in order to determine what role sediment stratification plays in the development of nearshore morphology. The approaches include, continued numerical methods for studying sediment turbulence interactions which include improvements to account for a distribution of sediment grain sizes, laboratory experiments focused around an oscillating grid turbulence tank which will be used to confirm and calibrate approximations used in the numerical approach, field measurements designed to validate the results of the numerical framework in field conditions, and finally application of a virtual surf-zone which tests of the ability to reproduce observed changes in nearshore morphology through the inclusion of sediment stratification.