3.3 Sediment transport processes over benthic ecosystems

Description

The fundamental aim of this project was to build on ‘conventional’ knowledge of sediment transport over bare sandy beds to develop new transport formulations applicable to a broad range of bottom types. Critically, it is shown here that conventional sediment transport models break down dramatically in canopy environments. These models typically either (1) ignore the flow attenuation provided by the canopy or (2) assign an enhanced bed stress to canopy regions (to diminish the flow), a correction which ultimately enhances sediment erosion (relative to adjacent regions without canopies). Both of these errors can result in massive over-prediction of the capacity for sediment to be mobilised within the canopy, and thus significant underestimation of the threat from dredging activities.

Aims

• To further our understanding of sediment dynamics in bioclastic environments by allowing for conversion of existing grain size distributions to settling velocities and thus allowing for the hydrodynamic properties and transport modes of grains to be established.
• To assess the ‘rough-wall’ boundary layer flow dynamics and turbulent shear stresses over a coral reef flat.
• To quantify the grain-size distribution and concentrations of suspended sediment in the water column.
• To evaluate how modifications to the mean and turbulent flow structure alter suspended sediment grain sizes, SSCs, and SSFs over a rough coral reef flat, including the implications for making robust sediment transport predictions within reef environments.
• To assess the variability of sea-swell waves, infragravity waves and mean currents at different locations and timescales in a fringing reef.
• To quantify the magnitude as well as the spatial and temporal variability of suspended sediment concentrations.
• To evaluate how the suspended sediment concentrations that were observed relate to the prevailing hydrodynamic processes.
• To build predictive capability for the two key hydrodynamic parameters, bed shear stress and near-bed turbulent kinetic energy, that are expected to govern sediment erosion and deposition in coastal canopies.