Bottom Boundary, Optics and Suspended Sediment

Oceans 2025 Theme 3, work package 3.3 & 3.4 combined

The aim of this research is to understand the basic turbulence and sediment processes from their source in the bottom boundary to their interactions and transport in the water column and then relate it to effects of light absorption and attenuation. The main output will be to use this understanding to improve current sediment transport and optical models to be used in Coastal morphodynamics and bathymetric evolution.

The understanding and mathematical representation of sediment transport process underpin our ability to predict the entrainment of sediments into the water column and the transport of sediments for forecasting seabed and coastal morphodynamic evolution. The difficulty in achieving accurate process models lies with the complex inter-dependence of sediment processes in the bottom boundary layer. Near the bed the fundamentals of sediment transport are governed by interactions between the sediment transport triad; the bed, the hydrodynamics and the mobile sediments. These three components interrelate, being mutually interactive and interdependent.

Sediment Process triad

Further up in the water column the processes controlling sediment transport relate to suspension, processes, advection and flocculation. Advection is the movement that results from the currents; suspension results from the competition between settling due to gravity and suspension due turbulent processes generated by these currents. Flocculation or aggregation is the result of the particle characteristics, the chemical and biological environment interacting with turbulence, which will sometimes help sometimes hinders this process. This modification of the properties of particles in the water column determine both the residence time of sediments in suspension (and so the potential transport rate) and the penetration of light through the water column.

Suspended sediment and turbulence at the mouth of the Dee Estuary over two tidal cycles. Top figure shows the volumetric concentration of fine particles, middle shows the turbulence shear production and bottom concentration of large particles. Concentrations are in µ1 1-¹ and production is in log10( W m-³). Figure show clear example tidal resuspension of fine and coarse material, tidal advection and the presence flocs near the surface.

A range of impacts from coastal sediment transport, seabed and coastal morphology, through to pollutant transport and the primary productivity of coastal waters are controlled by the behaviour of sediments in the water column. However, there is a recognised need for significant improvements in our capability to model the processes and their impacts, beyond the presently limited accuracy and applicability of widely-used empirical approaches. Pressing problems in the context of improving operational modelling around the UK coastline are the transport rates of sediments from estuaries into the coastal zone, and the impact that the sediments have on water opacity and the rates and distribution of primary production.

Numerical simulation of the Dee Estuary bed and suspended matter from a Lagrangean sediment transport model.

The Team

Alejandro Souza
Peter Thorne
Andrew Lane
Kyle Betteridge
Ben Moate
Jenny Brown
Judith Wolf
Laurent Amoudry