Benthic boundary layer processes and seabed dynamics in shelf seas are caused by both natural events (such as tidal currents, waves, winds, storms, and biological activity) and anthropogenic perturbations (for example dredging or trawling), and they are highly variable both in space and in time. This results in spatio-temporal variability of bed and suspended sediments, the latter of which are inherently controlled by the turbulent benthic boundary layer. Spatial variability of these bed and suspended sediments then has an important impact on overall sediment mobility, transport and pathways in shelf seas and estuaries, and thus on shelf seas ecosystems via the effect of suspended particulate matter (SPM) on light penetration for example. Indeed, a spatial gradient of suspended sediment in the presence of tidal currents results in sediment transport patterns that are distinct from the more typical resuspension-driven ones. This was first observed by Weeks et al. (1993), has since been reproduced in simple shelf sea models (e.g., Souza et al., 2007) and recently been shown to occur in tidally-dominated estuaries (Amoudry et al., 2014).
Nevertheless, the spatial variability of turbulent boundary layer processes and seabed dynamics, as well as its impact on sediment mobility and pathways, have yet to be fully characterised to the point of developing robust shelf seas predictive models. This is mostly due to a lack of necessary data sets, and the project will address this key gap via analyses of new extensive observational data collected in the Celtic Sea.
Spatial data on near-bed processes are difficult to obtain and are thus rare (e.g., Amoudry and Souza, 2011). However, new extensive observational data collected in 2014 and 2015 in the Celtic Sea as part of the NERC Shelf Sea Biogeochemistry programme will enable the studentship to focus on the spatial variability of benthic boundary layer processes in shelf seas. In particular, the studentship will seek to address how the inherent spatial variability of bed characteristics controls benthic boundary layer turbulent processes, seabed dynamics and transport of suspended sediment in shelf seas. The new observational data collected will enable the following hypotheses to be tested: 1) Spatial variability of near-bed turbulence and resuspension in shelf seas is generated by spatial variability in bed characteristics, independently of the influence of bathymetry (i.e. water depth). 2) Spatial variability in near-bed processes results in sediment transport pathways, and in feedback mechanism(s) linking spatial variability in bed characteristics and shelf seas sediment transport.
The studentship will focus on the following objectives: (i) characterize turbulent processes and dynamics in the benthic boundary layer for a series of contrasting shelf seas benthic environments, (ii) determine how spatial variability of the benthic environment controls these near-bed processes (iii) investigate the impact on transport and pathways of sediments at the shelf sea scale.
The studentship will use extensive in situ observational data from a number of deployments of a NOC instrumented benthic lander at several locations in the Celtic Sea, which present contrasting benthic environments in terms of sediment type along a permeable sand to cohesive mud gradient (sand, muddy-sand, sandy-mud, mud). Data from an additional deployment for a deeper sandy site will also be available. The lander is equipped with a suite of state-of-the-art instruments, which provide a unique and comprehensive series of data sets on near-bed hydrodynamics and sediment transport.
Near-bed hydrodynamic and turbulence is obtained from analysis of several acoustic Doppler velocimetry instruments, both at point locations (Acoustic Doppler Velocimeter) and for vertical profiles (Acoustic Doppler Current Profiler and high-resolution Nortek Pulse Coherent Acoustic Doppler Profiler). Analysis of these data will aim to resolve detailed characteristics of the turbulent tidal boundary layer (e.g., Souza et al., 2004; Souza and Howarth, 2005). Near-bed resuspension characteristics will be derived from data collected with the following instruments: Acoustic Backscatter System, LISST (Laser In Situ Scattering and Transmissometry) (e.g. Ramirez-Mendoza et al., 2014), and LISST-Holo (Holographic Particle Imaging System) (e.g. Davies et al., 2011). Small-scale bed topography (e.g., ripples) is measured with a 3D ripple profiler. The bed at each site has been fully characterised in terms of the sediment characteristics (grain size, bulk density, porosity) and benthic fauna from a series of cores, SPI (Sediment Profile Imaging) images and trawls.