Laurent O. Amoudry

An approach for the identification of exemplar sites for scaling up targeted field observations of benthic biogeochemistry in heterogeneous environments

Continental shelf sediments are globally important for biogeochemical activity. Quantification of shelf-scale stocks and fluxes of carbon and nutrients requires the extrapolation of observations made at limited points in space and time. The procedure for selecting exemplar sites to form the basis of this up-scaling is discussed in relation to a UK-funded research programme investigating biogeochemistry in shelf seas. A three-step selection process is proposed in which (1) a target area representative of UK shelf sediment heterogeneity is selected, (2) the target area is assessed for spatial heterogeneity in sediment and habitat type, bed and water column structure and hydrodynamic forcing, and (3) study sites are selected within this target area encompassing the range of spatial heterogeneity required to address key scientific questions regarding shelf scale biogeochemistry, and minimise confounding variables. This led to the selection of four sites within the Celtic Sea that are significantly different in terms of their sediment, bed structure, and macrofaunal, meiofaunal and microbial community structures and diversity, but have minimal variations in water depth, tidal and wave magnitudes and directions, temperature and salinity. They form the basis of a research cruise programme of observation, sampling and experimentation encompassing the spring bloom cycle. Typical variation in key biogeochemical, sediment, biological and hydrodynamic parameters over a pre to post bloom period are presented, with a discussion of anthropogenic influences in the region. This methodology ensures the best likelihood of site-specific work being useful for up-scaling activities, increasing our understanding of benthic biogeochemistry at the UK-shelf scale.

Modelling-based assessment of suspended sediment dynamics in a hypertidal estuarine channel

We investigate the dynamics of suspended sediment transport in a hypertidal estuarine channel which displays a vertically sheared exchange flow. We apply a three-dimensional process-based model coupling hydrodynamics, turbulence and sediment transport to the Dee Estuary, in the north-west region of the UK. The numerical model is used to reproduce observations of suspended sediment and to assess physical processes responsible for the observed suspended sediment concentration patterns. The study period focuses on a calm period during which wave-current interactions can reasonably be neglected. Good agreement between model and observations has been obtained. A series of numerical experiments aim to isolate specific processes and confirm that the suspended sediment dynamics result primarily from advection of a longitudinal gradient in concentration during our study period, combined with resuspension and vertical exchange processes. Horizontal advection of sediment presents a strong semi-diurnal variability, while vertical exchange processes (including time-varying settling as a proxy for flocculation) exhibit a quarter-diurnal variability. Sediment input from the river is found to have very little importance, and spatial gradients in suspended concentration are generated by spatial heterogeneity in bed sediment characteristics and spatial variations in turbulence and bed shear stress.

Tidal, Riverine, and Wind Influences on the Circulation of a Macrotidal Estuary

AbstractThe effect of tides, river, wind and Earth’s rotation on the three-dimensional circulation in the Dee, a macrotidal estuary, are investigated using a fine-resolution model. The interactions of the large tidal amplitude, currents, river, and wind-generated circulation require baroclinic and unsteady studies to properly understand the estuarine dynamics. Assessment of the model skill has been carried out by model–observation comparisons for salinity, which is the main control for density, surface elevation, current, and turbulence. Stationary nondimensional numbers were only partially able to characterize the dynamics in this (real) complex macrotidal estuary. At low water, tidal straining and constrained river flow cause stratification. Large spatial variability occurs in the current and residual patterns, with flood-dominated maximum values occurring within the tidal channels. The tides control residual circulation by modulating stratification through tidal straining and bathymetric constraint on ...

Modeling flocculation in a hypertidal estuary

When fine particles are involved, cohesive properties of sediment can result in flocculation and significantly complicate sediment process studies. We combine data from field observations and state-of-the-art modeling to investigate and predict flocculation processes within a hypertidal estuary. The study site is the Welsh Channel located at the entrance of the Dee Estuary in Liverpool Bay. Field data consist of measurements from a fixed site deployment during 12–22 February 2008. Grain size, suspended sediment volume concentration, and current velocity were obtained hourly from moored instruments at 1.5 m above bed. Near-bottom water samples taken every hour from a research vessel are used to convert volume concentrations to mass concentrations for the moored measurements. We use the hydrodynamic model Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) coupled with the turbulence model General Ocean Turbulence Model (GOTM) and a sediment module to obtain three-dimensional distributions of suspended particulate matter (SPM). Flocculation is identified by changes in grain size. Small flocs were found during flood and ebb periods—and correlate with strong currents—due to breakup, while coarse flocs were present during slack waters because of aggregation. A fractal number of 2.4 is found for the study site. Turbulent stresses and particle settling velocities are estimated and are found to be related via an exponential function. The result is a simple semiempirical formulation for the fall velocity of the particles solely depending on turbulent stresses. The formula is implemented in the full three-dimensional model to represent changes in particle size due to flocculation processes. Predictions from the model are in agreement with observations for both settling velocity and SPM. The SPM fortnight variability was reproduced by the model and the concentration peaks are almost in phase with those from field data.

Fate and pathways of dredged estuarine sediment spoil in response to variable sediment size and baroclinic coastal circulation.

Most of the worlds megacities are located in estuarine regions supporting commercial ports. Such locations are subject to sedimentation and require dredging to maintain activities. Liverpool Bay, northwest UK, is a region of freshwater influence and hypertidal conditions used to demonstrate the impact of baroclinicity when considering sediment disposal. Although tidal currents dominate the time-varying current and onshore sediment movement, baroclinic processes cause a 2-layer residual circulation that influences the longer-term sediment transport. A nested modelling system is applied to accurately simulate the circulation during a three month period. The hydrodynamic model is validated using coastal observations, and a Lagrangian particle tracking model is used to determine the pathways of 2 sediment mixtures representative of locally dredged material: a mix of 70% silt and 30% medium sand and a mix of 50% fine sand and 50% medium sand. Sediments are introduced at 3 active disposal sites within the Mersey Estuary in 2 different quantities (500 and 1500 Tonnes). Following release the majority (83% or more) of the particles remain within the estuary due to baroclinic influence. However, particles able to leave follow 2 distinct pathways, which primarily depend on the sediment grain size. Typically the finer sediment moves north and the coarser sediment west. Under solely barotropic conditions larger sediment volumes (up to 5 times more) can leave the estuary in a diffuse plume moving north. This demonstrates the necessity of considering baroclinic influence even within a hypertidal region with low freshwater inflow for accurate particle tracking studies.

Effects of Instrumented Bottom Tripods on Process Measurements

AbstractThe measurement and assessment of ocean bottom processes are important sources of information for understanding bedform evolution and sediment entrainment and for improving numerical models. Instrumented tripods have been used to investigate bottom boundary layer and sediment dynamics processes for several decades. In this paper, the effects of instrumented tripods on hydrodynamics and on the sea bed are investigated via numerical modeling and field data collected under moderate to strong tidal currents and mild surface waves. Under high currents, streamlines are modified and structure-induced vertical velocities are produced. To minimize this effect, a rotation of the three-dimensional current measurement under the frame is recommended. Acceleration of the flow under the frame is also significant (on the order of 10%–20%), which leads to an increase in bottom stress and can produce a large scour pit in energetic currents. Wave–structure interactions mainly increase turbulence near the frame. No s...

SEDIMENT TRANSPORT MODULE FOR A B-GRID COASTAL SHELF OCEAN MODEL

We present a three-dimensional sediment transport model implemented within the Proudman Oceanographic Laboratory Coastal Ocean System (POLCOMS). An unlimited number of sediment classes can be transported both as suspended load and bed load. Suspended sediment concentration is calculated following advection-diffusion schemes similar to those used for other scalar quantities. The location of the sediment bed is obtained from near-bed sediment mass conservation and the sediment bed is represented in the model by a layered structure. The ability to reproduce suspended concentration profiles and morphological evolution for a simple case is tested against laboratory experiments of trench migration in a flume

Working Toward a Common Strategy for U.K. Sediment Transport Research: U.K. Sediment Initiative 2009: Developing Multidisciplinary Sediment Dynamics Research in a Strategic Context; Liverpool, United Kingdom, 27–29 April 2009

U.K. Sediment Initiative 2009: Developing Multidisciplinary Sediment Dynamics Research in a Strategic Context; Liverpool, United Kingdom, 27–29 April 2009; A workshop funded by the U.K. Natural Environment Research Council (NERC) brought together U.K.-based researchers, stakeholders, and policy makers with an interest in sediment processes to foster collaborative links and to help NERC theme leaders develop future Theme Action Plans (TAPs). This could be achieved only by identifying gaps in knowledge and prioritizing research needs toward formulating a U.K. sediment transport research strategy. More than 50 participants from NERC research and collaborative centers, U.K. higher education institution researchers, industry consultants, and government departments and agencies attended the workshop. The workshop was divided into three parts. First, in an introduction, guest speakers discussed the importance of sediment transport from different perspectives. The speakers included Darius Campbell (U.K. Department for Environment, Food, and Rural Affairs) on policy drivers, Richard Whitehouse (HR Wallingford, Ltd.) and David Lambkin (ABPMer, Ltd.) on industry needs, John Rees (NERC theme leader) on the NERC TAPs and possible funding opportunities, Alan Davies (University of Bangor) on the academic research perspective, and Chris Sherwood (U.S. Geological Survey) on international insight on large sediment transport projects and on developments of the U.S. National Community Sediment Transport Model project. Second, in a series of breakout sessions, participants considered the stakeholders’ needs, the different dynamic areas of the coastal ocean, and the process time scales. Third, the workshop group discussed possible funding streams and ways to better formulate a concerted research plan for presentation to funding bodies

Parameterization of intrawave ripple‐averaged sediment pickup above steep ripples

Near-bed sediment pickup is critical for predictions of intrawave suspension and in turn net sediment transport in coastal models. In the present study, numerical results from a two-dimensional Reynolds-averaged Navier-Stokes model are used to assess the functional relationship of intrawave ripple-averaged sediment pickup above steep ripples. The numerical model provides intrawave time histories of ripple-averaged near-bed velocities and turbulence, which are qualitatively interrogated to determine pickup functional relationships. Several specific sediment pickup formulations are implemented within the numerical model: expressions relating pickup to near-bed velocity or near-bed turbulent kinetic energy via the bed shear stress; and expressions relating pickup to near-bed shear production of turbulent kinetic energy. These are then tested via model-data comparisons of near-bed suspended sediment concentration. The results show that the traditional functions relating sediment pickup to near-bed velocity cannot lead to reasonable intrawave suspension predictions above vortex ripples under a ripple-averaged framework. Instead, relating sediment pickup to near-bed turbulence quantities, such as turbulent kinetic energy or shear production of turbulent kinetic energy, significantly improves the numerical predictions for these conditions. This article is protected by copyright. All rights reserved.

Improving decadal coastal geomorphic predictions: an overview of the iCOASST project

Coastal areas are already at high risk from a range of geohazards. The cumulative effect of human intervention on soft coastlines has frequently left them far from equilibrium under today’s conditions, especially in densely populated areas. Future changes in marine forcing due to climate change reinforce the need to understand and predict processes of change in shoreline position and configuration at management (decadal) scales. The UK-based iCOASST project is developing new and improved methods to predict decadal geomorphic evolution, linked to coastal erosion and flood risk management. This is based on a framework that links several components to develop a system-level understanding of this change. The framework includes: (1) new methods for system-level analysis and mapping of coast, estuary and inner shelf landform behaviour; (2) well validated ‘bottom-up’ hydrodynamic and sediment transport shelf models which can be applied at shelf scales to investigate inner shelf-coastal interactions; and (3) model compositions formed of existing or new ‘reduced complexity models’ of selected coastal landforms and processes that are suitable for multiple decadal length simulations. This will ultimately allow multiple simulations of coastal evolution which can explore uncertainties in future decadal-scale coastal response, including the effects of climate change and management choices. This paper outlines the current state of progress in the iCOASST Project.