We present a two-dimensional, two-phase model for non-cohesive sediment transport. This model solves concentration-weighted averaged equations of motion for both fluid and sediment phases. The model ...accounts for the interphase momentum transfer by considering drag forces. A collisional theory is used to compute the sediment stresses, while a two-equation (
k–
ε) fluid turbulence closure is implemented. A benchmark sediment transport problem concerning the scouring downstream of an apron is carried out as an example and numerical results agree with existing experimental data.
Benthic sediments in continental shelf seas control a variety of biogeochemical processes, yet their composition, especially that of fine sediment, remains difficult to predict. Mechanisms for mud or ...fine sediment deposition and retention are not fully understood. Using sediment data and a hydrodynamic model of the Northwest European shelf seas, a relationship is shown to exist between fine benthic sediment composition and regions of cyclonic tidal current rotation. The reduced thickness of cyclonic tidal benthic boundary layers compared with the anticyclonic case promotes deposition of fine sediment and trapping of resuspended material. Adding the effects of the benthic boundary layer thickness, as influenced by ellipticity or not, sheds some light on the limitations of approaches only focusing on bed shear stress and sediment pathways to predict the location of mud deposits. A tidal boundary layer predictor that includes ellipticity alongside tidal current magnitude and depth was shown to spatially agree with maps of mud deposits.
•Regions of fine benthic sediment coincide with cyclonic tidal currents in shelf seas.•Currents rotating opposite the Coriolis force limit tidal boundary layer thickness.•Predicted thin cyclonic boundary layers spatially agree with fine sediment deposits.
The need for hydrokinetic turbine wake characterisation and their environmental impact has led to a number of studies. However, a small number of them have taken into account mobile sediment bed ...effects. The aim of the present work is to study the impact of the presence of a horizontal-axis three-bladed turbine with the flow and a mobile sediment bed. We use a series of laboratory experiments with a scaled modelled turbine installed in a flume with a mobile sandy bed at the bottom. Acoustic instruments were used to monitor flow, suspended sediment and bed behaviour. Results show a velocity decrease of about 50% throughout the water column and no flow recovery after a distance of 15 rotor diameters. Clearly visible ripples in the absence of the model turbine were replaced by horseshoe-shaped scour pit in the near wake region, and a depositional heap in the far wake. Suspended sediment differences were recorded in the streamwise direction with a possible effect of the wake as far as 15 rotor diameters. These results imply potentially important effects on the efficiency of turbine arrays, if the flow were to be lower than expected, on turbine foundations and modify coastal sediment transport.
•The relationship between a modelled turbine, flow and sediments is investigated.•A velocity decrease of about 50% was found in the presence of the rotor.•No flow recovery was recorded after a distance of 15 diameters from the rotor.•A horse-shoe shape scour pit was developed in the near wake region.•Suspended sediment patterns were affected as far as 15 diameters from the rotor.
The accurate parameterisation of momentum and heat transfer across the air-sea interface is vital for realistic simulation of the atmosphere-ocean system. In most modelling applications accurate ...representation of the wind stress is required to numerically reproduce surge, coastal ocean circulation, surface waves, turbulence and mixing. Different formulations can be implemented and impact the accuracy of the instantaneous and long-term residual circulation, the surface mixed layer, and the generation of wave-surge conditions. This, in turn, affects predictions of storm impact, sediment pathways, and coastal resilience to climate change. The specific numerical formulation needs careful selection to ensure the accuracy of the simulation. Two wind stress parameterisations widely used in the ocean circulation and the storm surge communities respectively are studied with focus on an application to the NW region of the UK. Model-observation validation is performed at two nearshore and one estuarine ADCP (acoustic Doppler current profiler) stations in Liverpool Bay, a hypertidal region of freshwater influence (ROFI) with vast intertidal areas. The period of study covers both calm and extreme conditions to test the robustness of the 10 m wind stress component of the Coupled Ocean-Atmosphere Response Experiment (COARE) bulk formulae and the standard Charnock relation. In this coastal application a realistic barotropic-baroclinic simulation of the circulation and surge elevation is set-up, demonstrating greater accuracy occurs when using the Charnock relation, with a constant Charnock coefficient of 0.0185, for surface wind stress during this one month period.
The extraction of power from the flow of water has become an important potential source of clean energy. In spite of significant interest in the interaction between energy extraction devices and ...water currents, comparatively little work has focused on flow asymmetry. Indeed, unusual wake behaviour and limits of turbine array efficiency have typically been attributed to boundary effects rather than the particular turbine geometry. The aim of the present study was to reveal the asymmetries in the hydrodynamic wake and the interactions with the sediment bed due to the presence of a hydrokinetic turbine. We combined: (i) computational fluid dynamics simulations; (ii) optical flow measurements from a series of flume experiments above a fixed rough bed; and (iii) acoustic measurements from a further series of flume experiments above a mobile sand bed. Results showed flow asymmetry due to the presence of the rotor which appeared to be related to the development of the wake and potentially to the gyre of the blades. Suspended sediments in the flume also exhibited asymmetrical characteristics due to the flow asymmetry. This imbalance in the flow field and sediment transport may decrease energy extraction efficiency in turbine arrays and also could have important environmental consequences.
•Flow asymmetries and their effects on the sea bed were investigated.•Near bed flow cross-stream differences were clearly seen far from the turbine.•Sediment scour and deposition showed a non-uniform morphology at each side of the wake.
Ripples and small-scale bedforms are ubiquitous in shallow water environments under the combined action of currents and waves. Small scale processes linked to their formation and migration are ...interconnected with sediment transport at larger scales (e.g. tens of metres to kilometres), both resulting in and being affected by large scale sediment transport and geomorphological evolution. The lower shoreface provides a key link between coasts and continental shelves, but the contribution of ripples and small-scale bedforms to sediment transport in this region has yet to be fully addressed. This work presents a study of sediment dynamic processes on the lower shoreface in the presence of small-scale bedforms. Observations were made during the winter of 2017 on the lower shoreface of Perranporth Beach, which is in the south west of the UK and exposed to Atlantic waves. The analysis of morphological expressions and the variability of ripples under waves, currents and wave-current conditions are assessed. Ripple morphology and associated dynamics are analysed for their potential contribution to the exchange of sediment between the lower and the upper shoreface. In the present study it was observed that even though ripples were evolving depending on the wave-current forcing, little ripple migration was observed due to low wave skewness. The implication is that ripple migration and bedload transport are only a small contribution to onshore sediment transport under low to moderate energy conditions. However, during more energetic conditions, ripples were washed out and the wave skewness increased, resulting in onshore sediment transport under a sheet flow regime. This suggests that ripple formation and migration can have little impact on the cross-shore supply of sediment from the lower shoreface to the upper shoreface and that more energetic wave conditions are required to significantly transport sediment towards the beach.
•High-resolution observations on the seabed reveal 2D and 3D wave vortex ripples in a macrotidal sandy lower shoreface.•The morphological ripple transitions provide assessment of sediment transport regime thresholds.•The hypothesis of ross-shore sediment supplied from the lower to the upper shoreface through ripple migration is not supported.
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 upscaling 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.
Results from a 1D setup of the European Regional Seas Ecosystem Model (ERSEM) biogeochemical model were compared with new observations collected under the UK Shelf Seas Biogeochemistry (SSB) ...programme to assess model performance and clarify elements of shelf-sea benthic biogeochemistry and carbon cycling. Observations from two contrasting sites (muddy and sandy) in the Celtic Sea in otherwise comparable hydrographic conditions were considered, with the focus on the benthic system. A standard model parameterisation with site-specific light and nutrient adjustments was used, along with modifications to the within-seabed diffusivity to accommodate the modelling of permeable (sandy) sediments. Differences between modelled and observed quantities of organic carbon in the bed were interpreted to suggest that a large part (>90%) of the observed benthic organic carbon is biologically relatively inactive. Evidence on the rate at which this inactive fraction is produced will constitute important information to quantify offshore carbon sequestration. Total oxygen uptake and oxic layer depths were within the range of the measured values. Modelled depth average pore water concentrations of ammonium, phosphate and silicate were typically 5–20% of observed values at the muddy site due to an underestimate of concentrations associated with the deeper sediment layers. Model agreement for these nutrients was better at the sandy site, which had lower pore water concentrations, especially deeper in the sediment. Comparison of pore water nitrate with observations had added uncertainty, as the results from process studies at the sites indicated the dominance of the anammox pathway for nitrogen removal; a pathway that is not included in the model. Macrofaunal biomasses were overestimated, although a model run with increased macrofaunal background mortality rates decreased macrofaunal biomass and improved agreement with observations. The decrease in macrofaunal biomass was compensated by an increase in meiofaunal biomass such that total oxygen demand remained within the observed range. The permeable sediment modification reproduced some of the observed behaviour of oxygen penetration depth at the sandy site. It is suggested that future development in ERSEM benthic modelling should focus on: (1) mixing and degradation rates of benthic organic matter, (2) validation of benthic faunal biomass against large scale spatial datasets, (3) incorporation of anammox in the benthic nitrogen cycle, and (4) further developments to represent permeable sediment processes.
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.
Key Points:
Functional relationship of sediment pickup above steep ripples is assessed
Traditional near‐bed velocity‐dependent pickup is shown as inadequate
Relating pickup to near‐bed turbulence significantly improves model results
•Identification of inherent problems with model nesting.•Demonstration of the importance of boundary conditions for ROFI expansion.•Demonstration of the impact of initial horizontal density gradients ...on annual ROFI dynamics.
A region of freshwater influence (ROFI) under hypertidal conditions is used to demonstrate inherent problems for nested operational modelling systems. Such problems can impact the accurate simulation of freshwater export within shelf seas, so must be considered in coastal ocean modelling studies. In Liverpool Bay (our UK study site), freshwater inflow from 3 large estuaries forms a coastal front that moves in response to tides and winds. The cyclic occurrence of stratification and remixing is important for the biogeochemical cycles, as nutrient and pollutant loaded freshwater is introduced into the coastal system. Validation methods, using coastal observations from fixed moorings and cruise transects, are used to assess the simulation of the ROFI, through improved spatial structure and temporal variability of the front, as guidance for best practise model setup. A structured modelling system using a 180m grid nested within a 1.8km grid demonstrates how compensation for error at the coarser resolution can have an adverse impact on the nested, high resolution application. Using 2008, a year of typical calm and stormy periods with variable river influence, the sensitivities of the ROFI dynamics to initial and boundary conditions are investigated. It is shown that accurate representation of the initial water column structure is important at the regional scale and that the boundary conditions are most important at the coastal scale. Although increased grid resolution captures the frontal structure, the accuracy in frontal position is determined by the offshore boundary conditions and therefore the accuracy of the coarser regional model.
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