24 research outputs found
Extension of k-ω turbulence closure to two-phase sediment transport modelling: application to oscillatory sheet flows
No full textA number of turbulence closure schemes can be employed to numerically investigate near-bed wave boundary layer and sediment transport problems. We present an extension of the widely used k-ω turbulence model to two-phase sediment transport modelling. In this model, a transport equation is solved for the turbulence specific dissipation rate ω. For two-phase models, this equation is similar to the one for clear fluids with an additional term due to inter-phase interaction terms, which we will discuss. The new k-ω model is then applied to wave and current sheet flows. We compare the numerical results from the k-ω model and from an existing k-ε model against sheet flow experimental data collected in oscillatory water tunnels. Both models provide broadly similar numerical results. Nevertheless, the k-ω and k-ε models display different behaviour around flow reversal, and neither is able to fully reproduce observed suspension peak at flow reversal
Parameterization of near-bed processes under collinear wave and current flows from a two-phase sheet flow model
No full textSediment transport models require appropriate representation of near-bed processes. We aim here to explore the parameterizations of bed shear stress, bed load transport rate and near-bed sediment erosion rate under the sheet flow regime. To that end, we employ a one-dimensional two-phase sheet flow model which is able to resolve the intrawave boundary layer and sediment dynamics at a length scale on the order of the sediment grain. We have conducted 79 numerical simulations to cover a range of collinear wave and current conditions and sediment diameters in the range 210–460 μm. The numerical results confirm that the intrawave bed shear stress leads the free stream velocity, and we assess an explicit expression relating the phase lead to the maximum velocity, wave period and bed roughness. The numerical sheet flow model is also used to provide estimates for the bed load transport rate and to inspect the near-bed sediment erosion. A common bed load transport rate formulation and two typical reference concentration approaches are assessed. A dependence of the bed load transport rate on the sediment grain diameter is observed and parameterized. Finally, the intrawave near-bed vertical sediment flux is further investigated and related to the time derivative of the bed shear stress
Salt Intrusion as a Function of Estuary Length in Periodically Weakly Stratified Estuaries
No full textEstuarine salt intrusion greatly threatens freshwater supply in surrounding lands. Physical barriers, which reduce the estuary length (L), are widely constructed to control salt intrusion. Yet, the role of L in salt intrusion remains unknown. Using a process-based, idealized, semi-analytical three-dimensional model, we systematically investigate for the first time this unknown for tide-dominated, periodically weakly stratified estuaries. Results show decreasing L significantly reduces salinities for short estuaries (L w/4, with Lw the dominant tidal wavelength), but not for long estuaries. Tidal pumping remains a key salt importer in most estuaries, regardless of L. However, substantial decreases in L relative to Lw/4 can change the dominant landward salt importer from tidal pumping to horizontal diffusion. The latter, together with gravitational circulation, weakens responses of salt intrusion to changes in tidal and river forcing in short estuaries. This study highlights the importance of considering L to understanding and mitigating salt intrusion
Intercomparison of the Charnock and CORE bulk wind stress formulations for coastal ocean modelling
No full textThe 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
Investigating how river flow regimes impact on river delta salinization through idealized modelling
No full textNetCDF files containing salinity output data from five Delft3D simulations. Annual flow distributions of equal water volume but different shape have been implemented. The data is named according to the input hydrograph's shape each time (e.g. leptokurtic, mesokurtic, platykurtic etc.
Residual circulation modelling at the national UK scale to identify possible sediment pathways to inform decadal coastal geomorphic evolution models
No full textA coastal area model of the UK has been setup to investigate the coastal residual circulation to identify possible sediment sources and sinks to inform coastal geomorphic models of alongshore and cross-shore sediment pathways. This study investigates the sensitivity of the annual residual to more realistic forcing and wave-current coupling. Two contrasting case study sites, one in the northwest the other in the southeast of the UK, are analyzed in more detail to identify the vertical variability within the residual circulation to suggest how the transport pathways for suspended and bedload material may differ to inform the regional management schemes
Impact of operational model nesting approaches and inherent errors for coastal simulations
No full textA 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 180 m grid nested within a 1.8 km 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
Multiscale temporal response of salt intrusion to transient river and ocean forcing
No full textSalt intrusion in surface waters endangers freshwater availability, influences water quality, and affects estuarine ecosystem services with high economic and social values. Salt transport and the resulting salinity distributions result from the non-linear interactions between salt and water dynamics. Estuaries are often considered under (quasi)-steady assumption or by focusing on specific timescales. Our understanding of their temporal multiscale response to transient forcing is limited, which hinders the implementation of effective mitigation strategies. We apply wavelet analyses to quantify the variability of salt intrusion from hourly to seasonal timescales and unravel the temporal variability of its response across scales. We focus on an estuary that undergoes significant transient forcing, the Modaomen estuary in the Pearl River Delta, and apply the wavelet analyses to year-long data generated by a coastal ocean numerical model. Our results show that this estuary responds to changes in tidal and riverine forcing throughout the year over interwoven timescales. Our results highlight the temporal variability of the salt intrusion response time both within a given regime and for the transition between regimes. They also suggest that tides control the response time more strongly than river discharge, even though river discharge determines the magnitude of the salt intrusion, and thus modulates the evolution of the salt intrusion response time. We propose a broadly applicable framework to calculate response times with simple data. These results can provide a first-order guidance for design and implementation of estuarine management strategies and mitigation measures that ensure water access and facilitate sustainable development
