Impact of representation of hydraulic structures in modelling a Severn barrage

In this study, enhancements to the numerical representation of sluice gates and turbines were made to the hydro-environmental model Environmental Fluid Dynamics Code (EFDC), and applied to the Severn Tidal Power Group Cardiff-Weston Barrage. The extended domain of the EFDC Continental Shelf Model (CSM) allows far-field hydrodynamic impact assessment of the Severn Barrage, pre- and post-enhancement, to demonstrate the importance of accurate hydraulic structure representation. The enhancements were found to significantly affect peak water levels in the Bristol Channel, reducing levels by nearly 1 m in some areas, and even affect predictions as far-field as the West Coast of Scotland, albeit to a far lesser extent. The model was tested for sensitivity to changes in the discharge coefficient, Cd, used in calculating discharge through sluice gates and turbines. It was found that the performance of the Severn Barrage is not sensitive to changes to the Cd value, and is mitigated through the continual, rather than instantaneous, discharge across the structure. The EFDC CSM can now be said to be more accurately predicting the impacts of tidal range proposals, and the investigation of sensitivity to Cd improves the confidence in the modelling results, despite the uncertainty in this coefficient

Representation and operation of tidal energy impoundments in a coastal hydrodynamic model

A methodology associated with the simulation of tidal range projects through a coastal hydrodynamic model is discussed regarding its capabilities and limitations. Particular focus is directed towards the formulations imposed for the representation of hydraulic structures and the corresponding model boundary conditions. Details of refinements are presented that would be applicable in representing the flow (and momentum flux) expected through tidal range turbines to inform the regional modelling of tidal lagoons and barrages. A conceptual tidal lagoon along the North Wales coast, a barrage across the Severn Estuary and the Swansea Bay Lagoon proposal are used to demonstrate the effect of the refinements for projects of a different scale. The hydrodynamic model results indicate that boundary refinements, particularly in the form of accurate momentum conservation, have a noticeable influence on near-field conditions and can be critical when assessing the environmental impact arising from the schemes. Finally, it is shown that these models can be used to guide and improve tidal impoundment proposals

Multiscale hydro-environmental modelling of marine renewable energy devices, with particular application to the severn barrage

This research study presents enhancements to the hydro-environmental model Environmental Fluid Dynamics Code (EFDC), improving the predictive capabilities of the impacts of tidal range renewable proposals and dissolved phosphate concentrations in estuaries. Refinements to the representation of turbines and sluice gates, including updates to the discharge relationships used and momentum conservation were applied to the Severn Tidal Power Group’s Cardiff-Weston Barrage, providing an accurate assessment of the barrage’s potential impacts and highlighting the importance of correct hydraulic structure representation. The Severn Barrage was found to have minor impacts on peak water levels as far-field as the west coast of Scotland. The refinements reduced predicted peak water levels by up to 1 m upstream of the barrage. The applicability of the updated model in assisting with the design and optimisation of tidal lagoons was then tested by running a suite of different configurations of the Bridgwater Bay Lagoon, varying the turbine numbers from 60 to 360. It was demonstrated that additional turbines can negatively impact energy output, by reducing average generating time and generating over a lower head difference. Previous laboratory and field studies demonstrated a link between salinity and phosphate sorption to sediments due to the competition for sorption sites between seawater anions and phosphate. Since sediment-associated nutrients are not readily available for biological uptake, the dissolved proportion of phosphate is of particular importance when trying to predict the grown of phytoplankton and the potential for eutrophication. The salinity-linked sorption relationship was incorporated into the EFDC model to improve the prediction for dissolved phosphate across the estuary by taking into account the salinity variation. The refinement to the numerical calculation for the phosphate partition coefficient in the model caused a measurable change to the predicted dissolved phosphate levels, bringing them closer to measured data from the estuary

Sensitivity of tidal lagoon and barrage hydrodynamic impacts and energy outputs to operational characteristics

The feasibility and sustainable operation of tidal lagoons and barrages has been under scrutiny over uncertainties with regards to their environmental impacts, potential interactions and energy output. A numerical modelling methodology that evaluates their effects on the hydro-environment has been refined to consider technical constraints and specifications associated with variable turbine designs and operational sequences. The method has been employed to assess a number of proposals and their combinations within the Bristol Channel and Severn Estuary in the UK. Operational challenges associated with tidal range power plants are highlighted, while also presenting the capabilities of modelling tools tailored to their assessment. Results indicate that as the project scale increases so does its relative hydrodynamic impact, which may compromise annual energy output expectations if not accounted for. However, the manner in which such projects are operated can also have a significant impact on changing the local hydro-environment, including the ecology and morphology. Therefore, it is imperative that tidal range power plants are designed in such a way that efficiently taps into renewable energy sources, with minimal interference to the regional hydro-environment through their operation

Implementation of a full momentum conservative approach in modelling flow through tidal structures

Understanding the impact of various hydraulic structures, such as coastal reservoirs and tidal range impoundments, has been one of the key challenges of hydro–environmental engineering in recent years. Over the last half-century, several proposals for tidal range schemes in the UK have been scrutinised and then abandoned due to the uncertainty over the environmental footprint and/or the cost of electricity. Therefore, it is essential to understand the fundamental assumptions for reliable hydrodynamic analysis of these projects. This study examined the impact of the fully conserved momentum through tidal structures using a novel approach. The method was applied to 2D and 3D versions of the regional model of Swansea Bay tidal lagoon, examining two different types of velocity distribution at turbine exit. A simplified distribution significantly increased the velocity and length of the jet exiting the turbines during power generation. A realistic distribution gave more accurate results, with jet velocities more closely resembling the situation without including the momentum. The 3D model with realistic distribution has markedly improved the resulting vertical velocity profile. The value of the improved methodology for momentum conservation has proved to be particularly useful in local-scale studies. It can be applied to other similar hydraulic structures and used for the analysis of sediment transport, water quality, et