Severn Barrage

Abstract
In recent years, there have been growing international challenges relating to climate change and global warming, with a conflict developing between the need to create a low-carbon economy and rapid depleting reserves of fossil fuels. In addition to these challenges there continues to be the added complexity of a significant global increase in energy demand. Marine renewable energy from tidal barrages is carbon-free and has the potential to make a significant contribution to energy supplies now and in the future. Therefore, it is appropriate to evaluate the total energy that can be extracted from such barrages. In this study two different methods are proposed to estimate the total annual energy output from a barrage, including a theoretical estimation based on the principle associated with tidal hydrodynamics, and a numerical estimation based on the solutions obtained from a 2D hydrodynamic model. The proposed Severn Barrage in the UK was taken as a case study, and these two methods were applied to estimate the potential annual energy output from the barrage. The predicted results obtained using the two methods indicate that the magnitude of the annual energy output would range from 13 to 16 TWh, which is similar to the value of 15.6 TWh reported by the Department of Energy and Climate Change, in the UK. Further investigations show that the total annual energy output would increase by about 15% if a higher discharge coefficient were to be adopted for the sluice gates, or if the turbine performance were to be improved. However, the estimated
annual energy output could exceed the value of 16 TWh if future technological advances in both sluice gate construction and turbine performance are included.

Many research projects in recent years have focused on marine renewable energy devices and structures due to the growing interest in marine renewable energy. These devices and structures have very different life spans. Schemes such as the Severn Barrage in the UK, as originally proposed by the Severn Tidal Power Group (STPG), would be the largest tidal renewable energy generation project in the world and would be operational for well over a century if built. Due to the long working life of some of these marine renewable energy schemes, it is important to study the impacts of climate change on such schemes, and particularly sea level rise. This study focuses on investigating the impacts of sea level rise due to climate change on the largest macro-tidal estuary in the UK, namely the Severn Estuary and Bristol Channel, and the alterations of the impacts and the performance of the Severn Barrage as a result of climate change. A hierarchy of computer models was implemented to identify the more localised impacts of climate change in the region of the study. Moreover, the potential benefits of the barrage on reducing flood risk, as well as the impact of climate change and the barrage on intertidal mudflats were investigated. The model predictions showed that the barrage would reduce flood risk due to the sea level rise. Furthermore, annual power output and the initial reduction in flood risk of the barrage would not be affected by sea level rise.

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 they efficiently tap into the renewable energy sources, with minimal interference to the regional hydro-environment through their operation.

The Severn Estuary has one of the largest tidal ranges in the world and has long been the subject of consideration for tidal energy generation. Whilst plans to build a tidal barrage across the estuary have existed in various forms since the 1920s, the 1989 Severn Tidal Power Group (STPG) proposal is commonly referred to as the original Severn Barrage. The UK government abandoned this ebb generation scheme as a public investment project in 2010. However, plans to build a two-way generation scheme were more recently put forward by a private consortium, namely Hafren Power. To assess the impact that a barrage would have on the hydro-environment in the estuary a number of numerical modelling studies have previously been conducted for the STPG scheme. As this design has now been superseded by the Hafren Power proposal, new studies have been conducted to investigate the impact of a two-way scheme. In this study the hydro-environmental impacts of both ebb-only and two-way schemes were assessed using physical and numerical modelling techniques. Scale model barrages were constructed and testing was carried out using a physical model of the Severn Estuary, located in the hydraulics laboratory at Cardiff University. A depth integrated numerical model, namely DIVAST, was applied to the physical model geometry and modifications were made to simulate the effects of the barrage structures. The numerical model predictions showed good agreement with the corresponding laboratory data. The results were consistent with conclusions from previous studies, relating to the ebb-only scheme. These included an increase in the minimum water levels upstream of the barrage, a reduction in the mean water levels downstream of the barrage and a general reduction in tidal velocities. For a two-way scheme changes in the tidal elevations and velocities depended on the exact operating conditions of the barrage. It was found that with no starting head the tidal regime was similar to the natural state, with little change in the elevations and a slight reduction in the mean velocities. As a starting head was introduced there was still little change in elevations downstream of the barrage, however, there was an increase in the minimum water levels upstream of the barrage, and a further reduction in the mean velocities.

Abstract
This paper presents details of a numerical modelling study of the Cardiff–Weston barrage proposal for Britain’s River Severn, previously studied by the Severn Tidal Power Group. The study focuses mainly on investigating the potential far-field and near-field hydroenvironmental impacts of the barrage by using an integrated one- and two-dimensional numerical hydroenvironmental model. The model was set up for a large domain, starting from the outer Bristol Channel close to Lundy Island to the River Severn tidal limit near Gloucester. Potential changes in water elevations, tidal
currents and consequently flood risk, sediment concentrations and background bacterial levels were assessed by comparing the model predictions for both the existing conditions and those with the barrage included.The results of the study show that the barrage has the potential to reduce maximum water elevations and tidal currents in this highly dynamic estuary. As a result of changes in tidal currents, a corresponding reduction in
suspended sediment levels will lead to a predicted reduction in the concentration levels of faecal bacteria,which are associated with sediment deposition/erosion and increased light penetration within the water column.
An increase and change in the benthic biodiversity of the estuary is also predicted to occur.

Abstract
Details are given herein of the current main proposals for tidal energy provision from the Severn Estuary, in the UK, with particular emphasis being focused on the Severn Barrage project, as originally promoted by the Severn Tidal Power Group.  In particular, emphasis has focused on assessing the potential hydro-environmental impacts and power outputs of a barrage across the estuary, with an unstructured grid, high resolution, model being developed and applied to the estuary to assess the implications of each of five shortlisted proposed schemes on the hydrodynamic, geomorphologic, flood risk and faecal indicator organism changes within the estuary.  An outline is given of recent research on power refinements to the model to assess the options for power generation.  The results show that the Severn Barrage has the potential to reduce the tidal currents in a highly dynamic estuary.  This leads to reduced suspended sediment loads (particularly upstream of the barrage), increased light penetration within the water column and, potentially, an increase in the benthic bio-diversity and the level of aquatic life in the estuary.  The results also show that the Severn Barrage will reduce the risk of flooding markedly upstream of the barrage and to a lesser extent downstream of the structure. In contrast the alternative options have far less impact on flood risk changes.  In addition to the Severn Barrage some results are shown herein for a typical lagoon option, namely the Fleming Lagoon.

Abstract
The Severn Estuary, located in the UK between south east Wales and south west England, is an ideal site for tidal renewable energy projects, since this estuary has the third highest tidal range in the world, with a spring tidal range approaching 14 m.  The UK Government recently invited proposals for tidal renewable energy power projects from the estuary and many proposals were submitted for consideration. Among the proposals submitted and subsequently shortlisted were: the Cardiff-Weston Barrage, the Fleming Lagoon and the Shoots Barrage, all three of which are national public interest.  Therefore a two-dimensional finite volume numerical model, based on an unstructured triangular mesh, has been refined to study the hydrodynamic impact and flood inundation extent, post construction, of all three of these proposed tidal power projects. The model predicted hydrodynamic processes have been analysed in detail, both without and with the barrage structures, including the discharge processes at key sections, the contours of maximum and minimum water levels, the envelope curves of high and low water levels, the maximum tidal currents, the local velocity fields around the structures and the mean power output curves. Simulated results indicate that: (i) although the construction of the Cardiff-Weston Barrage would have an adverse impact on a range of environmental aspects, due to there being approximately a 50% decrease in the peak discharge entering the upstream region, it would reduce the maximum water levels upstream of the barrage by typically 0.3-1.2 m, which could be positive in respect of coastal flooding; (ii) the construction of the Fleming Lagoon would have little influence on the hydrodynamic processes in the Severn Estuary; and (iii) the construction of the Shoots Barrage would decrease the maximum water levels upstream of the M4 bridge by between 0.3 and 1.0 m, but it could lead to an increase in the maximum water levels downstream of the barrage by typically 20-30 cm.

Marine renewable energy is playing an increasing significant role in many parts of the world, mainly due to a rise in the awareness of climate change, and its detrimental effects, and the increasing cost of natural resources. The Severn Estuary, located between South West England and South Wales, has a tidal range of up to 14 m which makes it the second highest tidal range in the world. There are a number of barrage proposals amongst various marine renewable energy schemes proposed to be built in the estuary. The Cardiff-Weston STPG (Severn Tidal Power Group) Barrage, which would be one of the world׳s largest tidal renewable energy schemes if built, is one of the most publicised schemes to-date. This barrage would generate about 17 TWh/annum of power, which is approximately 5% of the UK׳s electricity consumption, whilst causing significant hydro-environmental and ecological impact on the estuary.

This study mainly focuses on investigating the hydro-environmental impacts of the STPG barrage for the option of two-way generation, and compares this with the commonly investigated option of ebb-only generation. The impacts of the barrage were modelled by implementing a linked 1-D/2-D hydro-environmental model, with the capability of modelling several key environmental processes. The model predictions show that the hydro-environmental impacts of the barrage on the Severn Estuary and Bristol Channel, such as changes in the maximum velocity and reduction in suspended sediment and bacteria levels, were less significant for the two-way generation scheme when compared with the corresponding impacts for ebb-only generation.

The study investigates the impact that construction of a Severn Barrage in the Severn Estuary, on the west coast of the UK, might have on local wave conditions. Implementation of a barrage will impact on tidal currents and water elevations in the wider region. There is strong tidal modulation of wave conditions under the natural regime and therefore barrage-induced changes to tidal conditions could affect wave modulation in the region. This paper uses Swan, an open source 3rd generation spectral wave model, to investigate the possible impacts of construction of a barrage on tidal modulation of the wave conditions. It is found that current variations, rather than water level variations, are the dominant factor in tidal modulation of wave conditions. Barrage implementation does not substantially change the modulation of the wave period or direction. However, barrage implementation does affect the tidal modulation of wave heights in the area of interest. The tidal modulation of the wave heights is generally reduced compared to the natural case; the peaks in the wave heights on an incoming tide are slightly lowered and there is lesser attenuation in wave heights on the outgoing tide. This modulation leads to net changes in the wave heights over one tidal cycle. For all of the tested wave conditions, this net change is small for the majority of the tested domain, namely to within ±5% of the no barrage case. There are some areas of greater change, most notably larger net increases in the wave heights near the North Somerset coast where the post-construction net wave height increase over a tidal cycle approach 20% of the pre-construction conditions. These changes do not impact coastal flooding because the wave height increase is not co-incident with high tide. Importantly, the maximum wave height is not increased and thus the likelihood of extreme events is not increased. The area of greatest reduction is between Swansea and Porthcawl. Changes over a neap tidal cycle show similar patterns of net change, but the modulation over the tidal cycle is different; primarily the magnitude of modulation is half that for the spring tide case and the shape is altered in some locations.

Abstract
The Severn Estuary has a spring tidal range approaching 14m, which is among the highest tides in the world. Various proposals have been made regarding the construction of a tidal barrage across the estuary to enable tidal energy to be generated. The aim of the current study is to investigate the impact of constructing a tidal barrage on the hydrodynamic processes in the Severn Estuary using a numerical model. A two-dimensional hydrodynamic model based on an unstructured triangular mesh has been used in this study. The model employs a TVD finite volume method to solve the 2D shallow water equations, with the numerical scheme being second-order accurate in both time and space. The model has been calibrated by comparing model predictions with observed tidal levels and currents at different sites, for typical spring and neap tides, and it has also been verified using tidal level time series at four tide gauging stations measured in 2003. In order to predict the hydrodynamic processes with a barrage, the model domain was divided into two subdomains: one each side of the barrage. Details were given of the method used for representing the various hydraulic structures, including the sluices and turbines, along the proposed Cardiff-Weston barrage. The impact of constructing the barrage on the water levels and velocities was then investigated using this model. Model predicted hydrodynamic parameters, without and with the barrage, were analysed in detail. Model predictions indicated that with the barrage the mean power output could reach 2.0 GW with up to 25 GWh units of electricity being generated over a typical mean spring tidal cycle. At some cross-sections, the maximum discharges were predicted to decrease by 30 % to 50%, as compared with the corresponding discharges predicted without the barrage. The model also predicted that with the barrage, the maximum water levels upstream of the barrage would decrease by 0.5-1.5 m, and with the peak tidal currents also being reduced considerably. For different operating modes, complex velocity fields were predicted to occur in the vicinity of the barrage.

Marine renewable energy is playing an increasing significant role in many parts of the world, mainly due to a rise in the awareness of climate change, and its detrimental effects, and the increasing cost of natural resources. The Severn Estuary, located between South West England and South Wales, has a tidal range of up to 14 m which makes it the second highest tidal range in the world. There are a number of barrage proposals amongst various marine renewable energy schemes proposed to be built in the estuary. The Cardiff-Weston STPG (Severn Tidal Power Group) Barrage, which would be one of the world's largest tidal renewable energy schemes if built, is one of the most publicised schemes to-date. This barrage would generate about 17 TWhr/annum of power, which is approximately 5% of the UK's electricity consumption, whilst causing significant hydro-environmental and ecological impact on the estuary. This study mainly focuses on investigating the hydro-environmental impacts of the STPG barrage for the option of two-way generation, and compares this with the commonly investigated option of ebb-only generation. The impacts of the barrage were modelled by implementing a linked 1-D/2-D hydro-environmental model, with the capability of modelling several key environmental processes. The model predictions show that the hydroenvironmental impacts of the barrage on the Severn Estuary and Bristol Channel, such as changes in the maximum velocity and reduction in suspended sediment and bacteria levels,

The potential impacts of ebb-only (one-way) and flood-ebb (two-way) modes of operation of a tidal barrage on the eutrophication potential of the Severn Estuary were investigated in this study using the simple modelling approach adopted by the UK’s Comprehensive Studies Task Team (CSTT). The model predictions were compared against CSTT thresholds for assessing eutrophication in estuaries and coastal waters. For the no barrage scenario, as well as the ebb-only and flood-ebb operating modes, the estuary was found to be potentially eutrophic as the predicted equilibrium nutrient concentrations and potential maximum phytoplankton biomass chlorophyll concentrations exceeded the threshold limits. Potential maximum phytoplankton primary productivity under both ebb-only and flood-ebb operating modes were found to be noticeably higher than that for the no barrage scenario. This is believed to be due to a combination of increased water residence time and greater light availability behind the barrage. However, the conditions for phytoplankton production are likely to be more favourable under ebb-only operating mode as the potential for maximum phytoplankton primary production was found to be significantly increased under ebb-only operating mode, with largely decreased suspended particulate matter concentration compared to flood-ebb operating mode where only a small reduction in suspended particulate matter concentration is predicted.

Abstract
The Severn Estuary has a spring tidal range approaching 14 m and is regarded as having one of the highest tidal ranges in the world. Various proposals have been made regarding the construction of a tidal barrage across the estuary to enable tidal energy to be extracted. The barrage scheme originally proposed by the Severn Tidal Power Group (STPG) would be the largest project for tidal power generation in the world if built as proposed. Therefore, it is important to study the impact of different operating modes for this barrage on the tidal power output and flood inundation extent in the estuary. In this paper, an existing two-dimensional hydrodynamic model based on an unstructured triangular mesh has been integrated with a new algorithm developed for the estimation of tidal power output, which can account for three barrage operating modes, including ebb generation, flood generation, and two-way generation. The refined model was then used to investigate the impact of different barrage operating modes on the tidal power output and the associated extent of flood inundation along the Severn Estuary. Predicted results indicate that the mode of flood generation would produce the least electricity energy and cause a larger reduction in the maximum water levels upstream of the barrage. Two-way generation would provide an improvement to these conditions, and produce an equivalent amount of electricity to that from ebb generation, with a low installed capacity and a small loss of intertidal zones. Therefore, the mode of ebb generation or two-way generation would appear to be a preferred option for power generation, because both would offer benefits of acceptable electricity energy and reduced flood risk.

The Severn Estuary has the world’s second largest tide range and a barrage across the estuary, located just seawards of Cardiff in Wales and Weston in the South West England, has been proposed for over half a century, with the objective of extracting large amounts of tidal energy. A Severn Barrage, as previously proposed by the Severn Tidal Power Group (STPG), would be the largest renewable energy project for tidal power generation in the world, if built as proposed, and would generate approximately 5% of the UK's electricity needs. However, concerns have been raised over the environmental impacts of such a barrage, including potential increase in flood risk, loss of intertidal habitats etc. In addressing the challenges of maximising the energy output and minimising the environmental impacts of such a barrage, this research study has focused on using a Continental Shelf model, based on the modified Environmental Fluid Dynamics Code (EFDC) with a barrage operation module (EFDC_B), to investigate both the far and near field hydrodynamic impacts of a barrage for different operating scenarios. Three scenarios have been considered to simulate the Severn Barrage, operating via two-way generation and using different combinations of turbines and sluices. The first scenario consisted of 216 turbines and 166 sluices installed along the barrage; the second consisted of 382 turbines with no sluices; and the third consisted of 764 turbines and no sluices. The specification of the sluice gates and turbines are the same for all scenarios.  The model results indicate that the third scenario has the best mitigating effects for the far-field and near-field flood risks caused by a barrage and produces the most similar results of minimum water depth and maximum velocity distributions to those obtained from simulating the natural conditions of the estuary, i.e. the current conditions. The results also show that the flow patterns around the barrage are closest to those for the existing natural conditions with minimal slight changes in the estuary. Thus, the results clearly indicate that the environmental impacts of a Severn Barrage can be minimised if the barrage is operated for two-way generation and under the third scenario. Although it appears that the energy output for the third scenario is less than that obtained for the other two scenarios, if very low head (VLH) turbines are used, then the third scenario could generate more energy as more turbines could be cited along the barrage structure. Therefore, the study shows that a Severn Barrage, operating in two-way generation and with 764 turbines (ideally VLH turbines), would be the best option to meet the needs of maximising the energy output, but having a minimal impact on environmental changes in the estuary and far-field.

The Severn Estuary has the second largest tide range in the world and a barrage across the estuary from Cardiff in South Wales to Weston in South West England has been proposed for over half a century, to extract large amounts of tidal energy from the estuary. To assess the environmental impacts of the proposed tidal barrage requires accurate model predictions of both the near-field and far-field hydrodynamics, which can strongly depend on the model area and the appropriate boundary forcing. In this paper two models, based on the Environmental Fluid Dynamics Code (EFDC) numerical model with a recently-developed Barrage module (EFDC_B), were set up with different computational domains. The Continental Shelf model, which was centred on the Bristol Channel, has its open boundary extended to beyond the Continental Shelf. The Irish Sea model, which was also centred around the Bristol Channel, only has its open boundary extended to the Celtic Sea in the south and the Irish Sea in the north. In order to investigate the effects of the open boundary conditions imposed in the models on the near and far-field hydrodynamics for the case of the Severn Barrage, the Continental Shelf model was first run with and without the operation of the Severn Barrage. The Irish Sea model was then run, also with and without the operation of the Severn Barrage, and with the open boundary conditions provided by the Continental Shelf model. The results from both models were then analysed to study the impact of the tidal barrage on the near-field and far-field hydrodynamics in the Bristol Channel and Irish Sea. Detailed comparisons of the model results indicate that the hydrodynamic conditions along the open boundaries of the Irish Sea model are affected by the tidal barrage and that the open boundary conditions also have noticeable impacts on the far-field hydrodynamics, especially in the Irish Sea, with approximately an average 4–7 cm difference in the maximum water levels predicted in Cardigan Bay and with a maximum difference of 9 cm in the northern part of Cardigan Bay.

The Severn Estuary has a spring tidal range approaching 14 m and is regarded as having one of the highest tidal ranges in the world. Various proposals have been made regarding the construction of a tidal barrage across the estuary to enable tidal energy to be extracted. The barrage scheme originally proposed by the Severn Tidal Power Group (STPG) would be the largest project for tidal power generation in the world if built as proposed. Therefore, it is important to study the impact of different operating modes for this barrage on the tidal power output and flood inundation extent in the estuary. In this paper, an existing twodimensional hydrodynamic model based on an unstructured triangular mesh has been integrated with a new algorithm developed for the estimation of tidal power output, which can account for three barrage operating modes, including ebb generation, flood generation, and two-way generation. The refined model was then used to investigate the impact of different barrage operating modes on the tidal power output and the associated extent of flood inundation along the Severn Estuary. Predicted results indicate that the mode of flood generation would produce the least electrical energy and cause a larger reduction in the maximum water levels upstream of the barrage. Two-way generation would provide an improvement to these conditions, and produce an equivalent amount of electricity to that from ebb generation, with a low installed capacity and a small loss of intertidal zones. Therefore, the mode of ebb generation or two-way generation would appear to be a preferred option for power generation, because both would offer benefits of acceptable electrical energy and reduced flood risk.

The Severn Estuary has the world's second largest tide range and a barrage across the estuary, located just seawards of Cardiff in Wales and Weston in the South West England, has been proposed for over half a century, with the objective of extracting large amounts of tidal energy. A Severn Barrage, as previously proposed by the Severn Tidal Power Group (STPG), would be the largest renewable energy project for tidal power generation in the world, if built as proposed, and would generate approximately 5% of the UK's electricity needs. However, concerns have been raised over the environmental impacts of such a barrage, including potential increase in flood risk, loss of intertidal habitats etc. In addressing the challenges of maximizing the energy output and minimizing the environmental impacts of such a barrage, this research study has focused on using a Continental Shelf model, based on the modified Environmental Fluid Dynamics Code (EFDC) with a barrage operation module (EFDC_B), to investigate both the far and near field hydrodynamic impacts of a barrage for different operating scenarios. Three scenarios have been considered to simulate the Severn Barrage, operating via two-way generation and using different combinations of turbines and sluices. The first scenario consisted of 216 turbines and 166 sluices installed along the barrage; the second consisted of 382 turbines with no sluices; and the third consisted of 764 turbines and no sluices. The specification of the sluice gates and turbines are the same for all scenarios. The model results indicate that the third scenario has the best mitigating effects for the far-field and near-field flood risks caused by a barrage and produces the most similar results of minimum water depth and maximum velocity distributions to those obtained from simulating the natural conditions of the estuary, i.e. the current conditions. The results also show that the flow patterns around the barrage are closest to those for the existing natural conditions with minimal slight changes in the estuary. Thus, the results clearly indicate that the environmental impacts of a Severn Barrage can be minimized if the barrage is operated for two-way generation and under the third scenario. Although it appears that the energy output for the third scenario is less than that obtained for the other two scenarios, if very low head (VLH) turbines are used, then the third scenario could generate more energy as more turbines could be cited along the barrage structure. Therefore, the study shows that a Severn Barrage, operating in two-way generation and with 764 turbines (ideally VLH turbines), would be the best option to meet the needs of maximizing the energy output, but having a minimal impact on environmental changes in the estuary and far-field.

The potential impacts of a barrage on the eutrophic status of the Severn Estuary were investigated in this study using a simple modelling approach adopted by the UK’s Comprehensive Studies Task Team (CSTT). The model predictions were compared against CSTT thresholds for assessing eutrophication in coastal waters. Under both pre- and post-barrage (ebb-only generation) conditions, the estuary was found to be potentially eutrophic as the predicted equilibrium nutrient concentrations and potential maximum phytoplankton biomass exceeded the CSTT thresholds. Potential primary productivity under post-barrage conditions was found to be significantly higher than that under pre-barrage conditions. This is believed to be due to a combination of increased water residence time and greater light availability behind the barrage.