ABSTRACT: 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 As this design has now been superseded by the Hafren Power proposal, new studies have been conducted to investigate the impact of a two-way scheme.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. KEY WORDS: Severn Barrage, Numerical Modelling, Physical Modelling, Ebb generation, Two-way Generation 1 INTRODUCTION

The Severn estuary continues to be at the centre of debate with regards to renewable energy generation. With the third largest tidal range in the world it provides a vast resource for both tidal stream and tidal range technologies. The tidal stream industry is still in its infancy, with developers looking to deploy first generation arrays to become fully commercial. These arrays will have a capacity of the order

Physical and Numerical Modelling of the Severn Barrage

James Brammer PhD Research Student, School of Engineering, Cardiff University, The Parade, Cardiff, CF24 3AA, U.K. Email: BrammerJR@cf.ac.uk

Roger Falconer CH2M HILL Professor of Water Management, School of Engineering, Cardiff University, The Parade, Cardiff, CF24 3AA, U.K. Email: FalconerRA@cf.ac.uk

Christopher Ellis Graduate Engineer, Arup, 4 Pierhead Street, Cardiff, CF10 4QP, U.K. Email: Chris.Ellis@arup.com

Reza Ahmadian Lecturer, Cardiff University, The Parade, Cardiff, CF24 3AA, U.K. Email:AhmadianR@cf.ac.uk

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of 10 MW, such as the Marine Current Turbine’s Skerries project. Compared to tidal range proposals in the Severn, this is a modest contribution to the UK’s energy mix. For example, Tidal Lagoon Power’s Swansea Bay proposal will have a 240 MW capacity; however the scheme that is synonymous with tidal power in the Severn Estuary is the Severn Barrage.

Although existing in various forms from as early as the 1920s, the 1989 Severn Tidal Power Group (STPG) proposal is commonly referred to as the original Severn Barrage [1]. With an installed capacity of 8,640 MW, provided by 216 bulb turbines, it could deliver 17 TWhr of energy per annum. This scheme divided opinion however, as concerns were raised over its environmental impact. This was primarily due to the mode of operation: ebb-only generation, as this would have raised the minimum water levels significantly upstream of the barrage, resulting in a loss of intertidal habitat. A wide range of research has been conducted on this scheme [2], [3], and the UK government commissioned the Severn Tidal Power feasibility study [4] to assess its viability, amongst other proposals for tidal power in the Severn Estuary. With an estimated cost in the order of £20bn, at a time of global financial constraints, the UK government abandoned the proposals in 2010.

A private consortium was subsequently formed, namely Hafren Power, with the aim of privately financing a barrage. Due to the commercial nature of this new proposal, exact technical details are not publicly known at this stage, however, it is understood that the group has proposed a new design of barrage with the key difference being the mode of operation. In the scheme a two-way mode of operation is proposed which will use over 1000 turbines [5]. The turbines are a new very low head design, as opposed to the traditional bulb turbines that were considered in the STPG scheme. It is envisaged that this scheme will have less environmental impact, with a reduced loss of intertidal zones, and with the tides following the natural state more closely.

This paper describes a joint physical and numerical modelling study, comparing the hydrodynamic impact of the STPG and Hafren Power schemes, with the natural conditions. This study was undertaken using a scaled physical model of the Severn Estuary at Cardiff University, and a depth-integrated hydro-environmental model, namely DIVAST. To the Authors’ knowledge, there have been no physical modelling studies of the Severn Estuary investigating the hydrodynamic impact of tidal range energy schemes, using the scaled bathymetric data for such a specific estuary. However, Jeffcoate et al. [6] conducted experiments using an idealised barrage structure to investigate the near-field flow regime and assess the suitability of 2D and 3D numerical models. The lack of research in this field is partly due to the bespoke nature of each estuary model. Such models are not transferable and therefore this increases the cost of modelling, combined with the fact that most tidal range schemes are still at a concept stage.

In contrast, there have been a number of numerical modelling studies to assess not only the power output of tidal barrages, but also their hydro-environmental impact. 0D models (otherwise known as two-tank models) provide a simplified approach to estimating the performance of a barrage, with the turbine performance being specified using a Hill chart, where a relationship is defined between the tidal elevation and plan area of the basin to provide an estimate of the discharge through the barrage. Such models have been applied for a number of tidal barrage proposals around the UK coastline [7], [8]. The effects of a tidal barrage in the Severn Estuary have been modelled in a number of studies. Xia et al. [9] investigated the hydrodynamic characteristics of three proposed generation schemes, namely the Severn Barrage, Fleming Lagoon and the Shoots Barrage. As expected the schemes will have a varying impact on the hydro-environment as they differ greatly in size. The Severn Barrage is the largest proposed scheme with a rated capacity of 8.64 GW, and model predictions show a large reduction in the tidal currents for ebb-only generation, and changes in both the maximum and minimum water levels, particularly upstream of the structure. In this study all the three barrages were operated for ebb-only generation, however, other studies have considered other operating modes [3], with the operating conditions having a significant effect on the upstream water levels. Ebb-only generation would result in a 4-5 m increase on the minimum water levels upstream of the barrage, permanently flooding large intertidal areas, whereas both two-way and flood-generation only partially affect these levels. Ahmadian et al. [10] also studied ebb-only generation for the Severn Barrage, and investigated the hydro-environmental impacts using the 2-D model, DIVAST, and Zhou et al. [11] investigated the far field impacts using a continental shelf model. Other barrages around the UK have also been modelled to assess the potential power generation and hydro-environmental impacts [12], [13], and their interaction [14], as well as projects around the

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world [15].

2 PHYSICAL MODEL DETAILS

Figure 1: The Severn Estuary Physical Model (SEPM)

2.1 Severn Estuary Physical Model (SEPM) The physical model of the Severn Estuary extends between Gloucester and an imaginary line between Milford Haven and Hartland Point, as shown in Figure 1. In the horizontal direction a scale of 1:25000 was used to scale the geometry, whilst in the vertical the scale was 1:125, allowing the model to fit in a 6×4 m basin. Water was delivered to a holding tank via a pump, with the holding tank and basin separated by a mechanically operated weir. This allowed the incoming tidal wave to be specified, and for this study a 4 cm amplitude sinusoidal curve was used to represent a mean spring tide in the estuary. Based on the principle of keeping the Froude number consistent when scaling the model, a 12.4hr time period to represent a tide should scale to 20s. However, due to practical limitations a time period of 40s was used. The barrage structures had to be modified to take this limitation into account. Water velocities were measured in the model using Acoustic Doppler Velocimeters, whilst elevations were measured using calibrated wave probes, with further details being given in Ellis [16].

Figure 2: Scale Model of the Hafren Power Barrage

2.2 Barrage Design

The original STPG barrage proposal reached from Cardiff to Weston, passing in between the islands of Flat Holm and Steep Holm, and was 16km in length. Due to the commercial nature of the Hafren Power proposal precise details of the barrage location and technology used have not been released to the

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public. It is understood that the barrage will be 18km in length, passing around both islands. However for the scaled model tests in this study the size and location of the barrages were the same as those shown in Figure 2. Table 1 gives the details of the key design features of both barrages.

Table 1 Properties of barrage models

Barrage Scheme

STPG Hafren Power

Prototype Details

Ebb-only generation Two-way generation

216 turbines (9m diameter) 1026 turbines (9m diameter assumed)

166 sluices (22x17.5m) Holding period unknown

3hr holding period at high tide

Physical Model Details

24 holes (15mm diameter) to represent turbines

66 holes (20mm diameter) to represent turbines

2 flaps (160mm square) to represent sluices

Shutter to control holding periods

Note: Physical model areas were halved to account for tide period, as previously discussed

3 NUMERICAL MODEL DETAILS

In this study the hydro-environmental model DIVAST was used. This model solves the depth-integrated Navier-Stokes equations, and is based on a finite difference Alternating Direction Implicit (ADI) scheme. The model has previously been used in a wide range of studies to more specifically assess the impacts of the STPG scheme [10], and other marine energy technologies [17].

3.1 Governing equations and barrage modelling The governing equations as used in the DIVAST model, for the x-direction are given below:

t

qx

xqy

yQBx 0

3.1

qx

t Uqx

xVqy

y

fqy gH

x

xhH

U

x U

x

xhH

U

y V

x

1

xz

xz h FBx

3.2

where t= time, ζ is the water elevation, qx and qy are the depth-integrated discharges per unit width (or depth-integrated velocities) in the x and y directions respectively, U and V are the depth-averaged velocities, where qx = UH and qy = VH and β is the momentum correction factor to account for a non-uniform velocity profile. h is the horizontal eddy viscosity, f = Coriolis parameter, ρ = density, τ = components of stress tensor (where the subscripts denote planar direction), and g = acceleration due to gravity. Two source terms, QBx and FBx, were added to these equations to model the flow through a tidal barrage, thus the necessary fluid volume and momentum could be transferred from one cell to another. These terms are given as follows:

QBx 1

xyqorifice 3.3

5

FBx 1

xyHqorificeuorifice(1) 3.4

where η is the turbine efficiency, whilst the discharge was calculated using the orifice equation [18]:

qorifice CDAorifice(2gh)1

2 and uorifice qorifice

Aorifice 3.5

where CD is the discharge coefficient, Aorifice is the orifice area and h is the head difference across the barrage. A value of 1.0 was chosen for the discharge coefficient, as it was assumed that frictional losses through the model barrage would be negligible.

As these experiments were small in scale compared to the prototype, there was little value in trying to simulate the effects of individual turbines. Furthermore, specific performance and operating details of turbines were generally not in the public domain; hence no power take-off was simulated in the models, and a value of η=0 was used. Logical functions were implemented into DIVAST, so that the orifices could be switched on or off when necessary, and the orifices acted as either turbines or sluice gates etc. This depended on the mode of operation, and a range of parameters such as the holding period or starting head, could be specified. Whilst no power take-off was calculated, to make a comparison between the different operating modes the energy transferred across the barrage was calculated for each time step using the following equation:

Energy tgqorificeH 3.6

3.2 Severn Estuary model A like-for-like model of the SEPM was created. A grid size of 24mm was used, resulting in a domain

with a resolution of 229×167 cells, as shown in Figure 3. All the water elevation and velocity measurements were sampled at a number of points throughout the estuary (A to H). These points were the same locations as measured in the physical model, and were arbitrarily chosen to ensure that the measurements were spread throughout the estuary, but also had to be in deep enough water to ensure that the ADV was fully submerged. The seaward boundary was specified using a water elevation boundary condition. Other studies have included the discharge from the River Severn, either as a flow boundary or by linking a 1D river model. However, the SEPM does not take this discharge into account. Hence the upstream extent of the model was treated as a land boundary. All the land boundaries were defined using a no-slip condition. A time step of 0.02 s was used based on a Courant number restriction for accuracy, and the model was run for 40 tidal cycles to ensure solution convergence. Other parameters were set based on calibration with laboratory data, and recommendations from the user manual [19].

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Figure 3: Physical Model bathymetry, and sampling locations

4 RESULTS AND DISCUSSION 4.1 Natural Condition

-0.06

-0.03

0.00

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0 10 20 30 40 50 60

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er L

evel

[m

]

Time [s]

Pt A

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0 10 20 30 40 50 60

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er L

evel

[m

]

Time [s]

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Figure 4: DIVAST (blue line) and laboratory (red dots) predictions of water elevations in the SEPM

Figure 4 provides a comparison between the numerical and physical model predictions of water elevations in the SEPM, at two sites: Point H, which was located towards the open boundary, and Point A, which was the highest upstream measuring point. The main feature that can be observed in this figure is the tidal range amplification, which occurred due to the natural shape of the estuary. This increased with distance upstream; between Points H and A the amplitude increased by approximately 1.5 cm, which at a prototype scale translates to ≈ 1.9 m. This is lower than expected for the prototype scale, in which values

7

of > 3 m were anticipated. This reduced amplification is a result of doubling the time period to 40 s, as this affected the advective processes in the model. This limitation of the physical model therefore results in an under prediction of the tidal energy resource.

0.00

0.03

0.06

0.09

0.12

0 10 20 30 40 50 60

Spee

d [m

s-1]

Time [s]

Pt D

0.00

0.03

0.06

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0 10 20 30 40 50 60

Spee

d [m

s-1]

Time [s]

Pt F

0.00

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0 10 20 30 40 50 60

Spee

d [m

s-1]

Time [s]

Pt H

0.00

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0 10 20 30 40 50 60

Spee

d [m

s-1]

Time [s]

Pt A

Figure 5: DIVAST (blue line) and laboratory (red dots) predictions of velocities in the SEPM, for the natural

condition Very good agreement was observed between DIVAST and the laboratory measurements. This

finding is further evident in Figure 5, in which predictions are given of the tidal velocities in the SEPM. Points A and F showed the least agreement between the physical and numerical model predictions, which was thought to be due to the lower location of the ADV probe, and the interaction between the probe and the relatively shallow body of water in that area. Both models showed faster currents predicted on the ebb tide, as expected, and this can be clearly seen in Figure 6, which gives the velocity predictions across the domain for a flood and ebb tide, respectively.

The modelling results showed that for a flood tide the peak velocities are in the region of 0.07 m s-1, whereas for an ebb tide the peak velocities exceeded 0.12 m s-1. Using the appropriate scaling factor, which was modified to account for the time period scaling issue (see Ellis [16]), these velocities correspond to 1.6 and 2.7 m s-1, respectively, at the prototype scale. In general these agree well with the estimates of tidal velocities in the Severn Estuary, however, they are lower than expected, which is a result of the scaling issue already discussed.

8

Speed [m s-1]

Speed [m s-1]

Figure 6: DIVAST velocity predictions for natural condition at: (Top) mid flood tide and (Bottom) mid ebb tide

4.2 STPG Barrage

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-0.06

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0.00

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0 10 20 30 40 50 60

Wat

er L

evel

[m

]

Time [s]

Figure 7: DIVAST (blue line) and laboratory (red dots) predictions of water elevations both upstream and downstream of STPG barrage

Figure 7 shows the water elevations both upstream and downstream of the barrage, for the STPG

scheme. The minimum water levels upstream were increased significantly, from -4.7 cm to -1.2 cm - an increase of 4.4 m at the prototype scale. Downstream a small reduction was observed throughout the estuary as the natural frequency of the estuary was changed due to the presence of a barrage structure. These conclusions are the principal findings from previous studies, and the increase in the minimum water levels inside the basin would result in large areas of intertidal habitat being permanently flooded, a finding that has raised many environmental concerns. The predicted tidal velocities for points A, D, F and H are given in Figure 8, and velocity contour plots of the DIVAST predictions for the mid-flood and ebb tides are given in Figure 9. A reduction in the velocities throughout the estuary was observed, except for local increases around the barrage site due to the filling and emptying processes of the barrage. The magnitude of these local increases would not be expected to be transposable to the prototype scale, as they are dependant on the exact geometric details of the barrage structure, and modelling such fine details was not feasible at such a small scale. This reduction was significant on the ebb tide. Differences between the physical model measurements and DIVAST predictions were expected mainly because DIVAST used a regular grid; hence it was not possible to model the exact barrage geometry.

10

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0 10 20 30 40 50 60

Spee

d [m

s-1]

Time [s]

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0 10 20 30 40 50 60

Spee

d [m

s-1]

Time [s]

Pt H

Figure 8: DIVAST (blue line) and laboratory (red dots) predictions of velocities in the SEPM,

for the STPG barrage

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Speed [m s-1]

Speed [m s-1]

Figure 9: DIVAST velocity predictions for the STPG barrage at: (Top) mid flood tide and (Bottom) mid ebb tide

4.3 Hafren Power Barrage

With precise details of the Hafren Power scheme being still subject to design clarification, a number of operating conditions were modelled. Starting heads of 0, 3, 4, 5 cm were tested by manually opening and closing the shutter at different intervals. It was found that with 0cm head the tidal regime behaved much like the natural conditions, with a small decrease in elevations and velocities throughout the estuary. As the starting head was increased this affected the increase and decrease respectively in the maximum and minimum water levels, upstream of the barrage, as well as throughout the estuary. A similar behaviour was also observed with respect to the tidal velocities. Practically a head would have to be introduced for power generation, and therefore the following results are presented for a starting head of 3

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cm for both the ebb and flood generating phases. The reader is referred to Ellis [16] for the full results of the different starting heads. The DIVAST and physical model predictions of water levels both upstream and downstream of the Hafren Power barrage are given in Figure 10.

-0.06

-0.03

0.00

0.03

0.06

0 10 20 30 40 50 60

Wat

er L

evel

[m

]

Time [s]

Figure 10: DIVAST (blue line) and laboratory (red dots) predictions of water elevations both upstream and downstream of Hafren Power barrage

A 3 cm starting head led to a 2 cm increase and decrease of the minimum and maximum water levels,

respectively. This equates to a 2.5m change at the prototype scale. This would result in a smaller loss of intertidal habitat compared to the STPG scheme, as well as providing further flood protection. For all of the barrage schemes tested the velocities in the estuary were reduced, however, scaling issues were observed with larger starting heads, as opening and closing the shutter in a short time period led to a water hammer effect, resulting in instabilities in the free surface and velocities. This was particularly evident in the DIVAST predictions, as shown in Figure 11. It can be seen also, however, that further downstream at Point H these oscillations had dissipated. Finally, contour plots of the velocities at mid-flood and ebb tide from the DIVAST model are given in Figure 12, which further illustrates the reduction in velocities compared to the natural condition.

13

0.00

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0 10 20 30 40 50 60

Spee

d [m

s-1]

Time [s]

Pt A

0.00

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0 10 20 30 40 50 60

Spee

d [m

s-1]

Time [s]

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Time [s]

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0 10 20 30 40 50 60

Spee

d [m

s-1]

Time [s]

Pt H

Figure 11: DIVAST (blue line) and laboratory (red dots) predictions of velocities in the SEPM,

for the Hafren Power barrage

14

Speed [m s-1]

Speed [m s-1]

Figure 12: DIVAST velocity predictions for Hafren Power barrage at: (Top) mid flood tide and (Bottom) mid ebb tide

4.4 Energy Yield from a Barrage

As previously discussed in the physical model setup the turbines and sluice gates were represented by holes drilled through the barrage. Attempting to model power take-off is impractical at this scale and would also require further details of turbine performance and operating conditions. Hence, whilst it is acknowledged that the calculated energy values bear little resemblance to the prototype scale, comparisons can still be made between different schemes with regards to the total energy yield through the barrage structures. This was calculated using equation (3.6), and the results are given in Figure 13 and Table 2.

15

0

2

4

6

8

10

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-0.03

0.00

0.03

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0 10 20 30 40 50

Pow

er [W

]

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er L

evel

[m

]

Time [s]

0

2

4

6

8

10

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0.00

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0 10 20 30 40 50

Pow

er [W

]

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er L

evel

[m

]

Time [s]

(a) (b) Figure 13: Energy yield predictions for: (a) STPG barrage, and (b) Hafren Power Barrage

(Blue dotted line = upstream water levels, Red dashed line = downstream water levels, Black line = Power output) It was found that both the STPG and Hafren Power schemes could produce a similar amount of

energy at the physical model scale; as with a 3 m starting head, a total 38 Ws of energy was transferred through both the STPG and Hafren Power barrages in a single tidal cycle. The significant difference between the two schemes is that whilst with the STPG scheme the power generated occurs once per tidal cycle, whereas with the Hafren Power barrage this energy will be produced more evenly throughout the tidal cycle. This is advantageous in aspects such as connectivity to the electricity grid.

Table 2 Total energy yield across barrages

Scheme Starting

Head [cm]

Energy [Ws]

Flood tide Ebb tide Total

STPG 0 0 26.3 26.3

3 0 37.8 37.8

Hafren

Power

0 13.6 20.7 34.3

3 16.9 20.7 37.6

5 CONCLUSIONS

In this study the hydrodynamic impact of a Severn barrage was investigated in a joint physical and numerical modelling study. Two schemes were investigated, namely the ebb generation STPG scheme, and the two-way generation Hafren Power proposal. Scale model barrages were constructed and tested in a physical model of the Severn Estuary at the Hydro-environmental Research Centre’s hydraulics laboratory, at Cardiff University. The hydro-environmental model DIVAST was used to replicate the physical models, and modifications were made to include the operation of barrage structures.

The results confirm the conclusions made from previous studies with regards to the STPG scheme, with the main points being that upstream of the barrage minimum water levels will increase dramatically, permanently flooding areas of intertidal habitat, and downstream of the barrage a slight reduction in water levels was observed. Finally, the mean velocities throughout the estuary will decrease.

As exact details of the Hafren Power scheme are not in the public domain, conclusions are made more generally about the impact of a two-way generation scheme. The starting head dramatically affected the hydrodynamics of the estuary. With no starting head tidal elevations and velocities are similar to the natural state, although there is a reduction in both throughout the estuary. Practically a starting head would have to be used for power generation, which would result in the minimum water levels upstream of

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the barrage increasing. It was found that a 3cm head would increase these minimum levels by 2cm (a 2.5m rise at a prototype scale) and this could potentially provide an equivalent amount of energy as the STPG scheme.

The numerical model predictions showed very good agreement with the obtained physical model data, although some numerical instability was observed due to opening and closing the barrage over a short time period. It has been demonstrated that the modelling methodology is suitable to the assessment of the hydrodynamic impact of a barrage. Modelling at a prototype scale would enable further assessments to be made with regards to the hydro-environmental impact and energy yield from a barrage. ACKNOWLEDGEMENTS

This research has been carried out as part of the LCRI project, which is part funded by the Welsh Government, the Higher Education Funding Council for Wales, the Welsh European Funding Office, and the European Regional Development Fund (ERDF) Convergence Programme. The authors would also like to thank Mr Paul Leach for manufacturing the barrage models.

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