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Modelling of the optimal position of a fish recovery and return system for Hinkley Point C

Not Protectively Marked

TR197 Modelling of HPC FRR Outfalls declassified VSOSR ii

TR197 Modelling of HPC FRR Outfalls declassified VSOSR iii

Modelling of the optimal position of a fish recovery and return system for Hinkley Point C

Tiago Silva, Liam Fernand, Julian Metcalfe, Tony Dolphin and Berrit Bredemeier

TR197 Modelling of HPC FRR Outfalls declassified VSOSR iv

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Version and Quality Control

Version Author Date

Draft 0.01 Silva et al. 22/06/2011

Internal QC 0.02 B Robinson 23/06/2011

Revision 0.03

Final Draft 0.04 Silva et al. 24/06/2011

Executive QC 0.05 B Robinson 24/06/2011

Submission to EDF 1.00 27/06/2011

Comment from client CJL Taylor 27/06/2011

Revision 1.01 L Fernand 28/06/2011

Submission to EDF 2.00 29/06/2011

Distribution de-restricted at EDF request 2.01 BEEMS admin 13/09/2011

Approved VSOSR. Edition 2 in prep 2.02 22/09/2011

TR197 Modelling of HPC FRR Outfalls declassified VSOSR vi

TR197 Modelling of HPC FRR Outfalls declassified VSOSR vii

Table of contents

Executive summary ......................................................................................................................................... 1

1 Background ................................................................................................................................................ 3

1.1 Potential FRR discharge locations .................................................................................................. 4

2 Methodology .............................................................................................................................................. 4

2.1 Approximations and assumptions ................................................................................................... 5

3 Results ........................................................................................................................................................ 6

4 Sediment trap issues ............................................................................................................................... 9

5 Discussion and recommendation .......................................................................................................... 11

6 References ............................................................................................................................................... 12

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List of Tables and Figures

Tables

Table 1 Summary of location assessment .......................................................................................................... 2

Table 2 Reimpingement of passive tracers into the HPB intake for release points A to C. ............................... 8

Table 3 Summary of location assessment ........................................................................................................ 11

Figures

Figure 1 Locations of outfalls positions modelled for fish return and recovery system. Note position of Hinkley B intake ............................................................................................................... 4

Figure 2 Tidal curve and discharge simulation used .......................................................................................... 5

Figure 3 Trajectories of particles from position A, (top), B (middle), C (bottom). ............................................... 6

Figure 4 Density of particle positions in each 100 x100 m GETM grid cell for the length of the simulation according to release points A to C. .................................................................................. 7

Figure 5 Schematic design intent of discharge outfall to show good design and poor design .......................... 9

Figure 6 Cross section from land to discharge point D .................................................................................... 10

Figure 7 View East to West of position D (centre of cross wires) top, View from North position D. ................ 10

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Executive summary

A potentially substantive issue for the placement of the outfall of the Fish Recovery and Return system (FRR) from Hinkley C is re-impingement into Hinkley B. Three potential outfall locations were modelled (A, B, C in the figure below); the fourth position (D) was derived from interpolation. X marks the position of the Hinkley B intake.

Model simulations of the likely rate of re-impingement are low (< 1%) for all three positions considered, but site C has the lowest rate (0.25%). Other issues are relevant to the position of outfall such as predation, either by sea birds, fish, or by sea mamals, the length of the discharge tunnel and avoidance of the Hinkley B discharge plume. The shallower outfalls (A,B) are likely to have very low water (< 1m) at low tide and likely to suffer high mortality due to birds. The deeper outfall is less likely to suffer predation from birds, but more likely from sea mammals; however this is likely to be less extensive than due to the birds. Sediment accretion in the area, and the design of the outfall, also need to be considered.

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Table 1 Summary of location assessment

Locationof outfall A B C D

Model Reimpingement 2 nd 4th 1st 3rd

Tunnel Length 1st 2nd 4th 3rd Predation from Sea birds 4th 3rd 1st 2nd

Predation from fish, sea mammals 1st 2 nd 4th 3rd

Avoidance of Hinkley B plume 3rd 4th 1 st 2nd

Accretion/Siltation 4th 1st 3rd 2nd *1st is the best solution or least affected. While tunnel C would appear to be the best compromise, and is the most beneficial option which would reduce the likely mortality of returned fish, it does probably suffer from a potentially higher risk of siltation and possible accretion. Option D, while not being the best option for any particular issue, has no major disadvantages and is therefore worthy of consideration. In order to ascertain the exact position, to within a few metres, further examination of the bathymetric data at high resolution is required.

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1 Background

The proposed Hinkley C power station will extract approximately 125 m3 s-1.To minimise fish ingress and subsequent impingement on the station cooling water drum-screens, Hinkley C will have an Acoustic Fish Deterrent (AFD) system to allow fish to detect the intake in the turbid conditions present at Hinkley, and low velocity cooling water intakes designed to allow the larger fish to avoid entrapment by swimming away. These intakes in conjunction with an acoustic fish deterrent are designed to reduce losses due to impingement for fish that can respond to the sound cue eg hearing specialists such as herring, sprat and shad (BEEMS Technical Report TR148). Nonetheless, some fish, particularly those with poor hearing such as eels and lampreys and many flat fish, will not be deterred and will enter the cooling water system. Therefore a Fish Recovery and Return system (FRR) is planned that will be provided as a component of the cooling water infrastructure. The FRR will be designed in order to minimise physical trauma to the fish that are impinged and will permit the safe return of the more robust organisms directly into the marine environment. After being removed from the drum screens by a low pressure wash, the recovered organisms will be channelled to a pre-discharge basin, lifted above the level of the perimeter of the pumping station by Archimedean screw, and then returned to sea via a dedicated tunnel under the seawall and intertidal shore. The surfaces of all gullies, channels, and the discharge tunnel itself will be smooth in order to avoid abrasion, interruptions to flow that might damage fish through impact will be avoided, and any falls from one section to another will be into volumes of water. The AFD and FRR approaches described are complementary, as fish that do not respond to the acoustic cue (such as eels and flatfish) tend to have higher survival rates via FRR, whilst those fish that are sensitive to sound tend to be very much more fragile. However, one of the potential issues with the discharge of the FRR system is the proximity to the Hinkley B intake and the possibility that fish could be re-entrained into the Hinkley B intake.

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1.1 Potential FRR discharge locations

Figure 1 Locations of outfalls positions modelled for fish return and recovery system. Note position of Hinkley B intake

The three discharge positions shown in Figure 1 were modelled, all of which were feasible from an engineering perspective. Positions A and B are the shallowest and are just deep enough to reach the mean low water springs level.

2 Methodology

The BEEMS GETM model of Hinkley Point was used for this study to generate the forcing fields that drive the particle tracking. A lagrangian particle tracking model (Wolk, 2003 after Hunter et al 1993) was then used to generate particle tracks. The equation of motion was solved with a time step of 2 seconds and has two components. The advection is calculated using the Runge-Kutta method, while the turbulent diffusion is modelled using a stochastic differential equation for a random walk, where the length and direction of each step depends on the eddy diffusivity from GETM’s turbulence closure. The GETM model output used was Run E (BEEMS Technical Report TR177) which includes a thermal discharge from both HPC and HPB power stations. The HPB intake is to the east of the considered HPC FRR outfall positions and so it is during floodtide that a passive tracer is likely to be transported to the HPB intake. The simulation consisted of 4 hours during flood tide and a passive tracer with neutral buoyancy was released at the seabed for each of the three positions every minute (Figure 1). For each simulation 240 particles were released. A relatively short simulation of four hours was used because it was considered that animals which survive longer than 4 hours have responded and will start to exhibit their own behaviour. Animals that are not exhibiting near normal behaviour within this time are likely to have been predated.

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Figure 2 Tidal curve and discharge simulation used

2.1 Approximations and assumptions

It is likely that fish returned to sea via the FRR will be in a disorientated state and, given the very limited under water visibility at Hinkley, there will be minimal visual clues to guide fish. Consequently, any specific swimming behaviour (speed and direction) is difficult to predict but is most likely to be random in nature. Therefore the model includes a random walk element to account for this.

The GETM model does include vertical movements; however for this assessment all particles that pass within the grid square of the Hinkley B intake are considered to have been impinged. The grid square is 100m by 100m which is significantly larger than the intake structure, which is circular and of approximately 30m diameter.

There is evidence to suggest that the HP B intake structure does extract water from the bottom 4 metres. Thus during the first 3 hours of the flood tide the intake would be extracting water from the whole of the water column, whereas during the last 3 hours it is extracting from the bottom half.

The model estimates are therefore likely to be an over estimate of the likely impinged fish.

The model results have only been shown for the flood tide, because the ebb tide direction is from east to west and would carry discharged fish away from the HP B intake area. The model has shown results from one particular day when winds were close to mean conditions and direction. Running simulations for longer would produce greater variability but would be unlikely to significantly change the magnitude of the result or the order of ranking of the outfall position. It has been assumed that the water used to flush the fish from the intake screens is not chlorinated and that the discharge flow is not chlorinated, and has essentially the same water characteristics as the receiving water. Thus the discharge should mix quite rapidly as there will be no impediments to lateral or vertical mixing.

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3 Results

Figure 3 Trajectories of particles from position A, (top), B (middle), C (bottom).

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Figure 4 Density of particle positions in each 100 x100 m GETM grid cell for the length of the simulation according to release points A to C.

Figure 3 shows the individual trajectories of the 240 tracers for potential FRR outfalls A, B and C. Over a four hour period tracers move as far as 6 km eastward and 3 km northward. To assess the likelihood of tracers impinging on the HPB intake, as shown in Figure 4, we calculated the density of position points for each 100 x 100 grid cell (Table 2). These show that the release point C results in the lowest values of reimpingement, followed by A.

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Release point B suffers from being closer to HPB ─ 500 m against 1 km for A ─ and in the mean flow field that goes over HPB, while the position, C, is just north to miss the intake in this simulation. There is very little difference between the assessments for A and C. It should be noted that in the case of the discharge at A the highest concentration of particles (or fish) remains along the coast and are probably most vulnerable to predation from sea birds. Despite the conservative assumptions made in the model, all three positions result in less than 0.5% of the discharged particles taken into the area of the Hinkley B intake position. Table 2 Reimpingement of passive tracers into the HPB intake for release points A to C.

POSITION and depth of discharge ODN

OVERALL PROPORTION (%)

OSGB_E OSGB_N

A 6m 0.31% 319850 146380

B 6m 0.47% 320340 146600

C 8m 0.26% 320100 146710

D 7m 0.37% (by interpolation) 320230 146685

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4 Sediment trap issues

One of the potential problems of the discharge pipe is that sediment may become lodged in the pipe, and cause blockages. The main concern is that gravel could enter the pipe and not be flushed out unless via physical intervention. Storms are likely to move gravel and pebbles around; therefore a discharge away from the beach area is likely to be the best solution.

Figure 5 Schematic design intent of discharge outfall to show good design and poor design

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Figure 6 Cross section from land to discharge point D extracted from bathymetry at 25m resolution.

Figure 7 top; View East to West of position D (centre of cross wires) , Below; View from North position D. Note exaggeration of the vertical scale.

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5 Discussion and recommendation

The likely extent of re-impingement is low (< 1%) for all four positions considered. However, this is only one factor in considering the likely fish survival and thus the location of the FRR system. A further likely mortality risk is predation by sea birds, predatory fish or sea mammals, with bird predation being the main concern. The shallower potential FRR outfalls (A,B) are likely to have very low water (< 1m) at low tide, and fish returned at these locations are likely to suffer high predation from birds. Bird predation is likely to be reduced at the deeper outfall (C), but predation from predatory fish and from sea mammals is likely to be higher. Another factor to consider is the length of the tunnel and the possibility of increased abrasion and pressure change experienced by the fish, with shorter tunnels being more desirable. However the difference in FRR outfall tunnel lengths is not considered to be material environmentally; in particular the more robust fish without swimbladders that the FRR is intended to return safely to sea are relatively insensitive to pressure effects. Avoidance of the Hinkley B and C plumes is also important as fish exposed to the thermal shock of the plume when in a vulnerable state may suffer an increase in either mortality or predation. This comes from two forms; advection of the fish across the area of the B plume discharge where the temperatures are highest, and a much weaker effect where the Hinkley B plume is advected across the FRR discharge location on the ebb tide. However, the thermal plume is likely to have cooled considerably before reaching the discharge positions and is of less concern. The closer inshore a FRR discharge is sited, the more likely it is to experience the thermal plume discharge, as the shore attached plume tends to hug the coast. The proximity of FRR position B to the discharge means that it is probably the worst position. Table 3 Summary of location assessment

Locationof outfall A B C D

Model Reimpingement 2 nd 4th 1st 3rd

Tunnel Length 1st 2nd 4th 3rd Predation from Sea birds 4th 3rd 1st (least) 2nd

Predation from fish, sea mammals 1st 2 nd 4th 3rd

Avoidance of Hinkley B plume 3rd 4th 1 st 2nd

Siltation 4th 1st 3rd 2nd While tunnel C would appear to be the best compromise, and is the most beneficial option in terms of reducing the likely mortality of returned fish, it does probably suffer from a potentially higher risk of siltation and possible accretion. Option D, while not being the best option for any particular issue, has no major disadvantages and is therefore worthy of consideration. In order to ascertain the exact position, to within a few metres, further examination of the bathymetric data at high resolution is required.

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6 References

BEEMS Technical Report TR148: A synthesis of impingement and entrainment predictions for NNB at Hinkley Point, Cefas

BEEMS Technical Report TR177: Hinkley Point Thermal Plume Modelling: GETM Stage 3a results with the final cooling water configuration, Cefas

Hunter, J. R., P. D. Craig, and H. E. Philips, On the use of random walk models with spatially variable diffusivity, J. Computat. Phys., 106 , 366–376, 1993.

Frank Wolk 2003. Three-dimensional Lagrangian Tracer Modelling in Wadden Sea Area s. Diploma thesis. Carl von Ossietzky University Oldenburg. Hamburg, April 4, 2003