Far-Field Modelling of the Hydro-Environmental Impact of Tidal Stream Turbines

lable at ScienceDirect

Renewable Energy 38 (2012) 107e116

Contents lists avai

Renewable Energy

journal homepage: www.elsevier .com/locate/renene

Far-field modelling of the hydro-environmental impact of tidal stream turbines

Reza Ahmadian*, Roger Falconer, Bettina Bockelmann-EvansSchool of Engineering, Cardiff University, The Parade, Cardiff CF24 3AA, United Kingdom

a r t i c l e i n f o

Article history:Received 3 February 2011Accepted 2 July 2011Available online 19 August 2011

Keywords:Tidal stream turbinesHydro-environmental modellingMarine renewable energyMomentum sinkSevern estuaryBristol channel

* Corresponding author. Tel.: þ44 29 2087 5713; faE-mail addresses: [email protected] (R. Ahm

(R. Falconer), [email protected] (B. Bockelm

0960-1481/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.renene.2011.07.005

a b s t r a c t

Interest in the marine renewable energy devices, and particularly tidal stream turbines, has increasedsignificantly over the past decade and several devices such as vertical and horizontal axis turbines andreciprocating hydrofoils are now being designed around the world to harness tidal stream energy. Whiletidal stream turbines are being developed at a high rate and getting closer to commercialisation, it isimportant to acquire the right tools to assist planners and environmentalists, not only in finding a rightlocation for the turbines, but also in identifying their potential impacts on the surrounding marine andcoastal environment.

In this study, a widely used open source depth integrated 2D hydro-environmental model, namelyDIVAST, was modified to simulate the hydro-environmental impacts of the turbines in the coastalenvironment. The model predictions showed very good agreement with previously published 1D modelresults. Then, for demonstration purposes, the model was applied to an arbitrary array of tidal streamturbines in the Severn Estuary and Bristol Channel which has the third highest tidal range in the world.The model has shown promising potential in investigating the impacts of the array on water levels, tidalcurrents and sediment and faecal bacteria levels as well as the generated tidal power, which facilitatesinvestigating the relative far-field impacts of the arrays under various climate change scenarios ordifferent formations of the array.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Tidal energy extractors and, in particular, tidal stream turbineshave attracted considerable interest in recent years, due to theirmodularity, minimal visual impact and their predictable energygeneration. There are vast resources of tidal stream energy avail-able around the world which can be exploited; for instance in theUK the tidal current energy resource technically available forextraction is estimated to be around 22 TWh/y, representingaround 6% of UK electricity demand [1]. Likewise the tidal currentenergy resource technically available around Ireland is calculated tobe 10.46 TWh/y [2].

In considering that tidal stream turbine technology is still in itsinfancy, these turbines have attracted significant interest from bothacademic and industrial researchers. This interest is reflected in thenumber of research studies being carried out on ComputationalFluid Dynamics (CFD) and laboratory experimental modellingstudies of turbine design and performance etc., including researchstudies being undertaken by: Myers and Bahaj [3], Bryden and

x: þ44 29 2087 4939.adian), [email protected]).

All rights reserved.

Couch [4], Bahaj et al. [5], Batten et al. [6], O’doherty et al. [7] andWillis et al. [8] to mention but a few. These research studies havegenerally been complemented by trial scale and full-scale models,which have been installed and monitored in the real environmentin the past few years, such as: the two turbines manufactured byMarine Current Turbines Ltd., UK, with 11m diameter single rotor,300 kW, Seaflow being installed off the North Devon coast, UK, andthe grid connected double rotor SeaGen, which has been installedin Strangford Lough, Ireland [9,10], and the 10 m,1 MWOpenHydroturbine which has been installed in the Minas Passage of the Bay ofFundy, Canada, manufactured by OpenHydro Ltd., Ireland [11].

Although every fixed tidal stream device has a small footprint,the overall impact of each installation can only be investigated byconsidering both the number and size of the devices installedaround the coastline [12]. In addition, the environmental impact ofenergy extraction is not necessarily restricted to the immediatearea around the turbine site [4]. The preceding facts show theimportance of the far-field impact assessment studies, along withnear-field impact studies.

The current study mainly focuses on the far-field hydro-envi-ronmental impact assessment of tidal stream turbines. To under-take this assessment a 2D hydro-environmental model, namely theDIVAST (Depth Averaged Velocity And Solute Transport) model,

R. Ahmadian et al. / Renewable Energy 38 (2012) 107e116108

was modified so that the impacts of tidal stream turbines could besimulated by the model. Bearing in mind the lack of field measureddata to validate the model, the developed model was set up for anidealised channel, as modelled by Bryden and Couch [4], with thepredicted results in the current study being compared with theresults published by Bryden and Couch. Then, for the purpose ofdemonstration, the model was applied to an arbitrary site along theSevern Estuary and Bristol Channel.

The Severn Estuary has the third highest tidal range in theworld, with the spring tidal range exceeding 14 m and the peaktidal currents being well in excess of 2 m/s. These large currentshave made the estuary very attractive from a renewable energypoint of view and a number of tidal renewable energy schemes,including a 17 TWhr/year Severn Barrage, have been proposed to besited in the estuary. To investigate all potential impacts of an arrayof stream turbines in the selected site, the revised 2D DIVASTmodelwas linked to a 1D hydro-environmental model, namely FASTER(Flow And Solute Transport in Estuaries and Rivers), and theimpacts of the array on water levels, tidal currents, sedimenttransport and faecal bacteria concentrations have beeninvestigated.

2. Hydro-environmental model details

The DIVAST model was modified to study the hydro-environmental impacts of tidal stream turbines. This model hasbeen widely used in hydro-environmental modelling of the coastalenvironment [13e16] and was based on a finite-difference alter-nating direction implicit (ADI) solution of the depth integratedNaviereStokes and solute transport equations [17,18]. Morerecently, the model has been used in modelling the hydro-environmental studies of marine renewable energy devices[8,19,20]. The solute/sediment transport sub-model includes theeffects of dispersion, diffusion, erosion and deposition, adsorptionand desorption, as well as kinetic decay.

2.1. Governing equations

The 2D governing equations used in the model are briefly givenin this section, but further information on the 2D and 1D modelequations are given in Falconer [17] and Kashefipour [21]. Thehydrodynamic equations used in the 2D model are based on thedepth integrated three-dimensional Reynolds averaged equationsfor incompressible and unsteady turbulent flows, in addition to theimpacts of the other external forces such as: wind shear, bottomfriction, and the earth’s rotation to give for the x-direction [22]:

vx

vtþ vqx

vxþ vqy

vx¼ 0 (1)

vqxvt

þ b

�vuqxvx

þ vvqxvy

�¼ fqy � gH

vx

vxþ sxw

r� sxb

r

þ ε

"2v2qxvx2

þ v2qxvy2

þ v2qyvxvy

#ð2Þ

where qx, qy ¼ discharges per unit width in the x, y directions(m2 s�1), z ¼ water surface elevation above datum (m), H ¼ totalwater depth (m), b¼momentum correction factor for non-uniformvertical velocity profile (dimensionless), f ¼ Coriolis parameter(rad s�1), g ¼ gravitational acceleration(m s�2), sxw, sxb ¼ surfaceand bed shear stress components respectively in the x-direction(N m�2), and ε ¼ depth averaged eddy viscosity. The equation forthe y direction can be developed similarly to that given for thex-direction (i.e. Equation (2)).

The 2D advective-diffusion equation for predicting solutetransport was acquired by integrating the 3D solute mass balanceequation over the depth, giving:

vfHvt

þ vfqxvx

þ vfqxvy

� v

vx

�HDxx

vf

vxþ HDxy

vf

vy

�

� v

vy

�HDyx

vf

vxþ HDyy

vf

vy

�¼ H

XF (3)

where f ¼ depth averaged concentration (unit/volume) ortemperature (�C), Dxx, Dxy, Dyx, Dyy ¼ depth averaged dispersion-diffusion coefficients in the x, y directions respectively andSF¼ total depth average concentration of the source or sink solute.

The sediment modelling includes both cohesive and non-cohesive sediment erosion and deposition processes, which arereflected in Equation (3) through the source and sink term. Formore information on sediment modelling see Falconer et al. [23].The bacteria decay was modelled using a first order decay formu-lation according to the Chick’s Law [24] and due to the importanceof the sediment deposition and re-suspension on faecal bacterialevel [25e29], bacteria and sediment interaction was incorporatedin the model, with more details about the bacteria modelling beinggiven in Yang et al. [30] and Ahmadian et al. [19].

2.2. Turbine representation

The turbines have been integrated into the model by adding thereaction of axial thrust and drag force induced by turbines asexternal forces in the shallow water momentum equations (i.e.Equation (2)). Consequently, the momentum equation in thex-direction was revised as given below; with the y directionequation being written in a similar manner:

vqxvt

þ b

�vuqxvx

þ vvqxvy

�¼ fqy � gH

vx

vxþ sxw

r� sxb

rþ ε

"2v2qxvx2

þ v2qxvy2

þ v2qyvxvy

#þ FTx

rþ FDx

r(4)

where FTx ¼ reaction of axial thrust induced by the turbines on theflow; based on the Newton’s third law of motion, this reaction isequal to the thrust in the opposite direction and FDx ¼ drag forceinduced by the pile of a turbine per unit area normal to the flow inthe x-direction, respectively.

Assuming that the angle which the axis of the turbine makeswith the positive y direction is q (as illustrated in Fig. 1), then eachcomponent of the reaction of the axial thrust induced by theturbines on the flow can be written as:

FTx ¼ FT � jsin ðqÞj � signðuÞ (5)

FTy ¼ �FT � jcos ðqÞj � signðvÞ (6)

where FT, FTx and FTy ¼ total, x and y components of the reaction ofthe axial thrust induced by the turbines on the flow per unit area,respectively, u and v ¼ velocity components in the x and y direc-tions, respectively and sign(x) ¼ sign function which returns þ1when x is positive and �1 when x is negative.

The total reaction of the axial thrust per unit area is calculatedby dividing the axial thrust by the area of the cell, giving:

FT ¼ TDx� Dy

¼ 12� 1Dx� Dy

CTrAU2eff (7)

where CT ¼ thrust coefficient which can found from the literatureby knowing the Tip Speed Ratio (TSR), hub pitch and related

y

x x

yU

Turbineplane

Turbine axis

v

u

Δ

Δ

φ

θ

Fig. 1. Turbine and velocity direction schematic.

Fig. 3. Comparison of velocities in the channel without artificial energy extraction.Magenta line: predicted values. Blue dots: values reported by Bryden and Couch [4].(For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

R. Ahmadian et al. / Renewable Energy 38 (2012) 107e116 109

velocities [5] and it is considered to be constant in this study;Ueff ¼ effective flow velocity, which is the velocity normal to theturbine or parallel to the turbine axis. Turbines can be designed,either to rotate freely to be perpendicular to the flow or to rotateonly 180� to face the ebb and flood flow. When the turbine canrotate freely, the turbine is always normal to the flow and subse-quently the effective velocity is equal to the flow velocity. When theturbine is designed to rotate through only 180�, as illustrated inFig. 1, the turbine is not normal to the flow all the time during thetidal cycle. The velocity component in the x and y directions can bewritten as:

u ¼ U � sin ðfÞ (8)

v ¼ U � cos ðfÞ (9)

where U ¼ speed (m s�1) and f ¼ angle that the flow makes withthe positive y direction (degrees). Then the effective velocity can becalculated using:

Ueff ¼ jv� cos qþ u� sin qj (10)

Fig. 2. Comparison of depths in the channel without artificial energy extraction.Magenta line: predicted values. Blue dots: values reported by Bryden and Couch [4].(For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

Using Equations (8),(9)and (10), the effective velocity can berewritten relative to the velocity and the turbine axis direction, togive:

Ueff ¼ jU � cos ðq� fÞj (11)

As the piles are fixed, the components of the drag force inducedby the pile of a turbine on the flow are influenced by the flowdirection, giving:

FDx ¼ FD � jsin ðqÞj � signðuÞ (12)

FDy ¼ FD � jcos ðqÞj � signðuÞ (13)

where FD, FDx and FDy ¼ total, x and y components, respectively, ofthe drag force induced by the pile of turbine per unit area. Totaldrag force per unit area can be calculated in a similar manner to thedrag force induced by any other object giving:

Fig. 4. Comparison of depths in the channel with artificial energy extraction. Magentaline: predicted values. Blue dots: values reported by Bryden and Couch [4]. (Forinterpretation of the references to colour in this figure legend, the reader is referred tothe web version of this article.)

Fig. 5. Comparison of velocities in the channel with artificial energy extraction.Magenta line: predicted values. Blue dots: values reported by Bryden and Couch [4].(For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

R. Ahmadian et al. / Renewable Energy 38 (2012) 107e116110

FD ¼ 12� 1Dx� Dy

CDrAcU2 (14)

where CD ¼ the drag coefficient, and Ac ¼ the cross-sectional areaperpendicular to the flow.

The energy flux available for a turbine is [4]:

P ¼ 12CprAU3

eff (15)

where P ¼ energy flux (W m�2), and Cp ¼ power coefficient.

3. Model application

3.1. Idealised channel

Bryden and Couch [4] modelled the energy extraction process ina rectangular 1000 m wide and 4000 m long channel using a 1D

Fig. 6. Model domain extent and validation s

model. The refined 2D model was set up for an analogous channelwith a 100 m � 100 m grid. Similarly, upstream and downstreamboundaries were set to water level and flow boundaries respec-tively. The model was first run for the original channel without anyartificial energy extraction. The only difference between the twomodels was the bed roughness, which was used as a calibrationparameter, to reproduce the initial conditions without any energyextraction. Bryden and Couch [4] used a Manning’s number of0.035, while the Manning’s number used in this study was 0.037.Fig. 2 and Fig. 3 illustrate a comparison between the predictedwater levels and velocities during this study and the values pub-lished by Bryden and Couch [4]. As shown, both results match welland show that the channel used in the study carried out by Brydenand Couch was well reproduced in the current 2D model.

Fig. 4 and Fig. 5 illustrate a comparison between thewater levelsand velocities predicted by the current model and the resultsreported by Bryden and Couch [4]. There is good correlationbetween the predicted and published results and the comparisonsshow that the refined method integrated in the model to extractenergy performs appropriately.

3.2. Severn estuary and bristol channel

The 1D-2D linked model of the Severn Estuary and BristolChannel was set up from the outer Bristol Channel to the RiverSevern tidal limit, located close to Gloucester (Fig. 6). The simula-tion was carried out for 300 h including a spring and neap tidalcycle. The 1D model was from Gloucester to the Severn Bridge andthe 2Dmodel was set up from the old Severn crossing (M48 bridge)to an imaginary line between Milford Haven and Hartland Head.The upstream 1D model boundary condition at Gloucester was setas a flow rate varying between 60 and 106 m3/s (i.e. the normalRiver Severn condition) while the downstream boundary located inthe proximity of the Severn Bridge was set as a water levelboundary condition, where values for this boundary weredynamically derived from the 2D model. The time step for the 1Dmodel was 420 s. This model consisted of 351 cross-sections withan average distance of 240 m between two consecutive cross-sections. These cross-sections have shaped four reaches connect-ing to each other with two junctions. The inflows for all major

ites: A, Southerndown, B, Minehead [30].

Fig. 7. Comparison of predicted and measured water elevations at Southerdown (siteA).

Fig. 8. Comparison of predicted and measured current speeds at Southerdown (site A).

Fig. 9. Array formation

R. Ahmadian et al. / Renewable Energy 38 (2012) 107e116 111

inputs to the River Severn in the 1D model domain, including theRiver Wye, were treated as lateral inflows.

The bed elevations required for the 2D model were acquired byinterpolation of the bathymetric data digitised from the Admiraltycharts 1179, 1166, 1165 and 1152 at the centre of the cells. The 2Dmodel was based on a coarse (600 m) and fine (200 m) grid, witha maximum time step of 105 and 35 s for the coarse and fine grids,respectively. The model predictions for the different grid sizeswere compared and it was found that the model was not oversensitive to the mesh size [19]. In all 17 river and stream dischargeswere included into the Severn Estuary and Bristol Channel 2Dmodel domain, with these rivers and streams including the Taff,Ely, Parrett and Avon discharge locations, and with all sourcesincluding both flow and water quality point source inputs. More-over, flow and enterococci inputs for all the main wastewatertreatment works along the estuary and rivers were also included inthe mode [30].

The downstream boundary of the 2D model was specified asa water level boundary condition and the boundary values for thesimulation period were obtained from the Proudman Oceano-graphic Laboratory (POL) Irish Sea model [31]. Since this boundarywas so far seawards of the region of interest, the concentrations offaecal indicators and suspended sediments were set to zero alongthe downstream boundary. The upstream boundary of the 2Dmodel was set as a flow boundary condition and the flow andwater quality indicators at this location were acquired from the 1Dmodel.

Themodels were then calibrated using Admiralty Chart data andwere then validated against different sets of field data collected attwo sites illustrated in Fig. 6 by Stapleton et al. [32]. The modelpredictions and field measurements showed good agreement, withtypical comparisons between predicted water elevations and depthaveraged current speeds and measured data being shown in Fig. 7and Fig. 8 respectively. More information on the model details andmodel calibration can be found in Ahmadian et al. [19].

There are many protocols required for a site selected fordeployment of an array of turbines, such as: high currents anda minimum depth at low spring tide, distance from the navigationchannels, proximity to the national grid connections and supportinfrastructure, and finally not causing any significant damage to

for 2000 turbines.

Fig. 10. Water levels at P1 for a spring tide without and with the turbine array. Fig. 12. Current speeds at P1 for a spring tide without and with the turbine array.

R. Ahmadian et al. / Renewable Energy 38 (2012) 107e116112

the environment and marine habitat which cannot be easilymitigated against [8]. Since the main focus of this study was toinvestigate the hydro-environmental impact of tidal streamturbines, the site used in this study was solely selected for themodel demonstration purposes and none of the required consid-erations for selecting the site, apart from relatively high flowvelocity and minimum depth, have been taken into consideration.Finally, 10 m diameter turbines were used in this study, which isa relatively small size of turbine in comparison to some of themorerecent commercial horizontal axis turbines, to fulfil the minimumturbine depth requirements.

To investigate the potential far-field impacts of turbines, themodel was set up for an array of 2000 � 10 m diameter turbines inan area of 7.2 km2. This means that the turbines would be 50 m (i.e.5 � turbine diameter) away from the closest turbine in eachdirection. The array and the point for which results have beenpresented in this paper are illustrated in Fig. 9.

Fig. 11. Changes in water levels across the estuary at high water at Barry (red dot) after incllegend, the reader is referred to the web version of this article.)

Fig. 10 shows the water levels for a spring tide, at point P1.Moreover, Fig. 11 illustrates the differences in the water level acrossthe estuary with the turbine array included and excluded in themodel, at mean ebb tide at Barry (red dot). This figure shows thatthe changes inwater levels are less than 10 cm in the vicinity of thearray. Considering the mean depth of water, which is more than20 m in the area, it can be concluded that the array does not makea significant change in the water levels.

Fig. 12 illustrates the velocities for a spring tide at P1. This figureshows that the velocities predicted by themodel after including thearray of turbines was lower at P1 (located inside the array) than thepredicted velocities for the existing situation. Fig. 13 illustratestypical differences in the velocities across the estuary afterincluding the array of turbines in the model to the no turbinecondition, at mean ebb tide at Barry (red dot). This figure showsthat the velocities reduce inside the array, and both upstream anddownstream of the array.

uding the array of turbines. (For interpretation of the references to colour in this figure

Fig. 13. Changes in the velocities across the estuary at mean ebb at Barry (red dot) after including the array of turbines- vectors show the actual velocities. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)

R. Ahmadian et al. / Renewable Energy 38 (2012) 107e116 113

To study the sensitivity of the model to power extraction, themodel was run for a number of scenarios with the power coefficient(Cp) changing from 0.25 to 0.45. It was found that the model wasnot over sensitive to power extraction. Typical water levels andvelocities predicted during a spring tide at P1 for a number of thesecases are shown in Fig. 14 and Fig. 15, respectively.

Fig. 16 shows the suspended sediment (SS) concentrationsduring a spring tide at P1, without and with the turbine array. Thefigure shows that the SS concentrations are slightly lower inside thearray. Moreover, Fig. 17 illustrates typical differences in the SSconcentrations between with the array and the existing situationacross the estuary, at mean ebb at Barry (red dot). It was found thatthe SS concentrations were changed around 15 km away from thearray when they were compared to the existing situation. It wasfound that the SS levels reduced both upstream and downstream ofthe array and increased to the side of the array.

Fig. 14. Comparison of water levels at P1 for a spring tide without and with the turbinearray with different power coefficient.

Fig. 18 illustrates the concentrations of faecal enterococciduring a spring tide at P1. It can be seen that, as for the SSconcentrations, the faecal bacteria concentrations are slightlylower inside the arrays. Fig. 19 illustrates the typical differences infaecal enterococci concentrations across the estuary with theturbine array included and excluded in the model, at mean ebb atBarry (red dot). The results show that changes in the faecalenterococci concentrations are following the same trends as thechanges in the SS concentrations and changes in the faecal bacterialevels occur up to typically 15 km away from the array. Thesechanges are thought to be a direct consequence of changes in theability of the flow to transport water quality constituents and anindirect consequence of changes in the SS concentrations, whichaffects the transport of bacteria attached to the sediments and the

Fig. 15. Comparison of velocities at P1 for a spring tide without and with the turbinearray with different power coefficients.

Fig. 16. Suspended sediment levels at P1 for a spring tide without and with the turbinearray.

Fig. 18. Faecal bacteria levels at P1 for a spring tide, without and with turbine array.

R. Ahmadian et al. / Renewable Energy 38 (2012) 107e116114

decay rate. However, to acquire a better understanding of theimpacts of the tidal stream turbines on faecal bacteria concen-trations, more comprehensive studies are required.

Finally, Table 1 shows the amount of total, mean and maximumavailable power for extraction by the turbine array over the phasewhen the turbines were activated during the simulation period(287.6 h). In modelling the turbine impacts, the turbines were notactive during the first tidal cycle to avoid any instability whichcould be caused by the initial condition. Subsequently, the elec-tricity generation phase was equal to the simulation period (300 h),deduced by the first tide duration (12.4 h). The available energy toeach turbine at each time step was calculated using Equation (15)and total available energy was calculated by adding up the avail-able energy for all turbines during the simulation period.

The potential annual output of the turbine array can be calcu-lated using the mean generated power. Subsequently, the

Fig. 17. Changes in suspended sediment levels across the estuary at mean ebb tide at Barrycolour in this figure legend, the reader is referred to the web version of this article.)

estimated annual output of the turbine array would be 275 GWhr.The average value of the velocity magnitude at P1, which is insidethe array, over the simulation period was 0.947 m s�1. Using thisvalue as the effective velocity for all turbines, the mean generatedpower of the array using Equation (15) was equal to 26.72 MW.This value was 85% of the mean generated power predicted by themodel. The proximity of this value to the model predicted meanpower output confirms the model power predictions. Besides, asthe mean generated power predicted by the model was larger thanthe value calculated by using the average velocity at P1, it can beconcluded that the average value of the velocity magnitude for allthe turbines were higher than the average value of the velocitymagnitude at P1. Although the output values are discouraging inthe first instance, it is worth recalling that at the beginning of thissection it was stated that the array with 10 m diameter turbineswas not practical at this site and was only selected to demonstratethe model abilities.

(red dot) after including the array of turbines. (For interpretation of the references to

Fig. 19. Changes in faecal bacteria levels across the estuary at mean ebb tide at Barry (red dot) after including the array of turbines. (For interpretation of the references to colour inthis figure legend, the reader is referred to the web version of this article.)

Table 1Total, mean and maximum available power to the array.

Mean availablepower (MW)

Max. availablepower (MW)

Total availableenergya (MWhr)

31.4 245.5 9019

a In 287.8 h.

R. Ahmadian et al. / Renewable Energy 38 (2012) 107e116 115

4. Conclusion

A 2D depth integrated hydro-environmental model has beenmodified to simulate the impacts of tidal stream turbines on waterlevels, tidal currents and water quality constituent predictions inmarine and coastal water bodies. This modification was carried outby calculating the thrust and drag applied on each turbine andincluding the corresponding additional terms in the refinedmomentum equations as external force terms. In the absence of anyreal data published in the literature, the refined model was testedagainst published 1D modelling results in the first instance.

After it was found that the model results were close to thepublished results, the model was used to model the potential far-field impacts and power generated of an arbitrary array ofturbines in the Severn Estuary and Bristol Channel for demon-stration purposes. The hydro-environmental impacts of the turbinearray were investigated, as well as the array potential poweroutput. It was found that the impacts of the turbine arrays on thewater levels were not significant. The velocities were reducedinside and both upstream and downstream of the array, thisreduction was more than 25% in some places inside the array.Conversely, the velocities were increased along the sides of thearray, with these flow patterns indicating that the flow resistance ofthe turbine array was encouraging flow, and increasing currents,around the array as expected. Furthermore, the model predictionshave shown that the suspended sediment concentrations changednoticeably within 15 km from the turbine array. The suspendedsediment levels decreased upstream, downstream and inside thearray while there were predicted to increase less significantly alongthe sides of the array.

Since large populations of bacteria living in estuarine waters areattached to sediments, the interaction of bacteria and suspended

sediments was included in the model for faecal bacteria fluxpredictions. The faecal bacteria concentrations predicted by themodel indicated that the changes in the faecal bacteria concen-trations were very similar to the predicted changes in the sus-pended sediment levels. This was thought to be due to the complexprocesses involved in faecal bacteria transport, which were affectedby the changes in the bacteria attached to the suspended sedimentsas well as bacteria advection, diffusion and decay etc.

This study has shown that the refined hydro-environmentalmodelling tool developed as a part of this research programmecan be used in predicting the potential hydro-environmentalimpacts in marine and coastal water bodies, along with thepower output of tidal stream turbine arrays particularly for suchcases where the relative impacts and output of the arrays undervarious boundary conditions and array configurations are required.

Acknowledgements

The study is carried out as a part of MAREN project, which is partfunded by the European Regional Development Fund (ERDF)through the Atlantic Area Transnational Programme (INTERREG IV).

References

[1] Black & Veatch, Tidal Stream e Phase II UK Tidal Stream Energy ResourceAssessment. A report to the carbon trust’s marine energy challenge in, 2005.

[2] O’Rourke F, Boyle F, Reynolds A. Tidal current energy resource assessment inIreland: current status and future update. Renewable Sustainable Energy Rev2010;14:3206e12.

[3] Myers L, Bahaj AS. Simulated electrical power potential harnessed by marinecurrent turbine arrays in the Alderney race. Renewable Energy 2005;30:1713e31.

[4] Bryden IG, Couch SJ. ME1-marine energy extraction: tidal resource analysis.Renewable Energy 2006;31:133e9.

[5] Bahaj AS, Molland AF, Chaplin JR, Batten WMJ. Power and thrust measure-ments of marine current turbines under various hydrodynamic flow condi-tions in a cavitation tunnel and a towing tank. Renewable Energy 2007;32:407e26.

[6] Batten WMJ, Bahaj AS, Molland AF, Chaplin JR. The prediction of the hydro-dynamic performance of marine current turbines. Renewable Energy 2008;33:1085e96.

[7] O’Doherty T, Agaee DA, Mason-Jones A, O’Doherty D. An assessment of axialloading on a five-turbine array. P Civil Eng Energy 2009;162:57e65.

R. Ahmadian et al. / Renewable Energy 38 (2012) 107e116116

[8] Willis M, Masters I, Thomas S, Gallie R, Loman J, Cook A, et al. Tidal turbinedeployment in the Bristol channel e a case study. P Civil Eng Energy 2010;163:93e105.

[9] Fraenkel PL. Marine current turbines: pioneering the development of marinekinetic energy converters. P Mech Eng A-J Pow 2007;221:159e69.

[10] westwood A. SeaGen installation moves forward. Renewable Energy Focus2008;9:26e7.

[11] OpenHydro Press Release, OpenHydro successfully deploys 1MW commercialtidal turbine in the Bay of Fundy, in, 2009.

[12] SDC (Sustainable Development Commission). Turning the tide: tidal power inthe UK, in, London; 2007.

[13] Falconer RA, Hartnett M. Mathematical-modeling of flow, pesticide andnutrient transport for fish-farm planning and management. Ocean CoastManage 1993;19:37e57.

[14] Harris EL, Falconer RA, Lin B. Modelling hydroenvironmental and health riskassessment parameters along the South Wales Coast. J Environ Manage 2004;73:61e70.

[15] Bockelmann BN, Fenrich EK, Lin B, Falconer RA. Development of an ecohy-draulics model for stream and river restoration. Ecol Eng 2004;22:227e35.

[16] Kashefipour SM, Lin B, Falconer RA. Modelling the fate of faecal indicators ina coastal basin. Water Res 2006;40:1413e25.

[17] Falconer RA, Li GY. Modeling tidal flows in an Islands Wake using a 2-equationTurbulence model. P I Civil Eng-Water 1992;96:43e53.

[18] Kashefipour SM, LinB, Harris E, Falconer RA. Hydro-environmentalmodelling forbathing water compliance of an estuarine basin. Water Res 2002;36:1854e68.

[19] Ahmadian R, Falconer R, Lin B. Hydro-environmental modelling of proposedSevern barrage, UK. P Civil Eng Energy 2010;163:107e17.

[20] Ahmadian R, Morris C, Falconer R. Hydro-environmental modelling of off-Shore and coastally attached impoundments of The North Wales Coast.Edinburgh: The First IAHR European Congress; 2010.

[21] Kashefipour SM, Falconer RA, Lin B, Harris EL. FASTERmodel reference manual.In: Hydro-environmental research centre report. Cardiff University.; 2000.

[22] Falconer RA. An introduction to nearly horizontal flows. In: Abbott MB,Price WA, editors. Coastal, estuarial and harbour engineers’ reference book.London: E & FN Spon Ltd; 1993. p. 27e36.

[23] Falconer RA, Lin B, Kashefipour SM. Modelling water quality processes inestuaries. In: Bates PD, Lane S, Ferguson R, editors. Computational fluidmechanics: applications in environmental hydraulics. John Wiley and SonsLtd.; 1995. p. 305e28.

[24] Chick H. The process of disinfection by chemical agencies and hot water. J Hyg(Lond) 1910;10:237e86.

[25] Marshall KC. The effects of surfaces on microbial activity. Water PollutMicrobiol 1978;2:51e70.

[26] Burton GA, Gunnison D, Lanza GR. Survival of pathogenic bacteria in variousfresh-water sediments. Appl Environ Microbiol 1987;53:633e8.

[27] Valiela I, Alber M, Lamontagne M. Fecal-coliform loadings and stocks inButtermilk Bay, Massachusetts, USA, and management implications. EnvironManage 1991;15:659e74.

[28] Buckley R, Clough E, Warnken W, Wild C. Coliform bacteria in streambedsediments in a subtropical rainforest conservation reserve. Water Res 1998;32:1852e6.

[29] Obiri-Danso K, Jones K. Intertidal sediments as reservoirs for hippuratenegative campylobacters, salmonellae and faecal indicators in three EUrecognised bathing waters in North West England. Water Res 2000;34:519e27.

[30] Yang L, Lin BL, Falconer RA. Modelling enteric bacteria levels in coastal andestuarine waters. Proc. Inst Civil Eng Energy 2008;161:179e86.

[31] Heaps NS, Jones JE. 3-Dimensional model for tides and surges with verticaleddy viscosity prescribed in 2 layers .2. Irish Sea with bed friction Layer.Geophys J Roy Astr S 1981;64:303e20.

[32] Stapleton CM, Wyer MD, Kay D, Bradford M, Humphrey N, Wilkinson J, et al.in. Fate and transport of particles in estuaries, vol. IV: Numerical modelling forbathing water enterococci estimation in the severn estuary. Bristol: Envi-ronment Agency; 2007.