Numerical Modelling of the Interaction between Tidal Stream Turbines and the Benthic Environment David Haverson A thesis submitted in partial fulfilment of the requirements for the award on an Engineering Doctorate THE UNIVERSITY OF EXETER 2017 i IDCORE This thesis is submitted in partial fulfilment of the requirements for the award of an Engineering Doctorate, jointly awarded by the University of Edinburgh, the University of Exeter and the University of Strathclyde. The work presented has been conducted under the industrial supervision of Cefas as a project within the Industrial Doctoral Centre for Offshore Renewable Energy. ii I declare that this was composed by myself and that the material presented, except where clearly indicated, is my own work. I declare that that the work has not been submitted for consideration as part of any other degree or professional qualification. Signed: ________________________________________ iii Abstract The tidal stream industry has seen large growth in recent years, and the number of pre-commercial scale devices currently being tested reflects this development. However, commercialising this technology whilst showing that their environmental impacts is minimal remains a challenge. The impact on benthic communities is not considered to be a key strategic consenting issue, yet it is anticipated that the benthic habitat will change as a result of the presence of tidal turbines. To date, only single tidal turbine devices have been installed to demonstrate the application of tidal stream technology but despite successful tests there are still uncertainties surrounding the quantitative impacts these turbines have on local benthic communities. Unlike the wind industry, where physical effects of wind turbines have been catalogued through deployment of thousands of turbines, the tidal stream industry lacks these array scale quantitative data. Local impacts are known, but understanding the scale of the impacts and their relative significance of large arrays remains unknown. Tidal turbines (both single and arrays) interact with the hydrodynamics by decreasing the near field current flow directly in its wake through energy extraction and the drag caused by the physical structure. However, turbines may also affect the far field hydrodynamics, altering bed characteristics, sediment transport regimes and suspended sediment concentrations. As benthic habitats are closely linked to the physical seabed composition and the hydrodynamic conditions, the benthic environment is affected by to changes in the current flow. This thesis presents a series of studies investigating the interaction between tidal turbines and the benthic environment. Based on the hydrodynamic modelling software, TELEMAC2D, a numerical model has been developed to investigate the hydrodynamic impact of a single tidal array at Ramsey Sound, Pembrokeshire as well as the cumulative impact of multiple tidal developments in the Irish Sea. Based on the results of the models, the hydrodynamic outputs were used as inputs to drive a species distribution model, based on the software MaxEnt, to investigate how the distribution of benthic species altered in the presence of a 10MW tidal array at Ramsey Sound. Results of the study showed the development would have a minimal negative impact on the benthic environment. iv Acknowledgements Funding from the Energy Technologies Institute (ETI) and the RCUK Energy Programme for the Industrial Doctoral Centre for Offshore Renewable Energy (Grant number EP/J500847/1) is gratefully acknowledged. This work was carried out on the High Performance Computing Cluster supported by the Research and Specialist Computing Support service at the University of East Anglia I would like to thank my supervisory team Dr John Bacon (Cefas), Dr Helen Smith (University of Exeter), Dr Venki Venugopal (University of Edinburgh) and Dr Qing Xiao (University of Strathclyde) for their support, encouragement and guidance. I am most grateful to my colleagues at Cefas notably Dr Liam Fernand, for his support and wisdom, Dr Anna Downie and Dr Alex Callaway, for their time and expertise on species distribution modelling, Dr Markus Diesing and Dr David Stephens, for their knowledge and data for the sediment distribution modelling. I would like to thank Dr Paul Evans for the data he supplied for Ramsey Sound. To my family, I thank you for your support and encouragement throughout the years that lead to me to where I am today. Finally, to Rebecca, your unending love, support and patience over the course on my studies was immeasurable and without which I could not have succeeded. v “When one tugs on a single piece of nature they find the rest of the world is attached to it.” – John Muir vi Table of Contents Abstract………………………………………………………………………………...iv Acknowledgements……………………………………………………………………v List of Figures………………………………………………………………………….xi List of Tables…………………………………………………………………………xvii Abbreviations and Notations……………………………………………………….xviii 1 Introduction ................................................................................................. 1 1.1 Introduction ........................................................................................... 1 1.2 Benthic Environment ............................................................................. 4 1.3 Aims and Objectives.............................................................................. 7 1.4 Overview of Thesis ................................................................................ 9 2 Overview of Tidal Energy .......................................................................... 11 2.1 Introduction ......................................................................................... 11 2.2 Tide Theory ......................................................................................... 11 2.3 Harnessing Tidal Energy ..................................................................... 17 2.3.1 Tidal Barrages/Lagoons ................................................................ 17 2.3.2 Tidal Stream Turbines .................................................................. 20 3 Modelling Tidal Turbines ........................................................................... 25 3.1 Introduction ......................................................................................... 25 3.2 Modelling Tidal Turbines ..................................................................... 26 3.2.1 Solving Free Surface Flows .......................................................... 27 3.2.2 Representing Tidal Turbines ......................................................... 29 3.3 Hydrodynamic Modelling ..................................................................... 31 vii 3.3.1 TELEMAC-MASCARET Modelling System .................................. 32 3.3.2 Model Set-up ................................................................................ 33 3.3.3 Wave Modelling with TOMAWAC ................................................. 35 3.4 Conceptual Model ............................................................................... 38 3.4.1 Idealised Channel Model .............................................................. 38 3.4.2 Dragfo.f Subroutine ....................................................................... 42 3.4.3 Model Results ............................................................................... 45 3.5 Summary ............................................................................................. 49 4 Modelling the hydrodynamic and morphological impacts of a tidal stream development: Case Study of Ramsey Sound, UK ............................................ 51 4.1 Introduction ......................................................................................... 51 4.1.1 Ramsey Sound ............................................................................. 52 4.2 Methodology ........................................................................................ 55 4.2.1 Numerical model ........................................................................... 55 4.2.2 Modelling tidal turbines ................................................................. 57 4.3 Validation ............................................................................................ 58 4.3.1 Free Surface Elevations ............................................................... 59 4.3.2 Velocities ...................................................................................... 62 4.3.3 Harmonic Analysis ........................................................................ 67 4.4 Results and Discussion ....................................................................... 68 4.4.1 Array performance ........................................................................ 68 4.4.2 Influence of tidal array .................................................................. 71 4.4.3 Hydrodynamic far field effects ...................................................... 74 4.4.4 Morphological effects due to tidal velocities .................................. 78 4.4.5 Morphological effect due to waves ................................................ 87 4.5 Summary ............................................................................................. 92 5 Cumulative Impact Assessment of Tidal Energy in the Irish Sea .............. 93 5.1 Introduction ......................................................................................... 93 viii 5.2 Irish Sea Model ..................................................................................