Numerical investigation of tidal resource & far field effects of energy extraction in Lashy Sound, Orkney Simon Waldman 1 , Jonathan Side 2 , David Woolf 3 International Centre for Island Technology Heriot-Watt University Back Road, Stromness Orkney, KW16 3BA, UK 1 smw13@hw.ac.uk; 2 j.c.side@hw.ac.uk; 3 d.k.woolf@hw.ac.uk Abstract—Lashy Sound is a small channel in Orkney, Scotland, where a tidal stream energy development is planned. This study uses numerical modelling to investigate the energy resource of the Sound and the effects on the flow of removing this power. A new 3D regional-scale hydrodynamic model of the area was built using the MIKE software and was used to study Lashy Sound. A standard momentum sink approach was used to represent tidal energy converters. It is estimated that the maximum possible yield from this channel from the M2 tidal constituent alone is 23 MW mean and 61 MW peak power, although this would require an unrealistic and uneconomic arrangement of tidal turbines. The 30 MW capacity that is planned is predicted to be feasible, and the environmental effects of both large and small arrays are discussed. Keywords—MIKE, hydrodynamics, resource assessment, envi- ronmental impact I. I NTRODUCTION The Orkney archipelago in northern Scotland (Figure 1) contains a complex network of inter-island channels of which many, like the Pentland Firth to the south, experience rapid tidal flows and are hence of interest for tidal stream energy development. The strength of tidal activity in the region is due to the time taken for the M2 tidal wave to propagate around Scotland, and the resulting phase difference between the Atlantic and North Sea sides of Orkney [1]. Much research effort has been put into studying the Pentland Firth, which promises a great deal of tidal energy (e.g. [1]– [5]), and the large northern channel that includes the Fall of Warness, which is the location of the European Marine Energy Centre (EMEC) tidal test site (e.g. [6], [7]). Relatively little attention has been paid to the smaller channels within Orkney, what power may be available from them, and whether their exploitation could affect the major sites. In this work we use numerical modelling to examine one of these smaller channels, Lashy Sound, which has tidal energy development planned but has been largely uncovered in the academic literature. In this paper we describe a new three-dimensional numerical hydrodynamic model of the Pentland Firth and Orkney Waters (PFOW) area that was developed to study Lashy Sound and its surroundings. We relate validation of this model, and describe early work on using the model to address two questions: 1) How much power is available from Lashy Sound? 60.0°N 59.5°N 59.0°N 58.5°N 58.0°N 57.5°N 6.0°W 5.0°W 4.0°W 3.0°W 2.0°W 1.0°W Fig. 1. Situation of Lashy Sound with respect to the Orkney archipeligo and the north of Scotland. The black box shows the extent of the model domain, and the red box highlights the location of Lashy Sound. 2) What effect would exploitation of Lashy Sound have on other channels? II. THE CHANNEL Lashy Sound lies in the northern part of the Orkney archipeligo, situated between the isles of Eday to the west and Sanday to the east. At the southern end it becomes Eday Sound, which links to the large channel through Orkney that includes the Fall of Warness. Eday Sound also has a shallow and partially-obstructed exit to the east which sustains rapid flows at some states of the tide. To the north, Lashy Sound opens into open sea. For a labelled map of these channels, see Figure 2. It is notable that Lashy Sound has a north-south orientation, while the dominant tidal flow across the archipeligo as a whole is between east and west. Strong currents in Lashy Sound must, therefore, stem not directly from the hydraulic forcing between