Media Blog The Gulf of Corryvreckan: A Modern Perspective of an Ancient Folklore Written By Christian Armstrong, Scottish Association for Marine Science (SAMS), UHI PhD student The Gulf of Corryvreckan (Coire Bhreacain – Brecan’s cauldron) exists on the western coast of Scotland, and despite its fame, remains a source of mystery. Located between the islands of Scarba and Jura, Scottish folklore says the passage is home to a witch, the Cailleach (the old hag), and when she dons her handkerchief (the breaking standing waves), it is fatal to approach her. This folklore has evaded scientific investigation, until recent years. In 2012, researchers from the Oban-based Scottish Association for Marine Science (SAMS) entered the Gulf of Corryvreckan, and using a high-resolution multibeam sonar, revealed the shape of the seabed. Meanwhile, floating GPS transmitters, known as drifters, were released into the energetic tidal streams to track the paths and speeds of the currents. The combined investigation of these datasets has provided unprecedented insight into how the seabeds form and the tidal flows interact in dynamic equilibrium to inspire local tales of the mythical Cailleach. The energetic tidal currents occur due to the flow-constraining channel of the Gulf of Corryvreckan. As the tide propagates in and out of the Firth of Lorne, a difference in sea surface height is generated between the east and west approaches to the tidal strait. The flow speed through the passage increases as these sea surface heights become more unbalanced, with water racing from one side to the other to even out the water level. The energy of the tidal flows results from the geological history of the Gulf of Corryvreckan. Glacial erosion during the past two million years is thought to have periodically carved out the rock channel between the islands, resulting in depths of over 200m along its 3.2km length. At its narrowest, the gulf’s 1.1km width is characterised by a steep bedrock seabed composed of Jura Quartzite, that descends into the channel from the islands’ coastlines. Deep geological faults are contrasted by shallow upthrusts of outcropping rock with the shallowest of these outcrops being less than 30m deep. This outcrop has been locally defined as a ‘pinnacle’. However, recent bathymetric investigation into this feature revealed that the pinnacle plateaus considerably shoreward, suggesting the rocky outcrop is more precisely described as a ‘wall’. Atop the wall lies the famed standing wave train, formed during the flood tide. Waves can reach 5m in height during a westerly wind and are one of the most prominent flow features inspiring the Cailleach. Perhaps then, this submarine wall is the source of the area’s mythical legend, and the origin of the old hag. Following the flood stream west of here, the world’s third largest whirlpool occurs with a seemingly spontaneous and transient nature. As the steep seabed walls surrounding the Gulf of Corryvreckan recede to follow the coastlines of the islands, the flood tidal stream forms a westward projecting jet of water: The Great Race. Recording a maximum flow speed of over 4m/s, our drifters detailed the dynamics of the Great Race, revealing an arcing tidal current curving northward upon exiting the passage. The drifters map a series of loops indicating the northern flank of the Great Race. This characteristic is not reflected at the Great Race’s southern flank suggesting a hydrodynamic asymmetry. Researchers think that as the tidal streams round westward into the Gulf, the water is piled up against the northern boundary, Scarba’s southern coast. Driven by asymmetric bathymetry across the gulf’s west approach, the flow produces a clockwise rotating counter current (i.e. an eddy) as Scarba’s southern coast recedes. Along this northern eddyline, the friction between the Great Race and the eddy resembles the abyssal flow down a plug hole – the whirlpools of the Corryvreckan. The seabed not only steers the current but is also shaped by it. The transport of sand through the Gulf of Corryvreckan has formed sandy structures at the passage’s outflows which are themselves migratory – think sand dunes in a desert. Understanding the migratory dynamics of similarly unstable seabed structures found elsewhere on the continental shelf is essential to the maritime sector for safe navigation and construction. The most recent SAMS research into the Gulf of Corryvreckan was motivated in part to provide bathymetric data of seabed stability in extreme flow environments. However, this research also forms the most in-depth attempt at revealing the mechanism of the Corryvreckan whirlpools and the Great Race. It is with this research that the bathymetric-hydrodynamic interaction of the historically infamous Gulf of Corryvreckan is identified. Finally, the Cailleach is being unveiled.