COASTAL DISCOVERY

William Thomson explains some of the characteristics of the seas around our enchanting islands

For people who love the coast, there can surely be no better place than an island. Completely surrounded by the sea, nature sets the tone, and on an island the tides, winds and swell make the rules. Time is determined by the ebb and flow of streams, the rolling of waves. Being separated from the mainland, there are often cultural quirks to the place and islanders have their own accents, customs, rules and regulations – almost always more laidback and in tune with the natural world. Best of all, having a 360-degree coastline means there is always an optimum stretch of shore for your favoured activity. At any time there will be both an onshore wind and an offshore wind, depending on which side of the island you go to. For sailors looking to anchor in shelter, they will always find a refuge with the offshore breeze, while kitesurfers looking for an onshore wind that won’t blow them out to sea will find one on the other side. In short, islands are the perfect place for adventure.

Mid-ocean islands typically have small tides because they are surrounded by deep water so the ‘tide waves’ can flow past uninterrupted. But islands around Britain (especially the Channel Islands) have some of the world’s biggest tides because they are set within the shallow ‘Continental Shelf’ where the tide wave slows down and is compressed, pushing up the peaks of tide waves to make high tides. Added to this, the funnel-shaped coastline around the Cherbourg Peninsula squashes the tide into a corner, magnifying the vertical range from high to low tides. With all this water sloshing in and out every six hours, it comes as no surprise that the tidal streams here are also incredibly intense, highlighted by the ‘Alderney Race’, a stretch of water between Alderney and France where the tidal streams are squeezed through and accelerated. When these supercharged streams collide with an oncoming swell and/or wind, terrible seas suddenly appear.

While you can safely pass through these ‘tide waves’ at slack water, swell is ever present with island life. However, it is mainly found in specific parts of the island facing the prevailing winds, especially if there is a large body of water in that direction. This is because for a powerful swell to reach the beach there must have been a strong wind blowing over a large area of sea for a long time, ideally as far away as possible.

This gives the swell time to organise itself into clearly defined sets as it approaches, with the ‘best’ swell having a longer period (the time between two consecutive waves). A period of less than 10 seconds is classified as

‘Islands often have incredible biodiversity and provide some of the best wildlife encounters’

‘Islands around Britain – especially the Channel Islands – have some of the world’s biggest tides’

‘windswell’ and arrives with the wind that creates it, making messy conditions.

However, as the period extends over 10 seconds, it moves away from the storm where it was born, increasing the chances of a favourable offshore wind when it hits the beach. Looking at a map of the Channel Islands, you would expect the north-west beaches to experience more of this ‘groundswell’ because they are facing the Atlantic Ocean, where the prevailing westerly winds make the most consistent swell.