Channel Islands

The Channel Islands sit in the southeast corner of the Bay of Biscay approach, closer to the Normandy coast than to England. Jersey is the largest island; its south coast at St Helier records a mean spring tidal range of around 12 metres — consistently among the five largest tidal ranges anywhere in the world. The physics behind this comes from the shape of the Bay of Mont Saint-Michel, the wide, shallow embayment to the northeast that acts as a resonance chamber: incoming Atlantic tides are amplified as they funnel into the narrows between Jersey and the French coast. Mont Saint-Michel itself, a few nautical miles to the north, reaches 13 to 14 metres at equinoctial springs. The consequence for Jersey is that the sea remakes the coastline several times a day. Harbours that hold deep water at high tide dry completely at low water, with boats sitting upright on the sand or on harbour cradles. Reefs, islets, and rock platforms invisible at high water emerge at low tide to change the navigable geography entirely. Swimmers and walkers need to think in tidal states, not fixed geography. Elizabeth Castle in St Helier harbour — built on a tidal islet known as the Île au Guerdain — is the clearest illustration of this character. At high tide it stands isolated in water. At low water a sand causeway connects it to the waterfront; the walkable window is roughly four hours centred on low tide. The castle has been on that rock since the late sixteenth century; the St Aubin's Fort on the opposite side of the bay operates on the same principle. On Jersey's southwest coast, St Brelade's Bay is a protected sandy arc whose spring range of around 10.5 metres makes the beach a different place at different states of tide. At Gorey on the northeast coast, the harbour below Mont Orgueil Castle dries at low water — the castle has looked over a working tidal harbour since at least 1204.