Ebb and Flow in the Chausey Islands

Ebb and Flow in the Chausey Islands
Ebb and Flow in the Chausey Islands

Today’s story is the answer to the May 2023 puzzler.

France’s Chausey Islands (Îles Chausey), located in the English Channel, are subject to some of the largest tidal ranges on Earth. As a result, when tides are low, the islands number 365; when tides are high, only 52 remain above water.

These images show the archipelago at opposite tidal extremes: a low-tide view from July 22, 2018 (left), and a high-tide view from April 19, 2019 (right). Both images were acquired by the Operational Land Imager (OLI) on the Landsat 8 satellite.

Chausey lies approximately 18 kilometers (11 miles) off the coast of Normandy. The only inhabited island in the group is Grande Île, which is reachable via a short ferry ride. At low tide, it is possible to walk between some of the granite isles on sandy tidal flats. The area provides habitat for abundant seabirds and marine life and is protected as a European Environment Agency Natura 2000 site.

While the planet’s most extreme tidal swings are found across the Atlantic Ocean, in Canada’s Bay of Fundy, the tidal swings at Chausey rank among the world’s largest. The mean tidal range at the islands is 8.2 meters (27 feet), and during a spring tide it can exceed 14 meters (46 feet) .

Many factors, including the shape of coastlines, size of the ocean basin, and position of the Sun and Moon, govern the height of tides. At higher latitudes in the northern hemisphere, the relative closeness of the continents plays a large role in producing greater tidal ranges in the way it effectively constricts the oceans.

The English Channel’s energetic tides have been considered for tidal power schemes dating back to at least the 1960s. While no tidal generators were ever installed at the Chausey Islands, the world’s first tidal power plant in the nearby estuary of the Rance River came online in 1966 and continues to generate electricity today. With a capacity of 240 megawatts, it was the largest operating tidal power plant in the world until the Sihwa Lake Tidal Power Station in South Korea opened in 2011.

NASA Earth Observatory images by Allison Nussbaum, using Landsat data from the U.S. Geological Survey. Story by Lindsey Doermann.

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