Alderney breakwater


Duration:17 years

Cost:£1.2m (£140 today)

Country: Alderney, UK

What did this project achieve?

Construct a breakwater that goes out to sea for British naval ships to use

In the 1840s the British government saw France and its navy as a major military threat. The Royal Navy needed a bolt-hole for its ships near the French port of Cherbourg. Alderney, one of the Channel Islands, which are closer to France than England, was chosen as a good location.

The British didn't want the new harbour to be visible from the French side of the island. This meant building on the north west side of Alderney which had very little protection from the rough waves and storms of the Atlantic which have caused damage to the breakwater over the years.

The 1,430m-long breakwater – part of the new Braye Harbour - was built by engineer Thomas Jackson, who'd previously built railways and canals. It was completed by 1864 but the contract to repair breaches caused by stormy conditions in the Atlantic had to be extended.

In 1870 the government asked former ICE president Sir John Hawkshaw to report on the best way to secure the future of the breakwater, which had already undergone some major repair work. He recommended reducing its length to 870m.

Ownership of the breakwater passed from the British government to the Bailiwick of Guernsey in 1987. The Bailiwick (territory) consists of Guernsey, Alderney, Sark and some other smaller islands.

Bailiwick of Guernsey authorities have spent over £25m repairing breaches to the breakwater since 1987.

Difference the breakwater has made

Alderney breakwater has contributed to engineers' knowledge of how these structures behave. It's been the subject of many studies, including papers presented to ICE - the first of these was in 1863.

Despite the rough weather it sees every year the breakwater continues to shelter Alderney and boats in Braye Harbour.

It's also a tourist attraction, helping to bring income to the local economy.

How the work was done

It took a large team to build the Alderney breakwater. One of the first tasks by contracting engineer Thomas Jackson's was to build cottages to accommodate over 1,200 workers.

Jackson also constructed a railway from the construction site to Mannez, 2.25 miles away on the other side of the island. The government had bought land in the area to quarry rock for construction of the breakwater. Other building materials were brought to Alderney by steamboat.

The breakwater was formed of a rubble bank, topped with a masonry wall. The stones for the rubble weighed between 3 and 15 tonness while the masonry blocks weighed about 30 tonness each.

It was a mammoth task as the sea was up to 150ft deep in places and by the late 1850s 2-3,000 tonnes of rubble were being tipped each day. The masonry blocks could only be set between May and October to avoid winter storms. The breakwater took 17 years to build.


The force of the sea… was not understood at the commencement of the works.

Leveson Francis Vernon-Harcourt

From his paper on the breakwater presented to ICE in 1863.

Fascinating facts

The Bailiwick of Guernsey is one of 3 crown dependencies of the UK. Crown dependencies make their own laws and issue their own bank notes. The other crown dependencies are the Bailiwick of Jersey and the Isle of Man.

Alderney's entire population (about 1,500 people) was evacuated during World War 2. When German forces landed in June 1940 they found an empty island.

Plans for a marina in Braye Harbour were recently shelved amid worries that the breakwater could be permanently damaged by storms.

People who made it happen

  • Client: the British government
  • Designer: James Walker, a former ICE president
  • Contracting engineer: Thomas Jackson

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