Operation Chastity

A vital factor in the conception of Operation Overlord, the invasion of Northern France, was rapid capture of deep-water ports. These were considered essential to handle the large quantities of reinforcements and supplies needed for the campaign. Operation Chastity was devised in April 1944 to fulfill this need, by rapidly constructing a complete new port at Quiberon Bay, and was the final major revision to the invasion plan.

Quiberon Bay is a large anchorage, between Lorient and Saint-Nazaire on the southwest coast of the Brittany Peninsula, sheltered by the Quiberon peninsula and a line of small islands and containing four small ports. The Auray river drains into the bay near one of the ports, Locmariaquer, and has scoured a 3000-yard-long 80-foot (24 m)-deep pool just offshore, between 30 and 300 yards wide with nearly vertical sides.

Operation Chastity would have seen the construction of floating piers in the pool, allowing large ships to tie up alongside, with bridges to carry the cargo and troops to the shore. The plan was berths for offloading five ships simultaneously, providing a capability of 2,500 tons of supplies per day directly onto vehicles. A further 7,500 tons per day could be offloaded using lighters carrying supplies directly to the shore from 30 further ships moored in the pool.

This was seen as a very efficient scheme, since the two Mulberry prefabricated ports constructed on the Normandy beaches provided 6,000 tons of supplies a day at a construction cost of 120,000 man-months, whilst Operation Chastity facility would provide 10,000 tons per day (compared to 26,000 tons per day through the Normandy beaches and Cherbourg) but would only need 4,000 man-months to construct the prefabricated facilities. A further advantage was the ready access to local rail facilities and the relatively undamaged rail network away from the Normandy region.

The scheme required the rapid capture of Brest and Lorient, since shipping would be liable to attack as it passed these German-held ports on the way around the peninsula from the United Kingdom. The plan was approved on 22 April 1944 and the capture of Quiberon Bay accorded a high priority.

Cherbourg was the first major objective of the invasion, its capture originally scheduled for 14 June (D+8) but moved back to 21 June just before the invasion. However, it was only captured on 27 June, and the capture of further Brittany ports was further delayed, and the assault on Brest did not start until 25 August.

By the time that Quiberon Bay was on the point of being captured, the Allies had already seized Antwerp (on 4 September) with its port facilities intact. On 9 September, Eisenhower abandoned all plans for further exploitation of Brittany ports, including Operation Chastity, which were now seen as irrelevant since, unlike Antwerp, they were now hundreds of miles from the front of the Allied advance.