Admiralty scaffolding

[6] 20-foot (6 m) wide sections were assembled then carried to the sea to be placed in position at the half tide mark as an obstacle to boats.

[6] However, trials found that a 250-ton barge at 5+1⁄2 knots (6.3 mph; 10.2 km/h) or an 80-ton trawler at 7+1⁄2 knots (8.6 mph; 13.9 km/h) would pass through the obstacle as if it were not there and a trawler easily pulled out one bay with an attached wire rope.

[7] Tests in October 1940 confirmed that tanks could only break through with difficulty, as a result Z.1 was adopted as an anti-tank barrier for beaches thought suitable for landing tanks.

[7][9] Barriers varying in length from a couple of hundred feet to three miles were constructed consuming 50% of Britain's production of scaffolding steel[6] at an estimated cost of £6,600 per mile[6] (equivalent to £460,000 today[10]).

[9] After the war, the scaffolding got in the way of swimmers,[11][12] subsequently it was removed for scrap and remaining traces are very rare, but occasionally revealed by storms.