Built in 1944 to allow lorries to be off-loaded from ships at all states of the tide and to supply the Allied armies during the invasion of Europe. They formed a vital link in the supply chain, as conventional harbours were not available. The caissons used to build the breakwaters and flexible piers were built in various parts of England including Head Wrightson Ltd, Stockton-on-Tees in 1943/44. The order to build them was given by Prime Minister Winston Churchill in 1942 after the problems highlighted by the Dieppe raid. In one of the photographs Winston Churchill can be seen on a visit to Haverton Hill. Photographs courtesy of F Robinson, Eaglescliffe.Information courtesy of P Stevenson, Norton. 
Picture Stockton Archive
A Pictorial History of Stockton-on-Tees
For anyone interested you can see photographs of the cassions “Mulberry Harbour” referred to in previous comments in these photographs
As Mr Stevenson says, these cassions were built to form an artifical harbour to supply the Allied forces during the D-Day landings. Because the invading armies could not initially secure a deep water port, they needed somewhere to offload supplies directly from ships. The cassions made up what was code named the “Mulberry Harbour” and many were towed across the channel and sunk off the little town of Arromanches-les-Bains. They were desingned to last for only a few months, or until a deep water port was captured. However several of them can still be seen sticking out of the water in the bay off Arromanches. This harbour was nicknamed “Port Winston”. Another portable harbour was towed to Omaha beach by the Americans but because of bad weather this one only lasted a few weeks.