 The first components of the Mulberry harbours were brought across the Channel on D + 1 and the structures were in use for unloading by mid @-@ June . One was constructed at Arromanches by British forces , the other at Omaha by American forces . A severe storm on 19 June destroyed the Omaha harbour . The Arromanches harbour was repaired and remained in use for the next ten months , with a maximum capacity of 7 @,@ 000 long tons ( 7 @,@ 100 t ) of stores per day . Of the British supplies landed in Normandy by the end of August , 35 % arrived via the Mulberry harbour and 15 % came in via the small harbours at Port @-@ en @-@ Bessin and <unk> @-@ sur @-@ Mer . Most shipments were brought in over the beaches until the port of Cherbourg was cleared of mines and obstructions on 16 July . The most important use of the Mulberry harbour was the unloading of heavy machinery that could not be brought across the beaches . Artificial <unk> ( <unk> ) sheltered hundreds of ships during the storm of 17 – 23 June , and provided shelter for craft unloading stores at Juno and Sword . A joint Anglo @-@ American oil depot was constructed at Port @-@ en @-@ Bessin , fed via <unk> pipes known as " <unk> " from oil tankers moored offshore . Using this method , 175 @,@ 000 long tons ( 178 @,@ 000 t ) of petrol ( half for the Second Army ) was delivered by the end of August , by which time the underwater <unk> constructed in Operation Pluto were ready . 
