TheDover Barrage was an underwater blockade by England ofGerman submarines attempting to use theEnglish Channel duringWorld War I. The barrage consisted of explosive mines andindicator nets. A similar barrage was used in World War II.
The barrier consisted ofminefields laid betweenBelgium andDover at the outbreak of war, followed in February 1915 by 25 km ofindicator nets, steel netting anchored to the sea bed.[1] The first evidence of effectiveness was on 4 March 1915, when the German U-boatU-8 was taken, caught in the indicator nets.[1][2] The first stage of the barrage was completed in April 1915 and it was patrolled by ships of theDover patrol.[2] Both sides initially believed the barrier to be effective and the Germans attributed some submarine losses to it. However, in March 1916, the Germans found their submarines could traverse the Channel on the surface at night. The British moved the barrier to cover Folkestone–Cap Gris-Nez and used new mines andsearchlights, effectively closing the Channel to hostile submarines in August 1918.[1] Underwater scanning of the area covered by the Dover Barrage shows the wreck ofUB-109 broken in half.[3]
A mine barrage was also used in Dover during the 1940Siege of Calais.