These considerations, together with the abandonment of the proposed landing on the Belgian coast, owing to unfavourable military conditions, led to the decision late in 1917 to undertake blocking operations concurrently with an attack on the vessels alongside the Mole at Zeebrugge.
In order to carry out the general policy mentioned, the eastern end of the Straits of Dover had been heavily mined at intervals during the war, and these mines had proved to be a sufficient deterrent against any attempt on the part of surface vessels larger than destroyers to pass through.

Owing to the rise of tide enemy destroyers could pass over the minefields at high water without risk of injury, and they frequently did so pass.

Many attempts had been made to prevent the passage of enemy submarines by means of obstructions, but without much success; and at the end of 1916 a "mine net barrage"-- i.e.a series of wire nets of wide mesh carrying mines--was in process of being placed by us right across the Straits from the South Goodwin Buoy to the West Dyck Bank, a length of 28 miles, it being arranged that the French would continue the barrage from this position to the French coast.

The construction of the barrage was much delayed by the difficulty in procuring mooring buoys, and it was not completed until the late summer of 1917.

Even then it was not an effective barrier owing to the tidal effects, as submarines were able to pass over it during strong tides, or to dive under the nets as an alternative; it was not practicable to use nets more than 60 feet deep, whilst the depth of water in places exceeded 120 feet.
Deep mines were laid to guard the water below the net, but although these were moored at some considerable distance from the barrage, trouble was experienced owing to the mines dragging their moorings in the strong tide-way and fouling the nets.