Before the war navigation off this coast required the use of many facilities such as buoys and lighthouses. At the best of times buoys are not very dependable as navigational aids owing to their tendency to break away in heavy weather or to drag their anchors along the bottom. Their positions need to be "fixed" from time to time by means of angles to shore objects, or by methods of astronomical observation, and then compared with the positions shown on the charts. Lighthouses, however, unless they are of the small type without lighthouse-keepers, are more efficient aids to the navigator. During the war the lighthouses east of Nieuport were only used by the Germans during short periods when specially required for their own craft; the majority of the buoys were withdrawn and the remainder were moved to new positions which were frequently altered to prevent the Allies from making use of them. Thus, during the war, the charts available to the Allies were very soon obsolete; no others were obtainable. Navigation off the Flanders coast, for Allied vessels of any size, therefore, became decidedly hazardous.
The tides on the coasts, in addition to running alternately eastward and westward with considerable velocity, also caused large differences in sea-level amounting to fifteen feet between the highest and lowest states of the tide.
The Canal System
Mention has already been made that the only harbours on the coast eastward of Nieuport were situated at Ostende, Blankenberghe, and Zeebrugge. But these were not natural harbours. They had been artificially cut out of the coast-line by means of dredging. The entrance channels were preserved by piers built out into the sea and by dredging operations designed to retain the desired depths of water.
BIRD'S-EYE VIEW OF CANALS
Ostende, before the war, had been much used as a commercial harbour, and was therefore provided with numerous wharves, basins, and docks. It was a suitable harbour for all classes of submarines and torpedo craft. Blankenberghe was a little harbour about nine miles east of Ostende and three miles west of Zeebrugge. Its depth was exceedingly small; it could, therefore, only be used for shallow draught vessels such as fishing boats, motor boats, and the like. It was true that the rise of tide, amounting to approximately fifteen feet, would enable larger vessels to enter or leave near the time of high water, but any naval vessel stationed in a harbour from which it can only proceed to sea during a limited portion of the twenty-four hours at once loses much of its value. Zeebrugge could accommodate vessels up to a considerable size: the harbour works and depths will be described in detail presently. Although these three places provided the only harbours on the coast, there was a harbour of great importance at Bruges, about eight miles inland from Zeebrugge.