A short distance from Dunkirk lie the banks of Flanders, generally submerged less than five meters deep and separated by furrows ten to twenty meters deep. These sandbanks, also called sublittoral sandy sedimentary groups, are oriented from east-northeast to west-southwest and arranged parallel to the shore of the North Sea. They close the Dunkirk harbour, forcing ships to follow marked passages to enter or leave the port: the West Pass and the Zuydcoote Pass. It is to the north of the latter that the banks can be seen emerging at low tide.[1]
The continental base is relatively shallow in this area and the banks were formed by tidal currents. Their formation is linked to the flow gain, that is to say that the mass of materials brought by the flow is greater than that removed by the ebb. These banks can take on the appearance of real underwater dunes formed from the accumulation of shell sand, called hydraulic dunes, rising around twenty meters above the seabed.[1]