A vessel navigating close to a gently shelving bank will experience forces pushing the bow away from and drawing the stern towards the bank. If the forces are strong enough, it may cause the vessel to roll towards the obstruction which, because the draught has now increased on that side, may cause grounding on the low side.
It is often thought that it is the repelling action of the forward positive pressure area which is the sole cause of the bow being pushed away from the obstruction.
Inspection of the forces involved clearly show that this is not always the case. In certain circumstances there can be a greater suction area at the stern created by the faster flowing water in that area, which in turn creates a negative pressure area acting on a much greater turning lever.
It need not be a river or canal bank, the same effect can be observed where there is a shoal area which is significantly less on one side of the vessel than on the other or where a vessel is navigating near say a dredged channel where the depth is significantly deeper on one side.
The effect can only be controlled by constantly correcting the applied helm and through judicious adjustment of ship speed.