A number of numerical models of various sections of the North Sea have been developed, primarily for flood prediction; these simulate both the propagation of the astronomical tide and the generation and propagation of storm surges. The tides effectively originate in the deep ocean and their propagation into shallow seas can be simulated for an almost infinite period ahead. Surges are generated by the action of wind and pressure gradients, particularly over the shallow sea regions in the northern and central North Sea.