The interaction of surge and tide in the North Sea and River Thames

Summary. Although the tendency for surge peaks in the Thames to occur on rising tide has been recognized for some time, no satisfactory physical explanation has been presented. The phenomenon almost certainly results from non‐linear interaction between tide and surge and it is the mechanism of this interaction which is examined in the present study. A statistical analysis of surges recorded at 10 ports located along the east coast of Britain demonstrated the development of interaction as surges propagate southwards. This analysis showed that surges tend to develop a peak on the rising tide in the Thames irrespective of the phase relationship between tide and surge in the northern North Sea. A one‐dimensional model of the River Thames was used to examine how surge‐tide interaction varied for surges of differing types. In order to identify the mechanics of interaction, a new modelling technique was developed involving two models, one of tidal propagation and one of surge propagation, operated simultaneously with cross‐linkages in the form of perturbation terms providing the effects of interaction. By this means it was shown that quadratic friction is the dominant interaction mechanism in the Thames.