On the formation of tide-produced seiches and double high waters in coastal seas

We describe a mechanism by which the tide, forcing a coastal water body, produces an oscillation at the natural period of the water body - a seiche - around the time of high water. The seiche appears as a distortion in the tidal curve which can, under certain conditions, produce a double high water. The formation of the seiche is explored with a numerical simulation of a sea strait, forced by the tide at its open ends. A seiche, of wavelength equal to twice the length of the strait, is formed when the mean depth of the strait is similar to the tidal range: the rapid reduction of friction as high water is approached appears to be an important part of the formation process. Observations of water level in the centre of a shallow sea strait in north Wales confirm that there is a residual oscillation around high water with the form of a damped seiche. The crest of the seiche occurs just before, and the trough just after, the maximum in the fitted tidal curve. The seiche slows the fall of the tide after high water and on one occasion, at neap tides, the fall is reversed and a double high water is formed. It is possible that this mechanism, hitherto unidentified, contributes to the formation of double high waters in other shallow coastal locations.