I. Tidal friction in the Irish Sea

Abstract

The dissipation of energy in the tides has recently formed the subject of a paper by Mr. R. O. Street. In that paper it is assumed that the energy is dissipated by the viscous drag of layers of water which move parallel to the bottom of the sea. The assumption that tidal currents move in laminar motion is so opposed to ordinary observation of the surface of the sea in a tideway that I felt certain, on reading the paper, that if some other method could be found, which did not depend on any special assumptions as to the nature of the motion, it would be found that Mr. Street’s estimate of the dissipation is very much too small. This view is strengthened by the consideration that Reynolds criterion of stability would lead us to expect that eddies would form in any stream of sea-water flowing at a speed of 1 knot or more, when the depth is greater than some quantity of the order of magnitude of 1 or 2 cm. Since the mean depth of the Irish Sea is over 40 fathoms, mathematical considerations alone would lead us to suspect the existence of the eddies, which can in fact he seen marking the surface of the sea in places where the current runs exceptionally strongly, or over a particularly uneven bottom. Several of these places are marked as “ripples” on the chart of the Irish Sea, the sheet of water to which Mr. Street applied his calculations.