Tidal Currents versus Wind Waves

Abstract

IN NATURE, vol. xxiv. p. 286, a writer on “sea-shore alluvion” positively asserts that the travelling of sea beaches is due to wind-waves, and not to tidal currents, and calls a writer in the Engineer to task for having stated the latter. Notwithstanding this assertion, I would suggest that the writer in the Engineer is right. Twenty-five years ago, when an engineering student, I was taught that sea-beach travelling was due to wind-waves. Afterwards, while knocking about during fifteen years in the vicinity of the south and west coasts of Ireland, I noted facts that went to show that such a theory was not universally correct. This led me to study wind-action on the sea and lakes, also all I could find that had been written on the subject; the result being that as good evidence was so contradictory, no opinion could be come to from the evidence of others. But it was not till about ten years ago, when I was so circumstanced that I could properly study wave-action, and after six years' careful observation on the south-east coast of lreland, that I found that tidal currents were the principal motive power; and on again reading what had been written on the subject, that I found that nearly all the advocates for the driftage of sea beaches by wind-waves had studied on beaches where the most continuous and powerful winds acted in conjunction with the flow-tide current. As the results of my observations have been published in the Proceedings of the Royal Irish Academy, English and Irish Institutions of Civil Engineers, the Geological Societies of London, Dublin, &c., during the last six or eight years, it is unnecessary to repeat them here. I would, however, point out that when there are only wind-waves and no tidal currents, the beaches as a general rule are banked up, but do not travel (the writer in NATURE seems to have observed this, but does not appear to see the importance of it). This may be seen in the tideless Mediterranean, as pointed out by the late Dr. Ansted in his paper on the Lagoons at the Delta of the Rhone; it may also be seen in Malcombe, or any other bay where there is a “head of the tide” but no tidal current; and in the different freshwater lakes, when the wind-waves are the only motive power. But wherever there are tidal currents acting on a coast the beach must travel. Such tidal currents are those that most perplex the erectors of groynes. If there was only the travelling augmented by wind-waves, the erection of groynes would be very simple; but, as a general rule, they are most necessary where there are strong tidal currents (or conflicting currents) due to the regular “flow” tide, “half counter” tides, or “on-shore” tides; which conflicting currents, combined with the action of wind-waves, let them be direct or as “ground swells,” make up all the “cutting-out tides.” The greater the complications the greater the “cutting out,” and the more ingenious have to be the groynes. “Fulls” accumulate with the wind-waves, but rapidly disappear when the wind ceases. I presume the writer of the article in question is aware that the greatest rise of tide and the least current is at the “heads of the tides,” while the least rise and greatest current is at the “nodal or hinge lines”; and I would be interested to know where permanent beaches accumulate in the latter localities, as from what I have seen those that form rapidly disappear when the wind ceases.

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KINAHAN, G. Tidal Currents versus Wind Waves. Nature 24, 460 (1881). https://doi.org/10.1038/024460a0

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