Minikin’s equation mistake — a mystic art of systems of measuring units

This paper deals with one of the most controversial equations in coastal engineering — the so-called Minikin’s equation, describing the impact pressure due to wave breaking on a vertical-wall caisson of a composite breakwater. This equation has been used worldwide for many years, although it has been reported many times to overestimate real values of the impact pressure measured in nature and in the laboratory. Units of measurement, problems with dimensionality of parameters and conversion between different unit systems were suspected as causes. This paper explores in rigorous detail the reason behind the problems associated with Minikin’s equation. After a thorough literature review and a detailed analysis of all previously published forms of Minikin’s equation, a question of coherence/incoherence of systems of measuring units has been specially addressed. By adopting the incoherent English Engineering system of units, the most proper and satisfactory explanation of the curious mistake in Minikin’s formula has been found and formally correct forms of the equation, followed by appropriate units of measurement, are presented.