Recently Mitsuyasu proposed a new standard form of directional wave spectrum on the basis of many observed data. This is regarded as the most reliable one at present. But Mitsuyasu's spectrum is not usable for engineers, for it is described with a parameter of wind velocity. This parameter has been converted to another parameter which indicates the degree of concentration of energy, Smax. A method is presented how to infer the value of Smax from the steepness of deep water waves. As typical values of Smax for wind seas and swell, the values of 10 and 75may be used.
　This spectrum is applied to the computations of refraction and diffraction of irregular waves. Diagrams of refraction coeficient, Kr, and predominant wave direction, αp, are provided for the coast with parallel, straight depth contours by means of linear superposition of the spectral components. For the seashore with complicated bottom configulations, a solution by the equation of wave energy flux is used. Diffraction diagrams behind semi-infinite breakwaters and through breakwater gaps are also presentd for irregular waves with Smax=10 or 75.