Determination of Wave Angle in Shallow Water

Abstract

Information from a bidirectional electromagnetic current meter and surface‐piercing, continuous resistance wave staff is shown to provide good estimates of the angle of wave approach within the surf zone. Spectra of surface elevation fluctuations (η) define limits to the primary frequencies in the incident wave field, which are then used to define the wave‐driven components in the onshore (x) and alongshore (y) velocity spectra. Cross spectra between η and x and y give squared coherence values used to weight the wave contribution to the motion in any frequency band. A weighted orbital vector for each frequency band is calculated: the angles corresponding to each band are then further weighted according to the proportional contribution of the variance in the velocity in that frequency band to the total variance in the onshore and offshore velocities. Summation of the individual angle contributions gives a wave angle representative of the energy distribution in the peak of the incident wave spectrum. A test of the technique is made by comparing wind and wave angles, and wave angles calculated for adjacent instrument pairs.