Radiation Stress Estimators

Abstract

The radiation stresses Sij associated with the propagation of wind-generated waves are principal driving forces for several important surf-zone processes. The accurate estimation of the onshore flux of longshore-directed mean momentum Syx, using a linear array of pressure sensors, is considered here. Three analysis methods are examined: integration of two high-resolution directional-spectrum estimators [maximum likelihood (MLM) and a modified version (IMLM)], and a direct estimator of the Syx directional moment (DMMv) which is developed here. The Syx estimation methods are compared using numerical simulations and field data from two experiments at Torrey Pines Beach, California. In the first field experiment, IMLM and DMM, estimates of Syx (from a 3-element, 99 m long linear array) showed excellent agreement with a slope array (Higgins et al., 1981) in the frequency range 0.05–0.15 Hz. In the second experiment, IMLM and DMM, estimates of Syx (from a 5-element, 360 m long array) agreed with values of Syx obtained from a nearby orthogonal-axis current meter for the frequency range 0.06–0.11 Hz. The integration of the MLM directional spectrum estimates yields biased (low) values of Syx. Although the DMM method is used here for the estimation of Syx, it can easily be adapted for the calculation of any arbitrary directional moment. While conventional methods are shown to be deficient in Syx estimation, they provide accurate estimates of Sxx, the onshore flux of onshore-directed momentum.