Surface wave breaking over sheared currents: Observations from the Mouth of the Columbia River

Abstract

Measurements of waves and currents from freely drifting buoys are used to evaluate wave breaking parameterizations at the Mouth of the Columbia River, where breaking occurs in intermediate depths and in the presence of vertically sheared currents. Breaking waves are identified using images collected with cameras onboard the buoys, and the breaking activity is well-correlated with wave steepness. Vertical shear in the currents produces a frequency-dependent effective current that modifies the linear dispersion relation. Accounting for these sheared currents in the wavenumber spectrum is essential in calculating the correct wave steepness; without this, wave steepness can be over (under) estimated on opposing (following) currents by up to 20%. The observed bulk steepness values suggest a limiting value of 0.4. The observed fraction of breaking waves is in good agreement with several existing models, each based on wave steepness. Further, a semispectral model designed for all depth regimes also compares favorably with measured breaking fractions. In this model, the majority of wave breaking is predicted to occur in the higher frequency bands (i.e., short waves). There is a residual dependence on directional spreading, in which wave breaking decreases with increasing directional spread.