The Fraction of Broken Waves in Natural Surf Zones

Abstract

AbstractThis paper presents a novel quantification of the fraction of broken waves (Qb) in natural surf zones using data from seven microtidal, wave‐dominated, sandy Australian beaches. Qb is a critical, but rarely quantified, parameter for parametric surf zone energy dissipation models, which are commonly used as coastal management tools. Here, Qb is quantified using a combination of remote sensing and in situ data. These data and machine learning techniques enable quantification of Qb for a substantial data set (>330,000 waves). The results show that Qb is a highly variable parameter with a high degree of interbeach and intrabeach variability. Such variability could be correlated to environmental parameters: tidal variations correlated with changes in Qb of up to 70% for a given local water depth (h) on a low tide terrace beach, and increased infragravity relative to sea‐swell energy correlated to lower values of Qb at the surf‐swash boundary. Qb also correlates well with the Australian beach morphodynamic model: For more dissipative beaches Qb increases rapidly in the outer surf zone, whereas for more reflective beaches Qb increases slowly throughout the surf zone. Finally, when comparing data to existing models, three commonly used theoretical formulations for Qb are observed to be poor predictors with errors of the order of 40%. Existing theoretical Qb models are shown to improve (revised errors of the order of 10%) if the Rayleigh probability distribution that describes the wave height is in these models is replaced by the Weibull distribution.