Fraction of broken waves in natural surf zones

This paper presents a novel quantification of the fraction of broken waves (Q_b) in natural surf zones using data from seven microtidal, wave‐dominated, sandy Australian beaches. Q_b is a critical, but rarely quantified, parameter for parametric surf zone energy dissipation models, which are commonly used as coastal management tools. Here, Q_b is quantified using a combination of remote sensing and in situ data. These data and machine learning techniques enable quantification of Q_b for a substantial data set (>330,000 waves). The results show that Q_b 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 Q_b 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 Q_b at the surf‐swash boundary. Q_b also correlates well with the Australian beach morphodynamic model: For more dissipative beaches Q_b increases rapidly in the outer surf zone, whereas for more reflective beaches Q_b increases slowly throughout the surf zone. Finally, when comparing data to existing models, three commonly used theoretical formulations for Q_b are observed to be poor predictors with errors of the order of 40%. Existing theoretical Q_b 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.