Nearshore wave height variation in unsaturated surf

The nearshore evolution of wave height is presented from field observations during unsaturated surf conditions from 10 different beaches characterized by microtidal conditions and predominantly swell-dominated wave climates. Wave evolution is presented in terms of wave height to water depth ratio (gamma) for comparison with previous data from saturated surf. Both conventional time-averaged (gamma(rms)) and a new wave-by-wave analysis (gamma(w)) are performed. Values of gamma increase with increasing offshore wave height, indicating unsaturated surf. The observations show a variation in gamma values from near constant values in the mid surf zone to rapidly and asymptotically increasing gamma values in the inner surf zone. In contrast to previous data from saturated surf, gamma shows no dependence on either the absolute beach slope or the relative beach slope beta/k (h) over bar. The skewness of the distributions of gamma(w) is consistent with waves that are not depth limited. The inner surf zone wave heights are approximately equally dependent on the water depth and offshore wave height. The previous observations of gamma from saturated surf are shown to be consistent with a terminal bore height at the shoreline which is in excellent agreement with a previously derived value for the Miche parameter. In contrast, for the present unsaturated surf conditions, the terminal bore height at the shoreline can be approximated by H-b approximate to 0.12H(o), which is consistent with recent laboratory data sets.