Statistical distribution of swash maxima on natural beaches

Graduation date: 1992Beach response to overwash processes is a topic of significant importance. Two\ud particular aspects of this topic were chosen for detailed analysis: the distribution of\ud maximum wave runup elevations and the cross-beach celerity gradient of overwash bores\ud on natural beaches. Data were collected using both traditional nearshore instrumentation\ud and recently developed video-based techniques.\ud Field data from three separate experiments suggest that swash elevations may be\ud regarded as a stochastic process whose maxima have a specific probability distribution\ud function. The exact form of the maxima distribution depends solely on the relative\ud bandwidth of the swash power spectrum and the root-mean-square value of the swash\ud time series. Numerical simulations indicate that the linear assumptions required by the\ud distribution model are commonly violated. Nevertheless, the qualitative trends suggested\ud by the model are applicable to the probability statistics for all of the data.\ud Overwash celerity data were collected at a site on the Isles Dernieres, LA barrier island\ud chain during Hurricane Gilbert in September 1988. A video technique was applied that\ud allowed the quantification of overwash bore celerity vectors along several cross-shore\ud transects. Maximum celerities were found to exceed 2 m/sec. The cross-beach velocity\ud structure can be characterized generally as having a maximum celerity at the berm crest\ud with a linear decrease in velocity across the washover flat. Using the celerity results, a\ud simple model of cross-beach overwash sediment transport is discussed.\ud Results from both investigations demonstrate that important attributes of runup and\ud overwash processes can be sufficiently sampled using video techniques. More work is\ud needed in terms of understanding the influence of overwash processes, specifically in the\ud areas of runup trajectory and celerity characteristics, the interaction between fluid flow and\ud bed permeability, and the regional scale forcing of sea level elevation