Maximum Wave Run-up Measured on a Natural Beach Owing to Extreme Waves

Monday, 15 December 2014: 5:45 PM
Edward B Thornton and James H MacMahan, Naval Postgraduate School, Monterey, CA, United States
Unique field data indicative of maximum run-up owing to extreme wave conditions with a 50 year return period are obtained from the distribution of sea-glass on 10-18 m high dunes. The hypothesis that sea-glass is an indicator of maximum run-up is verified by the observations that new sea-glass on a beach is found at the rackline, the highest point of run-up. The source of the sea-glass is a garbage dump on the dune in southern Monterey Bay from 1937-1951. It is estimated that the dump, located on an erosive shoreline, was falling into the ocean by at least 1960, so that the maximum run-up values have a return period of at least 50 years. Various empirical run-up models based both on extensive laboratory and field measurements are assessed to include contributions from sea-swell and infragravity waves, setup and tidal elevation, which are parameterized on wave height and surf parameter, P, which is a function of wave height, period and beach slope. Deep water hindcast waves (1958-2011) refracted to 4m water depth are used as input to the models. Beach and dune slopes averaged over the run-up region from mean water level to the maximum run-up ranged 0.1 – 0.63 (angle of repose). Reasonable comparison with model predicted run-up with distribution of sea-glass on the dune were obtained for P <2 events, but were underpredicted for large P. Large P events are associated with long period swell waves characteristic of the Pacific Ocean that are outside the empirical parameter space from which the model equations were derived, suggesting a possible deficiency in the models.