Multiple Scales of Beach Morphodynamic Processes: Measurements and Modelling

Multiple scales of beach morphodynamic processes ranging from those of wave-breaking induced turbulence, individual wave, storm, seasonal, to inter-annual are examined in this dissertation based on both laboratory and field data. These processes were simulated using process-based numerical models and data-driven models. At a microscale, separating turbulence from orbital motion under breaking waves in the surf zone is essential to understanding wave-energy dissipation. Velocity data under monochromatic and random waves in the large-scale sediment transport facility (LSTF) were analyzed. Moving averaging provides a simple method for extracting turbulence from velocity measurements under random breaking waves collected at a reasonably high frequency. Various moving averaging time intervals were examined. An optimum moving averaging interval of approximately 30° to 42° phase angle (relative to peak wave period) allows a reasonable extraction of turbulence. An adaptive moving averaging with variable averaging time at wave crest and trough are proposed to improve the effect of turbulence extraction. At a mesoscale, hydrodynamic conditions associated with onshore migration of a sandbar and the subsequent equilibrium state of a stable bar were examined in the LSTF. Wave and near bottom velocity across the surf zone were measured during the onshore sandbar migration. The near-bottom velocity skewness indicates that before the sandbar reached equilibrium, the velocity was skewed offshore in the nearshore region, and skewed onshore seaward of the bar. The velocity skewness pattern reversed when the beach profile reached equilibrium and the sandbar became stable. The peak onshore directed acceleration was greater than the peak offshore directed acceleration throughout the surf zone during the periods of both onshore migrating and stable sandbar. The macroscale portion of the study examines the beach processes, particularly the morphodynamics of nearshore bar, at storm and seasonal scales. The bar height and bar position were extracted from bimonthly surveyed beach-profiles spaced at 300 m along the 22-km long Sand Key barrier island, West-Central Florida from October 2010 to August 2015. Seasonal beach cycle in the study area is illustrated by onshore sandbar migration during the summer and offshore sandbar migration during the winter, while subaerial beach remains rather stable. Alongshore variations of onshore and offshore sandbar migration were observed over storm events. The water depth over the pre-storm sandbar crest, or the bar crest elevation, is a major factor controlling the onshore or offshore sandbar movement. The offshore moving sandbar tends to have a shallower pre-storm bar crest, while the onshore moving sandbar tends to have a deeper pre-storm bar crest. A dynamic equilibrium bar height of 0.5 m for the study area was identified. The sandbar tends to evolve toward this equilibrium height during the seasonal cycle. The energetic conditions associated with Tropical Storm Debby caused a deviation from the above dynamic equilibrium conditions. The sandbar at most of the profile locations became higher than the pre-storm bar height regardless of the initial height of being greater or less than 0.5 m. After the storm, the higher and shallower bar experienced substantial erosion, the eroded sand was deposited in the trough landward. This resulted in a lower sandbar height, returning to the dynamic equilibrium height of 0.5 m. The Unibest-TC model (Walstra et al., 2012) is able to capture the measured trend of bar migration. The Modelling results suggest that offshore bar migration is dominated by suspended sediment transport. While onshore bar migration is driven mainly by bedload transport. At megascale, a data-driven model was developed to predict beach-profile evolution at multiple-annual scale. Empirical Orthogonal Function analysis was conducted on a time-series beach profile (R61) to identify temporal and spatial trends. Trends in the temporal EOF are modeled using a simple curve fitting. In this case, logarithmic and linear trends were identified. After the trend in temporal EOF values are identified, the curve fitting can be calibrated with 14-month data. The calibrated temporal EOF curve yielded accurate reproduction of profiles. The close examination of multiple scales of beach processes provides a comprehensive understanding of nearshore morphodynamics