Asymmetric Wave Shape Parameterization Estimates From Fire Island Observations

The transformation from generally linear deep-water waves to non-linear shallow water waves causes near-bed orbital velocities to become asymmetrical and contributes to net sediment transport. This non-linearity manifests itself in a variety of cross-shore sediment transport processes: ripple migration, growth and migration of shoals on ebb-tidal deltas, and onshore sandbar migration leading to beach accretion. These processes occur under mild to strong wave conditions and indicate the importance of accurately representing the non-linear wave shapes in the near-bed environment to estimate sediment transport. In this work, we estimate the non-linear wave shapes from observational data collected from an oceanographic field study in 15 m water depth approximately 2 km offshore from the southern shoreline of western Fire Island, New York during February to May 2014. Near-bottom pressure and single-point velocity sensors, along with upward-looking profiling velocity sensors, were used to measure wave conditions. Burst-averaged wave shapes were estimated directly from near-bottom measurements, and were found to agree well with non-linear wave shapes constructed from bulk surface-wave statistics using parameterizations described in the literature. We then used the burst-average wave shapes to estimate bedload sediment transport using a formula that calculates transport separately for crest and trough phases, and compared the results with estimates using the traditional excess-stress formulations forced with the time series of near-bottom currents. The long-term objective of this work is the development and validation of an operational morphodynamic model that can estimate wave asymmetry from bulk surface-wave statistics produced by phase-averaged wave models, and incorporate the wave shape in calculations of sediment fluxes.