Measuring and Modeling Temporal Changes in Beach and Swash Stratigraphy
Heidi M Wadman1, Edith L Gallagher2, Jesse McNinch3, Ad Reniers4 and Melike Köktąs4, (1)US Army Corps of Engineers, Field Research Facility, Kitty Hawk, NC, United States, (2)Franklin & Marshall College, Lancaster, PA, United States, (3)US Army Engineer Research & Development Center, Coastal and Hydraulics Laboratory, Duck, NC, United States, (4)Delft University of Technology, Hydrolic Engineering, Delft, Netherlands
Abstract:
Although many beaches appear to consist of uniform sands, upon closer inspection, over an order of magnitude of grain size range can exist on a beach. Recent studies have shown that not only does grain size vary on the surface of beaches, it is also correlated with morphology. In addition, grain size patterns change both at the surface and with depth as the morphology evolves. A comprehensive computer model (Xbeach) has been used with a multi grain size module to predict transport of different grain sizes, their sorting and separation, and to determine the importance of grain size variation on the beach, particularly in the swash zone.
Grain size is both observed and modeled to be greatest in the high-energy swash zone. In this shallow area, breaking waves effectively suspend and transport finer sediments, moving them on- and offshore. This leaves coarser material in the swash, which can act as armor, reducing erosion, altering the morphology, and potentially changing the hydrodynamics. During tidal- and storm-induced sea level fluctuations, the swash (as a sorting engine) moves up and down the beach profile, reworking the sediments and potentially creating stratigraphic layers of sorted coarse material.
In March and April of 2014, an experiment was performed at the US Army Corps of Engineers Field Research Facility. Over a week, 36 stratigraphic cores were collected along a cross-shore profile from the dune to the lowest extent of the swash. Grain size stratigraphy in the beach and its variations with location and in time are being examined. These data, together with observations from two large trenches, and another coring and remote sensing experiment set for early Oct 2015, are being used to examine layering in the beach owing to storms, rising and falling tides, and wave-by-wave transport mechanisms. These data will be used to drive model predictions to assess the importance of spatial and temporal grain size variation, differential grain transport, and layering on the morphodynamics of beaches.
