Abstract
Grain size on the surface of natural beaches has been observed to vary spatially and temporally with morphology and wave energy. The stratigraphy of the beach at Duck, North Carolina, USA was examined using 36 vibracores (~1–1.5 m long) collected along a cross-shore beach profile. Cores show that beach sediments are finer (~0.3 mm) and more uniform high up on the beach. Lower on the beach, with more swash and wave action, the sand is reworked, segregated by size, and deposited in layers and patches. At the deepest measurement sites in the swash (~−1.4 to −1.6 m NAVD88), which are constantly being reworked by the energetic shore break, there is a thick layer (60–80 cm) of very coarse sediment (~2 mm). Examination of two large trenches showed that continuous layers of coarse and fine sands comprise beach stratigraphy. Thicker coarse layers in the trenches (above mean sea level) are likely owing to storm erosion and storm surge elevating the shore break and swash, which act to sort the sediment. Those layers are buried as water level retreats, accretion occurs and the beach recovers from the storm. Thinner coarse layers likely represent similar processes acting on smaller temporal scales.
Highlights
Grain size varies on a natural beach and observations of different sand sizes and their spatial and temporal distributions are well documented (e.g., [1,2,3,4,5,6,7,8,9,10,11])
When the core is removed from the ground, water can flow out of the barrel, potentially disturbing the sediment layers within the barrel
It is important to know the range of grain sizes on a beach for sediment transport and morphological modeling, in part so that each fraction can be moved around accurately (e.g., [11,20])
Summary
Grain size varies on a natural beach and observations of different sand sizes and their spatial and temporal distributions are well documented (e.g., [1,2,3,4,5,6,7,8,9,10,11]). Coarse sediment was found around rip channels, where flows were strong, and the finest sediment was found high on the intertidal beach where wind was the most common transporting mechanism. These spatial patterns were observed to change with changing morphology. To examine grain size variations at a higher temporal resolution, an experiment was carried out along two cross-shore profiles in Monterey, CA [11] where the beach was relatively steep (slope 1:7.5) and had a vigorous shore break (where surf zone waves crash dramatically and driving the swash run-up, and where there is often a morphological step). Most experiments acknowledge that stratigraphy sampling should have been deeper [5,9], or more time or higher frequency measurements would have been useful [8], or they didn’t have direct hydrodynamic measurements [12]
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