Tracking of Small Discrete Objects Submerged in Surf and Swash Zones on Sand Beaches

An experiment was conducted in a wave flume to measure the trajectories of 20 small objects (gravel and microplastics) in the surf and swash zones on an equilibrium beach and a nourished foreshore beach with erosion and accretion in the swash zone. The trajectory of each of the 20 objects was measured in six tests consisting of 100 runs, with each run lasting 400 s. Gravel particles were mobile within the swash zone only. Large and small microplastics moved onshore from the surf zone and accumulated in the lower swash zone of wave rundown or in the upper swash or berm zone of wave runup. Plastic particles remained on the evolving sand surface of the nourished foreshore. A simple tracking model was developed to predict the measured trajectories of the 20 particles of gravel and microplastics. The calibrated model also predicted limited net displacements of hypothetical sand particles on the equilibrium profile beach. The tracking model may eventually be generalized and become applicable to track and clean up microplastics on beaches and predict the destinations of sand particles placed on eroding beaches.