The Grounding of Floating Objects in a Marginal Sea

Beaches, especially at or above the high tide line, are often covered in debris. An obvious approach to understanding the source of this debris elsewhere in the ocean is to use Lagrangian methods (observationally or in numerical simulations). However, the actual grounding of these floating objects, that is, the transition between freely floating near the coast and motionless on land, is poorly understood. Here, 800 groundings from a recent circulation project using expendable tracked drifters in the Salish Sea are statistically analyzed. Although the grounding process for individual drifters can be complex and highly variable, suitable analyses show that the complications of coastlines can be statistically summarized in meaningful ways. The velocity structure approaching the coastline suggests a quasi-steady "log-layer" associated with coastline friction. Although groundings are marginally more likely to occur at higher tides, there are many counterexamples and the preference is not overwhelming. The actual grounding process is then well modeled as a stationary process using a classical eddy-diffusivity formulation, and the eddy diffusivity that best matches observations is similar to that appearing in open waters away from the coast. A new parameter in this formulation is equivalent to a mean shoreward velocity for floating objects, which could vary with beach morphology and also (in theory) be measured offshore. Finally, it appears that currently used ad hoc beaching parameterizations should be reasonably successful in qualitative terms, but are unlikely to be quantitatively accurate enough for predictions of grounding mass budgets and fluxes.