Development and Experimental Validation of a Lagrangian Particle-Tracking Model to Simulate Wave-Driven Transport of Coastal Driftwood

Abstract

Wood deposits on beaches can contribute to coastal ecosystem function. However, driftwood and debris mobilized by storm waves pose hazards to coastal communities, infrastructure, and valued assets, including sensitive ecosystems. Predictive numerical tools are needed to guide risk assessment and management and opportunities to leverage the benefits of coastal driftwood. Models developed to predict driftwood fate and transport in rivers or by tsunami do not incorporate the effects of wind waves, surf-zone processes, or the wave–driftwood–shore interactions that exert important controls on coastal driftwood dynamics. Lagrangian transport models, developed to simulate oil spill and marine debris transport at oceanic scales, typically do not resolve beaching and washoff processes despite sensitivity to these mechanisms. Here, a novel Lagrangian model for simulating coastal driftwood transport by waves, which includes an efficient, dynamics-based beaching and washoff algorithm, was developed and compared to observations from a previous experimental study by the authors. Hydrodynamic forcing by two phase-resolving, nonlinear shallow water equation solvers (XBeach and SWASH) reveals that driftwood dynamics are sensitive to the vertical resolution of wave-induced velocities in the surf zone and swash zone velocity residuals. SWASH performed better than XBeach in driving the onshore-directed transport of driftwood observed in the experiments. For simulations where beaching occurred, the driftwood model (WOODRIFTSIM) reasonably reproduced mean transport and dispersion rates from the experiments, with some tuning of driftwood-beach interfacial friction coefficients. The sensitivity to friction coefficients confirmed that driftwood roughness is an important factor controlling mobility in wave-dominated settings. Residence time distributions of beached driftwood generally fit well to existing stochastic washoff models, except when extreme wave runup event interactions with beach morphologies resulted in fat tails in the washoff probability distributions. The driftwood model provides insight into factors affecting beaching and parameterization of probabilistic washoff algorithms for simulating buoyant debris transport on wave-dominated coasts. Driftwood on beaches and shorelines can provide environmental benefits. As a result, there is interest in trying to incorporate wood in sustainable (nature-based) shore protection works. However, large amounts of driftwood can be moved along the coast by storms, tides, and waves, causing damage to infrastructure and sensitive natural features, such as salt marshes. The costs of managing driftwood impacts on the coast are high. To understand risks, coastal zone managers and designers of infrastructure need to be able to predict where driftwood will go when mobilized by storms. This paper presents a new computer model developed to predict driftwood movement along the coast by waves. The model simulates how driftwood becomes beached and remobilized by waves and is efficient enough to be applied at large scales. However, the study showed that the wave models used as input to the driftwood model are as important as the driftwood model itself and that it is important to accurately predict water velocities near the surface and at the shore where waves are breaking. The model may be a useful tool for practitioners interested in understanding driftwood or floating debris impacts and accumulation on the coast.