Driftwood Transport and Fate in Coastal Waters
Creators: name: Murphy, Enda
Publisher: name: Université d'Ottawa | University of Ottawa
Year: 2024
Subjects: Driftwood
Woody debris
Physical modelling
Numerical modelling
Nature-based solutions
Dispersion

description: Driftwood, which consists of fragments of wood transported by water, is abundant in many of the world’s coastal zones. The origins of coastal driftwood include inland and coastal forests, and industry (e.g., forestry and construction). Wood is thought to provide a number of ecosystem services in coastal zones and estuaries, including habitat, structure, nutrients, and carbon storage. These perceived benefits have led to the increasing utilization of wood in so-called nature-based shore protection and coastal restoration schemes. However, the evidence to support this practice is limited. Large quantities and accumulations of driftwood are known to be detrimental to the health and performance of some sensitive coastal ecosystems and nature-based features, such as newly establishing or restored salt marshes. Moreover, buoyant coastal driftwood (or “woody debris” when referring to negative impacts) mobilized by natural disturbances, such as tsunami or storm waves, poses hazards to communities, the environment, infrastructure, and other valued assets; and can incur substantial cleanup or mitigation costs. While the fate and transport of wood in fluvial systems or debris mobilized by tsunami has received substantial attention in the literature, comparatively few studies have investigated coastal driftwood transport and mobility in open-coast settings, where wind waves are an important driver. A thorough understanding of coastal driftwood fate and transport mechanisms and pathways is needed to inform risk management practice, and to weigh the risks and benefits of using wood in nature-based solutions, and implications for coastal structures. This study characterized physical processes governing the transport and fate of coastal driftwood. Scale physical model experiments and optical tracking of model driftwood were conducted to quantify wave-driven transport and dispersion on a sandy shoreline with fringing reefs and various coastal structures. A numerical, Lagrangian particle-tracking model was developed to simulate driftwood transport in wave-dominated environments, and was validated using the experimental data. The physical and numerical modelling results provided unique insights to factors affecting coastal driftwood mobility, pertaining to driftwood characteristics (e.g., length, roughness, buoyancy), hydrodynamics (e.g., sea state parameters, wave-induced circulation, surf zone and swash zone dynamics), and coastal structures (e.g., length of groynes at the water line). The results highlighted the sensitivity of driftwood dynamics to surf zone and swash zone hydrodynamics, and the capabilities and limitations of two “state-of-the-art” nonlinear shallow water equations wave models (the reduced 2-layer XBeach non-hydrostatic model, and SWASH) as tools for simulating these processes were assessed. The numerical driftwood transport model (WOODRIFTSIM) incorporated a novel, physics-based beaching and washoff algorithm, which was used to assess the suitability of probabilistic formulae incorporated in several existing marine debris models, which typically assume an exponential decay in the probability of driftwood remobilization following a beaching event. The findings from the study were synthesized to describe practical implications for the use of coastal driftwood in nature-based shore protection with respect to service life, quantity, stability, and effects on wave run-up and morphodynamics. Considerations for coastal structure design in regions with mobile driftwood were also discussed. Limitations of the work, and needs for further research to develop a more comprehensive understanding of coastal driftwood fate and transport were identified.