Internal waves are oscillations that propagate along density interfaces within the ocean, commonly at the pycnocline between warmer surface water and colder deep water. These waves can be generated by tidal flow over topography or by wind-driven displacements of stratified layers. When internal waves shoal, steepen, or break near the continental shelf, they induce strong horizontal velocities, vertical displacements of isopycnals, and enhanced turbulence that directly affect seabed processes.
Mechanisms of sediment mobilization
The most direct influence of internal waves on sediment transport is through resuspension. As internal waves propagate onto the shelf their orbital motions near the bottom increase bottom shear stresses. Research by Michael H. Alford at Woods Hole Oceanographic Institution documents energetic internal tides that shoal on continental shelves and elevate near-bottom velocities, producing shear sufficient to lift fine and medium sands into suspension. Laboratory and theoretical work by Christopher Garrett at University of Victoria described how nonlinear steepening produces internal bores and solitary waves that create intense, short-lived bursts of flow capable of mobilizing sediment into the water column. Those turbulent bursts mix sediments upward and can form nepheloid layers that are advected coastward or alongshore.
A related process is the formation of density-driven boluses and return flows. Internal wave breaking can generate vertically coherent pulses of lighter or denser water that trap suspended sediments and transport them as a package. José Pineda at University of California Santa Cruz provided field evidence that internal bores can carry plankton and particulate material shoreward in pulses, enhancing cross-shore transport beyond what steady currents alone would achieve. Over repeated tidal cycles, these episodic transports accumulate, altering sediment budgets.
Consequences for coastal morphology and ecosystems
The cumulative effect of repeated internal-wave-driven resuspension and transport influences coastal morphodynamics. Persistent internal tidal activity can shift sand on inner shelves, modify the shape of sand bars, and influence rates of erosion and accretion along particular segments of coastline where seabed topography amplifies internal waves. For coastal ecosystems the consequences are twofold. Suspended sediment can reduce light penetration, stressing benthic photosynthetic communities, while simultaneous nutrient fluxes associated with internal mixing can boost productivity in the photic zone. This coupling of turbidity and nutrient delivery has been documented in observational studies by investigators at Scripps Institution of Oceanography that link internal tide-driven mixing to episodic nutrient injections on shelves.
Human and territorial considerations are significant. Fisheries and shellfish aquaculture sited near shelf breaks or in narrow channels may experience variable sedimentation and turbidity tied to internal wave activity, affecting gear, harvests, and habitat restoration efforts. Seasonal changes in stratification and anthropogenic alterations of coastal bathymetry, such as dredging, can modulate internal wave impacts, complicating management.
Understanding and predicting these effects requires integrated observations and models that resolve stratification, bathymetry, and nonlinear wave dynamics. Advances by investigators at institutions like Woods Hole Oceanographic Institution and Scripps Institution of Oceanography have improved process-level understanding, but local assessments remain essential because internal-wave impacts depend sensitively on regional topography, stratification, and human uses.