Dynamics of Sand Dunes beneath the World s Largest Internal Solitary Waves, South China Sea
Abstract
The goal is to understand the processes that form large sand dunes, with wavelengths of some 300 m and heights up to 16 m, which were discovered on the upper slope in the South China Sea (SCS), the site of the world#s largest internal solitary waves (ISWs). Similar dunes have been observed elsewhere in the SCS and in other ocean basins. Dune formation has been attributed in some cases to ISWs and in others to internal tides, with a potential role of the seafloor slope relative to the critical slope for internal tidal reflection. Dune formation might be facilitated by vortex shedding and resulting intense turbulence in bottom boundary layers (BBLs) beneath ISWs, which has been proposed theoretically and observed in laboratory experiments and direct numerical simulations (DNS). A dynamical mechanism by which ISWs and internal tides might form dunes has not been proposed, and a quantitative framework for predicting their occurrence and scales does not exist for the SCS or elsewhere. The hypotheses are that (1) inflected velocity profilesin BBLs beneath ISWS are unstable to vortex shedding and produce intense turbulence, which enhances rates of energy dissipation andsediment transport an order of magnitude above values in boundary layers with uninflected structures; (2) ISWs form dunes via flow-seabed interaction, in which spatial perturbations in seafloor elevation produce perturbations in fluid velocity and sediment flux, which lead, under favorable conditions, to sediment erosion on dune troughs and deposition on crests, resulting in dune growth; and (3) internal tides either form dunes or participate in their formation through flow-seabed interaction, particularly where the seafloor slope is near the critical value for internal tidal reflection, increasing near-bottom velocities produced by internal tides. Hypothesis (1) has been confirmed in DNS and laboratory experiments but has not been tested against quantitative fluid dynamical measurements in the ocean. Hypotheses (2) and (3) formalize suggestions that have been advanced in the literature but have not been tested quantitatively.The hypotheses will be tested by analysis of existing measurements in the BBL beneath ISWs, which were collected in the SCS in 2017 and 2019, and by mathematical and numerical analyses of the flow-seabed mechanism for dune formation by ISWs and internal tides, including the potential role of the critical seafloor slope. The anticipated outcomes of the research, if successful, are (1) a quantitative understanding of the dynamics of the BBL forced by ISWs, and (2) a quantitative ability to predict the occurrence and scales of dunes formed by ISWs, potentially in concert with other process including internal tides, in the SCS and elsewhere. The results will be documented in publications in scientific journals.Impacts on Department of Defense capabilities derive from impacts of sand dunes and BBL turbulence on acoustics and physical oceanography. Sand dunes in the SCS have been shown to refract sound and couple acoustic modes, thus impacting acoustic propagation, acoustic communication, and sonar system performance. ISWs in the SCS lose a large fraction of their energy while propagating over dunes, possibly because of intense BBL turbulence. Large sand dunes present resistance to the overlying flow, impacting regional-scale physical oceanography.Approved for Public Release
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 15, 2023
- Source ID
- N000142312472
Entities
People
- John Trowbridge
Organizations
- Office of Naval Research
- United States Navy