Characterizing small scale turbulence and wave-current-ripple interactions in the coastal bottom boundary layer
The coastal ocean bottom boundary layer (BBL) constitutes a relatively small portion of the water column. However, being a zone of increased mass and momentum transfer, it plays an important role in ocean dynamics, influencing various processes such as sediment transport, shelf circulation, tidal energy balances, and nutrient mixing. Hence, a thorough understanding of the mechanisms that drive the BBL as well as small-scale processes that occur within is essential.
Submersible particle image velocimetry system
In its simplest form, implementing 2-D particle image velocimetry (PIV) in the laboratory involves these following steps: (a) illuminating the flow field of interest with a thin laser sheet; (b) seeding the flow with particle tracers; (c) recording the particle traces on a camera over successive exposures with a known time delay; and (d) using a correlation algorithm to then generate velocity distributions over the entire field of view. In the ocean however, the aquatic particles that are already present, act as natural flow tracers. The submersible PIV system used a 595 nm wavelength, flashlamp pumped dye laser as the illumination source. The package also carried a pencil beam sonar for high-resolution bottom topographical mapping, which helped elucidate the bottom ripple interactions with the wave/current flow. Field measurements were carried out near the LEO-15 site in the Atlantic Ocean, off the New Jersey coast. A substantial database under different wave, mean flow and bottom topographical conditions was created as part of this study. Significant results included: (a) Mean velocity profiles over the ripple were in agreement with laboratory rough wall boundary layer studies; (b) wave-induced stresses were significant near the seabed (gradients of wave and Reynolds stresses were similar in magnitude), highlighting the importance of waves in nearbed momentum transport; (c) Measured mixing length and eddy viscosity profiles agree well with the classical zeroth order model predicting linear increase with elevation. These results could potentially help coastal engineers and scientists better validate their models applied in various nearshore sediment transport/coastal circulation studies.
Reference: Nayak, A. R., C. Li, B. T. Kiani, and J. Katz (2015), On the wave and current interaction with a rippled seabed in the coastal ocean bottom boundary layer, J. Geophys. Res. Oceans, 120,4595–4624, doi:10.1002/2014JC010606.