A subsea tower of six Nortek Vector velocimeters monitors turbulent mixing and internal tides on south shore Oahu in real-time
On the fringing coral reef of Mamala Bay, Hawaii, researchers from Stanford University, University of Hawaii, and Oregon State University are investigating turbulent mixing associated with shoaling internal tides. The project installed an eight meter high tower into the University of Hawaii's Kilo Nalu near-shore reef observatory. The tower was equipped with six cabled Nortek Vectors for high frequency measurements of velocity and density within and above the bottom boundary layer.
Mamala Bay, the dominant bay on the south shore of Oahu, has prevalent internal tides. Shoaling internal tides are created when offshore internal tides become non-linear and "break" as they enter the near-shore environment. It is this mechanism that makes them an important source of deep water nutrients for many of the world's coastal ecosystems, particularly coral reefs. These internal tides have been shown to strongly affect the near-bottom density stratification, and with it, the hydrodynamics. The ability to describe the hydrodynamics of mixing in these regions is critical for understanding their impact on reef ecosystems. This type of mixing controls the transport of many passive scalars such as nutrients, spores or larvae, and oxygen. While shoaling internal tides are widely common feature, their behavior in the near-shore is not well understood.
Tower of Vector's in Hawaii The tower of Vectors gave direct observations of the turbulent density fluxes, Reynolds stresses, and dissipation rates during internal tide events. These measurements will help answer key questions about the how shoaling internal tides affect vertical mixing and transport above the reef. They will be valuable for comparison with current models of the mixing efficiency of stratified turbulence. The tower's fast sampling measurements are also well suited for investigating the interrelationships between surface waves, internal tides, and turbulence.
Cabled into the observatory, the Vectors streamed their data to shore in real-time for two month long experiments in the spring of 2010 and 2011. Located at a site 23 meters deep, the Vectors were positioned at 0.1, 1, 2, 4, 6, and 8 meters above the seabed. This allowed the Vectors to sample both within and above the bottom velocity and mixed density layers. The Vector control volumes were spaced vertically from one another to aid with surface wave removal techniques. Precision Measurement Engineering Inc. (PME) fast-sampling salinity and temperature ("FastCT") sensors were integrated into the top four Vectors. This setup allowed for both the synchronization of the two instruments as well as for the FastCT measurements to be sent back with the Vector data stream. The Vector-FastCT setup sampled continuously at 64 Hz. The FastCT's temperature and conductivity probes were positioned next to the control volume of the Vector. "ChiNode" fast-sampling temperature sensors, developed at Oregon State University, were also cabled into the observatory and gave 100 Hz temperature sampling both at and between the Vector sample volumes.
In addition to the tower, two autonomous moorings were deployed in the alongshore and across-shore directions. These moorings consisted of a bottom mounted ADCP and an 18 m long T-chain. These monitored the larger spatial variability as well as allowing for the calculation of directionality of the shoaling internal tides. Knowing the direction of travel of these internal tides may act as a starting point for understanding the source locations for this internal tidal energy.
Report provided by Michael Squibb, Stanford University