Dynamics of shoaling internal waves in the near-shore [electronic resource] : Mamala Bay, Hawaii
Keywords: Civil and Environmental Engineering.

Shoaling internal waves are common in many inner-shelf regions. Their arrival is characterized by dense pulses of water propagating into shallower coastal zones. These nonlinear internal waves have been investigated as a source of enhanced diapycnal mixing. Mamala Bay, on the south shore of the Hawaiian island Oahu, experiences consistent internal wave action. As the waves in this bay approach shore, they form into highly nonlinear boluses, or "bores, " which are observed up to the 12 m isobath. Two month long field experiments in the spring of 2010 and 2011 observed numerous shoaling bore arrivals at the 23 m site of the University of Hawaii's Kilo Nalu Near-shore Observatory. This project was designed to monitor the regional flows and the vertical structure associated with these internal wave events. It also measured fine-scale turbulence including direct measurements density fluxes using fast sampling acoustic Doppler velocimeters (ADVs), conductivity, and temperature (FastCT) probes. Measurements of turbulent velocities in coastal areas are subject to bias from surface wave induced flows. These must be removed to have an accurate estimate of the Reynolds shear stresses and turbulent kinetic energy (TKE) dissipation. Several different methods currently exist to identify and remove this wave bias. A novel method was created using the wave-tracking mode of a Nortek Acoustic Wave and Current Profiler (AWAC). A comparison of these methods shows that the most consistent, effective removal comes the Feddersen and Williams (2007) and Bricker and Monismith (2007) methods. However, the novel AWAC removal method is effective under certain wave conditions. The effectiveness of removal is assessed using Ogive curves, and comparison to a theoretical curve for turbulence in the absence of waves (Kaimal et al. 1972). The tower designed for this experiment tested assumptions about the optimum spacing of velocimeters for use with differencing methods. Wave removal methods lose effectiveness when the ratio of wave orbital velocity to mean velocity is above 0.8. Shoaling internal waves in Mamala Bay were identified by sudden drops in temperature and increases in salinity followed by high frequency fluctuations of both. The shape and composition of these high density pulses is characteristic of boluses of deeper offshore water which supports the hypothesis that internal tides are a mechanism for cross shore transport of deep water onto fringing reefs. These bores tended to arrive from the southeast, increased Reynolds shear stresses, and skewed bottom roughness estimates. Rapid fluctuations in density in trailing edges of bores are thought to be a place of enhanced mixing. The tower of ADVs directly measured the buoyancy flux, TKE production and dissipation at four different elevations in the bottom 8 meters of the water column. The arrival of bore events brought levels of stratification strong enough to control the fine scale turbulence. Furthermore, these measurements show the trailing edge of these bores was a location of increased vertical mixing, turbulent kinetic energy production, and dissipation. The efficiency of this mixing is similar to existing laboratory and computational models for mixing in a stratified fluid. It also matches with observations at other field sites.