Nearshore Canyon Experiment
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Vertical Structure
Surfzone turbulence injected into the water column by breaking waves and generated by flows over the seafloor result in complex vertical structure of cross- and alongshore velocity, bubbles, and suspended sediment. Measurements to investigate the modulation of eddy viscosity in the water column by oscillatory flow over the rippled bed, the mean current boundary layer, and episodic strong turbulence events injected from the surface during wave breaking will be made with a vertical array of current meters and conductivity sensors, altimeters, wave staffs, pressure gages, and other sensors (FIGURES 1 and 2). The vertical array will be deployed within a cross-shore transect of wave and velocity sensors (FIGURE 3). Example time series are shown in Figure 4. Data acquired with the vertical and cross-shore arrays also will be used to calibrate models for cross-shore wave transformation and the corresponding set-up and set-down of the mean sea surface.

FIGURE 1

FIGURE 1: Schematic of the Vertical Array Sensor System used to investigate the vertical structure of velocity, bubbles, and suspended sediment in the surf zone.

FIGURE 2





FIGURE 2: A photograph of the Vertical Array Sensor System showing stacks of two-component, electromagnetic current sensors and conductivity cells to measure velocities and void fraction (air bubble content) from the bed to the surface. Local bottom morphology is mapped over a 2 m square by a raster-scanning altimeter, and out to 10 m radius by a rotating side scan sensor. All sensors are sampled at 20 Hz and the data streams are sent to shore-based computers through a fiber optic link. The sensors sample the entire surf zone at a fixed location as the 2 m tide rises and falls (see Figure 3)

FIGURE 3: Cross-shore array of pressure gages and current meters (puv's) to place the fixed vertical array measurements (Figures 1 and 2) in temporal and spatial perspective. Additionally, 5 puv's in an alongshore array (not shown) will measure infragavity waves and shear instabilities of the longshore current.

FIGURE 4: Sea-surface elevation versus time (upper panel) and vertical velocity (middle panel) and sediment concentration (bottom panel) as a function of depth and time. Sediment concentration and 3-components of velocity were acquired at 20 Hz in 1-cm (vertical) bins over the lower 60 cm of the water column with a Bistatic Coherent Doppler Velocity and Sediment Profiler (BCDVSP).

FIGURE 3

FIGURE 4
 
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