Wave height and period data were collected on electrical capacitance wave
gauges, which were calibrated daily with a computer-controlled procedure
incorporating a least-square fit of measurements at 11 steps. This averaging
technique, using 21 voltage samples per gauge, minimizes the errors of slack in
the gear drives and hysteresis in the sensors. Typical calibration errors are less
than 1 percent of full scale for the capacitance wave gauges. Wave signal
generation and data acquisition were controlled by a DEC MicroVax I computer.
Velocimeters (ADV) with a side-looking probe that is oriented to measure x-y
horizontal velocity information in a horizontal plane. Samples were collected at
20 Hz, though the instrument makes 250 pings/sec and averages for each output
sample. Accuracy is 0.5 percent of the measured velocity, with resolution of
0.1 mm/sec and threshold of 0.1 cm/sec. The probe samples a 0.25-cu-cm
volume located 5 cm from the sensor heads. The velocity data were analyzed for
wave direction.
The sensor placement locations are discussed in the next section. A gauge
rack was designed to hold the wave sensors in a co-linear manner, with a 0.61-m
(2.0-ft) separation between gauges. The rack was then moved to other locations
for various test runs.
Experiment Design
Four idealized "structural" arrangements, constructed of 3/4-in. marine
plywood, were examined. The first was a shore-parallel breakwater (Structure
1), the second was a typical dogleg jetty (Structure 2), and the third and fourth
structures were arranged for measurements of diffraction-refraction into the bay
for an unjettied (Structure 3) and jettied (Structure 4) inlet. Wave conditions
consisted of two irregular waves (0.8 sec, 0.2 ft (6.1 cm) and 1.6 sec, 0.15 ft
(4.6 cm)) and one regular wave (0.8 sec, 0.15 ft (4.6 cm)) for all model structure
configurations. Shore-normal waves were generated for all structures, and a
20-deg wave from shore normal was created for Structures 1 and 2. The 20-deg
wave was created from the same wave-generator location, resulting in increased
wave energy for Structure 1 relative to the shore-normal wave and decreased
energy for Structure 2 relative to the shore-normal wave.
A flood current was created for the Structure 3 (no jetties) condition. The
determination to create a flood current for this condition arose from the
observation that currents created by the breaking waves on each beach adjacent
to the inlet flowed into the inlet and exited seaward, interacting with the
incoming waves. This wave-current interaction caused breaking waves and
reduced wave energy entering the bay. Data were collected for both the flood
current and no-flood current conditions. For Structure 4, the jetties prevented the
longshore currents from entering the channel.
9
Chapter 2 Laboratory Facility, Equipment, and Experiment Design
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