each wave generator to measure the characteristics of the offshore waves. All 14
wave gauges are attached to linear actuators and can be simultaneously
calibrated, as described in Chapter 8.
Acoustic-doppler velocitimeters
Orbital wave velocities and unidirectional longshore currents are measured
using 10 acoustic-doppler velocitimeters (ADV). The ADVs are co-located with
the wave gauges in a cross-shore array along the length of the instrumentation
bridge. Each ADV is mounted to a 1.8-m-tall vertical strut, which is mounted to
the downstream side of the bridge. This allows the elevation of the ADV to be
easily adjusted to locate the sampling volume at the required elevation in the water
column (Figure 7). Each sensor has a 1.0-m-long stainless steel stem with a
permanently engraved scale consisting of 0.1-m major increments and 0.01-m
minor increments. Therefore, the sampling volume can be accurately positioned in
the water column by using the still-water level as a reference datum. Hence, the
vertical structure of the mean longshore current can be accurately measured.
However, for most experiments, the ADVs are positioned so that the sampling
volumes are located approximately one-third of the water depth above the bed to
measure a representation of the depth-average velocity.
ADVs 3 through 10 are standard three-dimensional (3-D) down-looking
sensors, which are ideal for measurements in water depths greater than 10 cm. The
down-looking sensor minimizes flow impedance when horizontal velocities are
dominant. However, this probe orientation cannot be used to measure the velocity
in the upper 5 cm of the water column. Therefore, ADVs 1 and 2 are two-
dimensional (2-D) side-looking sensors for measurement in shallow water close to
the shoreline. The sampling volume is located 5.0 cm from the acoustic transmitter
on all 10 systems.
Fiber-optic Backscatter Sensors
Four vertical arrays of Fiber-optic Backscatter Sensors (FOBS) are located
along the length of the instrumentation bridge. Each array is co-located about
midway between the wave gauges and the ADVs as shown in Figure 7. The
FOBS simultaneously measure suspended sediment concentration at 19
elevations in the water column. Each of the 19 measurement locations consists
of an optical fiber to transmit modulated white light into the water column and a
second optical fiber to receive and transmitted light backscattered from the sand
grains to a detector. Each of the four sets of sensors consists of a lower and an
upper array. The lower array consists of eight emitter/receiver pairs bundled
together in a circular, pipe-organ fashion with a 1.0-cm vertical spacing. The
upper array has 11 emitter/receiver pairs with a nearly logarithmic vertical
spacing.
Bathymetric survey system
The moveable bed needs to be surveyed on a regular basis to quantify spatial
and temporal beach profile evolution. Beach profiles are measured every 0.5 m
10
Chapter 2
Overview of the Laboratory Facility
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