of a submerged shoal. The amount of sand that accumulates in this submerged
shoal (during the test segment) is quantified volumetrically using the bathymetric
data measured immediately prior to and after each test segment. The shoal
volume is separated into 0.75-m-wide slices running alongshore, and the quantity
of sand in each slice is added to the sediment trap immediately downstream of
each slice. The method used calculate the quantities will be discussed in detail in
Chapter 10.
Water Level Control
Measuring the still-water level in a laboratory facility is relatively simple,
because it can be measured when the fluid is at rest and the water surface is
stationary. However, future experiments in the LSTF may involve the simulation
of tidal fluctuations, which complicates this measurement. Tides would require
simulation with a system to control the inflow and outflow of water.
Sophisticated systems have been developed whereby the rate of water level
change in the model is controlled by an automated system that relies on feedback
from the water level gauge (Hughes 1993). Although a system to simulate tides
does not presently exist in the LSTF, a water level control gauge was built to
make this possible in the future.
Because of the long-term nature of conducting sediment transport
experiments in a large-scale laboratory facility, it is important to maintain a
known water level during several months (or possibly a few years) of laboratory
studies. For example, unknown variations in the still-water level can cause
undesirable variations in the offshore wave height from one experiment to the
next. Therefore, a simple but robust system was constructed to provide an
accurate and consistent method of measuring the still-water level in the facility
over long periods of time.
A custom designed graduated glass cylinder was built for the LSTF and was
mounted inside of a protective support frame on the outside of the exterior wall
of the facility. The graduated cylinder has an inside diameter of 0.1 m. A
polyurethane tube, with an inside diameter of 0.003 m, connects the graduated
cylinder to the reservoir of water in the facility. Therefore, the surface area of
the water in the graduated cylinder is three orders of magnitude larger than the
cross-sectional area of the inflow/outflow tube. This was done to essentially
eliminate water surface elevation fluctuations while waves are generated.
Therefore, the graduated cylinder would be ideal for measuring the mean water
level in the facility if tides are simulated in the future. The zero reading on the
graduated cylinder was set at the average elevation of the concrete floor in the
facility in the region where waves are generated. By considering the meniscus of
the water surface on the inside face of the graduated cylinder, the water level in
the facility can be maintained within + 1 mm of the desired water level.
Summary
The LSTF is a high-quality laboratory facility that has been designed with
the capability to measure wave, current, sediment concentration, bathymetric,
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Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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