8.2. MEASUREMENT TOOLS AND TECHNIQUES
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level gauges.
Another type of water level measuring device consists of a pointed rod
equipped with a sensor and feedback that keeps the end of the rod just at
the water surface. As water level changes, the servo-system raises or lowers
the rod accordingly, and the elevations are digitized from the attached
potentiometer at preselected intervals. 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.
Any electronic gauge that registers changes in resistance or capacitance
as water levels slowly vary must feature good resolution over the projected
range of water levels, and it must be stable (no drift) over the time span of
the experiment. Also, there is the possibility that long-wave energy present
in the model could influence the water level in the stilling basin, and this
possibility should be investigated.
An alternate method of determining still water levels in models with
waves and slowly varying water levels is to record wave data throughout
the experiment, and then apply a low-pass filter to the recorded signal to
eliminate the short-wave frequencies. The remaining signal represents the
time-varying record of the variation in mean water level. The success of this
method depends on proper selection of the low-pass filter cutoff frequency
and a thorough understanding of possible long-wave seiching in the model
facility that might be contained in the low-passed record.
8.2.2 Sea Surface Elevation (Waves) Measurement
Practically all coastal engineering physical models and laboratory studies
feature some type of oscillatory wave motion; and in the overwhelming majority of cases, waves are the most important forcing function in the model.
The time varying, unsteady nature of oscillatory waves makes it more difficult to give an accurate description of the fluid flow throughout the model.
However, theory has provided means for estimating flow characteristics beneath waves at a point based on the variation of the sea surface elevation
at that location. In addition many empirical design procedures are based
on statistical parameters of the wave field.
Measurements of sea surface elevation variations are commonly referred
to as “wave measurements”, and the various instruments used to obtain the
measurements are called “wave gauges”. Wave measurements at a point
have been made since the days when models first featured mechanically
generated wave motion. Since that time, wave measurement techniques
have progressed to the point that reliable wave measurements are considered
a routine laboratory capability.
Although very rough visual estimates of regular wave height and perio
473
level gauges.
Another type of water level measuring device consists of a pointed rod
equipped with a sensor and feedback that keeps the end of the rod just at
the water surface. As water level changes, the servo-system raises or lowers
the rod accordingly, and the elevations are digitized from the attached
potentiometer at preselected intervals. 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.
Any electronic gauge that registers changes in resistance or capacitance
as water levels slowly vary must feature good resolution over the projected
range of water levels, and it must be stable (no drift) over the time span of
the experiment. Also, there is the possibility that long-wave energy present
in the model could influence the water level in the stilling basin, and this
possibility should be investigated.
An alternate method of determining still water levels in models with
waves and slowly varying water levels is to record wave data throughout
the experiment, and then apply a low-pass filter to the recorded signal to
eliminate the short-wave frequencies. The remaining signal represents the
time-varying record of the variation in mean water level. The success of this
method depends on proper selection of the low-pass filter cutoff frequency
and a thorough understanding of possible long-wave seiching in the model
facility that might be contained in the low-passed record.
8.2.2 Sea Surface Elevation (Waves) Measurement
Practically all coastal engineering physical models and laboratory studies
feature some type of oscillatory wave motion; and in the overwhelming majority of cases, waves are the most important forcing function in the model.
The time varying, unsteady nature of oscillatory waves makes it more difficult to give an accurate description of the fluid flow throughout the model.
However, theory has provided means for estimating flow characteristics beneath waves at a point based on the variation of the sea surface elevation
at that location. In addition many empirical design procedures are based
on statistical parameters of the wave field.
Measurements of sea surface elevation variations are commonly referred
to as “wave measurements”, and the various instruments used to obtain the
measurements are called “wave gauges”. Wave measurements at a point
have been made since the days when models first featured mechanically
generated wave motion. Since that time, wave measurement techniques
have progressed to the point that reliable wave measurements are considered
a routine laboratory capability.
Although very rough visual estimates of regular wave height and perio