474 CHAPTER 8. LABORATORY MEASUREMENT AND ANALYSIS
can be obtained using a linear scale and a stop watch, most studies require more precise measurements of the variation in sea surface elevations.
Sampling of sea surface elevations at regular, short intervals provides time
series records at a point. These records reveal the shape and dimensions of
the waves as they pass by the gauge position, and statistical or frequency
domain analyses of the time series can be used to characterize the waves.
Sea surface elevations are generally obtained as analog records of voltage
(or frequency), and then converted to digital time series records having
uniform spacing between data points. The records are stored in a computer
file that has a “header” containing sufficient information to link the file with
a particular gauge and experiment number. Having the data in digital form
greatly facilitates analysis of the data, and it provides the capability to have
real-time data displays on the computer screen.
At some point, the wave data time series must be converted to engineering units of length by combining the record with the appropriate calibration
function. If this is not performed as the data are being stored on the computer, there must be some way to associate the proper conversion factors
with the data file for later conversion.
Great care must be taken when selecting the sampling time interval
for digitizing sea surface elevations. A common “rule of thumb” is that
an absolute minimum of 10 evenly-space samples must be obtained over
one wavelength (time equal to one wave period). Fewer samples may miss
the wave crest, particularly for steep waves. For a typical laboratory wave
period of 1 s, wave data would need to be collected at a 10-Hz rate at
a minimum; however, most investigators prefer to collect at higher rates
(e.g., 20-30 Hz). Provided the instrument system has sufficient capacity,
higher collection rates give better delineation of the waves and aliasing of
the signal is reduced (Mansard and Funke 1988).
The most common wave gauges are the twin-wire resistance wave
gauge and the single-wire capacitance wave gauge which appeared in
the early 1950’s (see references given in Hudson, et al. 1979). These instruments are mounted so the wire(s) are vertical and piercing the water
surface to a depth below the lowest wave trough (i.e., some portion of the
wire must remain submerged at all times).
Resistance Wave Gauge
The resistance wave gauge has two parallel wires separated by a fixed distance and aligned perpendicular to the direction of wave travel. When the
gauge is operational, high frequency alternating voltage passes through the
wires, and the conductance between the wires is recorded. The measured
conductance is proportional to the length of wire beneath the water surface
can be obtained using a linear scale and a stop watch, most studies require more precise measurements of the variation in sea surface elevations.
Sampling of sea surface elevations at regular, short intervals provides time
series records at a point. These records reveal the shape and dimensions of
the waves as they pass by the gauge position, and statistical or frequency
domain analyses of the time series can be used to characterize the waves.
Sea surface elevations are generally obtained as analog records of voltage
(or frequency), and then converted to digital time series records having
uniform spacing between data points. The records are stored in a computer
file that has a “header” containing sufficient information to link the file with
a particular gauge and experiment number. Having the data in digital form
greatly facilitates analysis of the data, and it provides the capability to have
real-time data displays on the computer screen.
At some point, the wave data time series must be converted to engineering units of length by combining the record with the appropriate calibration
function. If this is not performed as the data are being stored on the computer, there must be some way to associate the proper conversion factors
with the data file for later conversion.
Great care must be taken when selecting the sampling time interval
for digitizing sea surface elevations. A common “rule of thumb” is that
an absolute minimum of 10 evenly-space samples must be obtained over
one wavelength (time equal to one wave period). Fewer samples may miss
the wave crest, particularly for steep waves. For a typical laboratory wave
period of 1 s, wave data would need to be collected at a 10-Hz rate at
a minimum; however, most investigators prefer to collect at higher rates
(e.g., 20-30 Hz). Provided the instrument system has sufficient capacity,
higher collection rates give better delineation of the waves and aliasing of
the signal is reduced (Mansard and Funke 1988).
The most common wave gauges are the twin-wire resistance wave
gauge and the single-wire capacitance wave gauge which appeared in
the early 1950’s (see references given in Hudson, et al. 1979). These instruments are mounted so the wire(s) are vertical and piercing the water
surface to a depth below the lowest wave trough (i.e., some portion of the
wire must remain submerged at all times).
Resistance Wave Gauge
The resistance wave gauge has two parallel wires separated by a fixed distance and aligned perpendicular to the direction of wave travel. When the
gauge is operational, high frequency alternating voltage passes through the
wires, and the conductance between the wires is recorded. The measured
conductance is proportional to the length of wire beneath the water surface