Measurement of sediment concentration
The FOBS can be relocated at various positions along the length of the
bridge in a similar manner in which the wave gauges and current meters are
distributed. Each array of FOBS is co-located about midway between the wave
gauges and the current meters. Figure 40 shows, one of the vertical arrays
midway between the wave gauge and the current meter, directly beneath the
person standing on the bridge. In this particular case, a large portion of the upper
array was above the water surface, since the water depth was significantly less
than the vertical range of the 19 sensors on the vertical array.
One of the most common problems associated with sediment concentration
measurements is determining the elevation of the sensors relative to the surface
of the sand bed. A standard procedure was developed to solve this problem. The
procedure requires two people, one to monitor the FOBS data acquisition
computer and the other to adjust the elevation of the vertical array of FOBS. The
vertical array is positioned so that the lowest sensor is well above the elevation of
the sand bed. The vertical array is slowly and carefully lowered until the output
signal from the lowest sensor gives a full-scale reading, which indicates that the
lowest sensor has penetrated the surface of the bed. If the vertical array is
lowered too far, the second lowest sensor will penetrate the sand and also will
output a full-scale reading. If no waves are present, the array is raised until the
output signal from the second lowest sensor returns to zero. However, if waves
are present and sediment is present in the water column, the array must be re-
deployed. After the array is positioned correctly, the elevation of the vertical
array is locked into position, and this procedure is repeated for the remaining
vertical arrays. At this point, data collection can begin.
Using this method, the maximum vertical positioning error is +1.0 cm.
However, with experience, it was determined that the elevation of the lowest
sensor can be positioned with a vertical accuracy of +0.5 cm relative to the
surface of the bed. However, ripples are always present and are constantly
moving; therefore, the vertical positioning accuracy will decrease somewhat. In
general, this method works well while waves are generated, even if the person
lowering the vertical array cannot see the lowest sensor or the elevation of the
sand bed, which is usually the case. Prior to moving the bridge to the next
transect, the four vertical arrays of FOBS are raised so that the lowest sensors are
not touching the surface of the sand bed.
During an experiment, the four arrays of FOBS are measured simultaneously
(i.e., a total of 76 sensors) at a frequency of 16 Hz to provide sufficient temporal
resolution to measure sediment resuspension events, which often occur over
small fractions of a wave period. The sampling duration is equal to the duration
of the wave machine drive signal, so that the average sediment concentration
obtained from the measurement is an accurate representation of the average
concentration generated by the sequence of irregular waves in the wave machine
drive signal. Therefore, the sampling duration of the FOBS is always the same as
the sampling duration for the wave and current data.
86
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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