Analysis of sediment concentration
The first step in processing FOBS data is to remove all of the data points
with a count value less than that obtained during the calibration procedure with
no sand in the water (i.e., the x-intercept in Figure 47). This is based on the
assumption that sediment concentration should not be less than clear water with
zero concentration. These low values occur if the water surface drops below the
elevation of a sensor, which occurs frequently for the upper sensors located in
shallow water.
The second step is to calculate the average count value for the remaining
time series. This average count value is used as input into the appropriate
calibration equation, and a sediment concentration is obtained.
The last step is to calculate the average concentration during the entire
sampling duration, which can be much lower than the average concentration
obtained from the time series after the unrealistically low values have been
removed. Essentially, this step replaces the unrealistically low values with zero
counts (representing clear water) and is averaged over the sampling duration.
To illustrate the steps involved in processing the data, Figure 48 shows the
same vertical concentration profile at the three different stages of post-
processing. These measurements were obtained using Vertical Array No.1 in
relatively shallow water during an LST experiment. The first data set represents
the average concentration obtained using the time series prior to any post-
processing. The second data set was obtained after the unrealistically low values
in the original time series had been removed, resulting in a significant increase in
the average concentrations measured in the upper water column. The third data
set represents the average concentrations during the entire sampling duration,
resulting in a significant decrease in the average concentrations measured in the
upper water column. This figure also shows that the concentrations measured
lower in the water column remain unchanged, because these original time series
did not contain any unrealistically low count values.
Bathymetric Data
Conducting a high resolution, bathymetric survey of a moveable-bed beach
with cross-shore dimensions of 18 m and longshore dimensions of 30 m could be
very laborious and time-consuming. Therefore, as discussed previously in
Chapter 2, a fully automated, 3-D, bathymetric survey system was developed to
quantify spatial and temporal changes in the moveable-bed beach. Since the
facility holds a large volume of water, it is not practical to drain the facility every
time bathymetric measurements are needed. Therefore, an amphibious profile
indicator is used so that changes above and below the still-water level could be
measured. The profile indicator is shown in Figure 8.
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Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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