Figure 46. Comparison of longshore current measured with ADV No. 2, before
and after removing spikes caused by air bubbles in shallow water
Sediment Concentration Data
As mentioned previously in Chapter 2, four vertical arrays of Fiber-Optic
Backscatter Sensors (FOBS) are located along the length of the bridge. Each
array of FOBS simultaneously measures sediment concentration at 19 elevations
above the sand bed. Each vertical array consists of a lower and an upper array.
The lower array is designed to measure sediment concentration close to the bed,
where the gradients in concentration are largest. This array has eight sensors
with a vertical spacing of 1.0 cm and has sufficient mechanical strength to
penetrate slightly into the sand bed without being damaged. The bottom sensor is
deployed directly on the bottom to serve as an elevation reference for the
remaining sensors. The upper array has 11 sensors with a nearly logarithmic
vertical spacing and measures the sediment concentration in the upper portion of
the water column where the gradients are generally weaker. Each vertical array,
consisting of 19 sensors, covers a total vertical range of 0.53 m. Therefore, when
located in shallow water, the upper array may be located above the water surface.
Calibration of FOBS
Each of the four vertical arrays was calibrated in a calibration tank,
consisting of a sealed vertical chamber which holds a water-sand mixture and has
a known volume. A pump recirculates the water-sand mixture through the
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Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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