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 ERDC TN-DOER-E7
April 2000
calculate the concentration of total suspended solids (TSS) present in the water column if the
following conditions are met:
The particle size distribution of the suspended material is known.
The salinity and temperature of the water are known.
The acoustic system has been calibrated.
In practice, when dredging operations are monitored, reliable TSS concentration measurements with
known and acceptable accuracy cannot be derived acoustically in routine field operations (see
Tubman, Brumley, and Puckette 1994 for a discussion of the theory and Puckette 1998 for a
discussion of the problems relating to calculating TSS from the measurements). However, acoustic
measurements used with other methods of measuring dredging-related plumes can produce valuable
information that, in most cases, could not be obtained if acoustic measurements were not included
in the monitoring.
Acoustic measurements of suspended sediment plumes provide the unique capability to produce
three-dimensional images of plumes during a relatively short time. Acoustic measurements can
rapidly produce images of the relative distribution of suspended-sediment concentrations in the
water column. These images can be used to locate the positions of other measurements relative to
boundaries of a plume and the spatial distribution of suspended-sediment concentrations. In
Mobile, fishery hydroacoustic measurements were made synoptically with the plume-monitoring
acoustic measurements. In Boston, additional measurements were optical backscatter and light
attenuation, as well as water samples from which TSS values were determined by filtering the
samples and weighing the filters. The study areas are shown in Figure 1.
transmits 1,200-kHz acoustic signals through the water and measures the acoustic signals that are
returned to the instrument. Four of the five beams point down at a 20-deg angle from the vertical.
These four beams measure the water velocity and the velocity of the boat across the bottom.
Subtracting the velocity of the boat across the bottom from the water velocity gives the current speed
and direction relative to the bottom. The fifth beam points straight down, and its measurements of
backscattered acoustic energy are used solely for detecting the presence of suspended sediment in
the water column. The BBADCP system used for this monitoring effort has design features
developed and implemented in the PLUmes Measurement System (PLUMES) specifically for
identifying and tracking suspended-sediment plumes from dredging related activities. The devel-
opment of PLUMES is discussed in Puckette (1998).
The BBADCP records data for both current and fifth-beam backscatter in bins that represent
25-cm-thick slices across each beam, continuously along the beam. They start 50 cm from each
beam transducer and produce valid data to near bottom. The data from all five transducers and all
bins for each transducer are recorded every 2 sec. The first step in processing the fifth-beam acoustic
backscatter data is to correct them for the geometric spreading and viscous attenuation losses of
acoustic energy. These losses decrease the amount of backscatter received from each of the 25-cm
slices by an amount that increases with distance along each beam. The losses are predictable, and
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