Environmental Engineering Reference
In-Depth Information
Measuring a stream discharge using the traditional method, such as with a Price current meter, is
time consuming and expensive, with a single low measurement often taking several hours.
A method that is replacing the traditional method is based on the use of acoustic Doppler cur-
rent proilers (ADCPs). The development of ADCPs has been the most important development in
streamlow measurement in the last 10 years (Simpson 2001; Hirsch and Costa 2004). Using acous-
tic energy (typically in the range of 300-3000 kHz) an ADCP measures the water velocity through
the water column using the Doppler shift, or the shift in frequency, of the acoustic signals relected
from materials suspended in and moving with the water. The water depth is determined by the
time-of-travel of the signals relected from the channel bottom; boat velocities are determined
using the Doppler shift of separate acoustic pulses relected from the riverbed; and the channel
width is computed using the instantaneous boat velocities and the time between each measurement
(Figure 4.12).
The ADCP has made many direct contributions to low measurements, including (Hirsch and
Costa 2004):
•
A conventional discharge measurement using a mechanical current meter can take several
hours to complete, while measurements with an ADCP can take on the order of minutes
and be equally accurate (Morlock 1996; Mueller 2003).
•
The ADCP allows measurements of lows in areas where the traditional velocity area
method does not work (no controls), such as the backwater areas of rivers or estuaries.
•
The ADCP can be deployed to obtain continuous proiles of water velocities, rather than
discrete measurements.
Also, measurements with ADCPs do not require any assumption regarding the vertical veloc-
ity proile, such as was used in the one-, two-, or three-point velocity measurement (Table 4.1).
That is, the assumption of a vertical velocity distribution is no longer required where ADCPs
are used.
Discharge measurement
Distance
=
33 m
0.2 0.3 0.4 0.5 0.6
0.7 0.8
0.8 1.0
1.0
0.2
0.1
0.3 0.4 0.5 0.6
0.3
0.3 0.3
0.6
0.6
0.8 0.8
1.0
1.0
Depth
0.4
0.5 0.5
0.7 0.7
0.9
0.9
0.4 0.4
0.5 0.5 0.5
0.7
0.7
0.2
0.3 0.3 0.3
0.4 0.4 0.4
0.5
0.5
0.2 0.2
0.3 0.3 0.3
0.4
0.4
0.2 0.2
0.2
FIGURE 4.12
River low proiling using a boat-mounted ADCP. (From Winkler, M.F., Acoustic Doppler
current proilers broadband band and narrowband technology explained, USACE Engineering Research and
Development Center, Vicksburg, MS, 2006.)
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