January 13, 2004
D. M. Fitzgerald, G. A. Zarillo & S. Johnston
Table 1. Operation range of commercially
available ADCP technology.
Bin Size (m)
Table 2. Coastal, inshore and nearshore ADCP deployment
Buoy -- mooring (waves)
ADCP. The ADCP gives the average velocity vector over each cell. A weight func-
tion is applied, so that the signal in the center of each cell has more weight. Data
produced by a broadband ADCP includes current velocity in Earth coordinates,
echo intensity (depends on sound absorption, beam spreading, transmitted power
and scatters), correlation coefficient (depends on data quality), and percent good
parameter (depends on correlation coefficient and vertical error velocity). The length
and range of the pulses (pings) are limited by the operating frequency. The pulse
length determines the minimum cell size. Table 1 lists the typical operating frequen-
cies of commercially available ADCP units along with the minimum cell size and
range associated with each frequency range. Single-ping random errors are relatively
large. Therefore, time averaging is required for reliable measurements. Typically an
averaging period lasts 1 minute or longer. During this period, ADCP data are logged
along with navigation data. Conversion of ADCP measurements to absolute current
velocities is typically performed afterwards with post-processing, but they can also
be processed on-line.
2.7.2. Shallow water current meters
ADCP measurements have been extensively used to quantify three-dimensional cur-
rent fields at tidal inlets, navigational channels, and in other shallow marine and
estuarine environments. Table 2 summarizes deployment configurations that are ap-
plicable to coastal engineering surveys in relatively shallow water.
Figure 11 illustrates the moving deployment configuration on a small coastal ves-
sel. In this case, the ADCP can be interfaced with navigation software and accounts
for the vessel motion by tracing the bottom. An ADCP sensor can also be deployed
on a taught line mooring as shown in Fig. 12. Bottom mount configurations often
use trawl-resistant moorings as shown in Fig. 13.