Table 2. Coastal, inshore and nearshore ADCP deployment configurations

Fig. 10. Schematic view of bottom mounted ADCP configuration used to measure current velocity in a series of vertical bins through the water column.

Fig. 11. Side-mounted ADCP sensor for moving deployment application (photo from the US Geological Survey).

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D. M. Fitzgerald, G. A. Zarillo & S. Johnston

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Table 1. Operation range of commercially

available ADCP technology.

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.

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