Moving-Boat Deployments

ADCPs can be mounted on either side of manned boats, off the bow, or in a well through the hull. Advantages and disadvantages for mounting locations on manned boats are listed in table 3. The ADCP should not be mounted in close proximity to any object containing ferrous metal or sources of strong electromagnetic fields, such as generators, batteries, and boat engines to minimize ADCP compass errors. A good rule of thumb is that an ADCP should not be mounted any closer to a steel object than the largest dimension of that object. This is a rule of thumb, however, and there are large variations in the magnetic fields generated by different metals. Even stainless steel varies appreciably in the amount of ferrous material contained in the steel.

ADCP mounts for manned boats should:

• allow the ADCP transducers to be positioned free and clear of the boat hull and mount;
• hold the ADCP in a fixed, vertical position so that the transducers are submerged at all times while minimizing air entrainment under the transducers;
• allow the user to adjust the ADCP depth easily;
• be rigid enough to withstand the force of water caused by the combined water and boat speed;
• be constructed of non-ferrous materials;
• be adjustable for boat pitch-and-roll; and
• be equipped with a safety cable to hold the ADCP in the event of a mount failure.

Photos of ADCP mounts and boats equipped with ADCP's.

CLICK ON PHOTO TO VIEW LARGER VERSION.

A tethered boat can be defined as a small boat (usually less than 2 m long) attached to a rope, or tether, that can be deployed from a bridge, a fixed cableway, or a temporary bank-operated cableway. The tethered boat should be equipped with an ADCP mount that meets all of the specifications outlined in the previous section on manned boats. The tethered boat should also contain a waterproof enclosure capable of housing a power supply and wireless radio modem for data telemetry. A second wireless radio modem attached to the field computer enables communication between the ADCP and field computer without requiring a direct cable connection. The radio modems should reliably communicate with the ADCP using the ADCP data-acquisition software; have a rugged, environmental housing; operate on a 12-volt direct current (DC) power supply; and have at least 38,400 baud data-communication capability to maximize ADCP data throughput (Rehmel and others, 2002).

Examples of tethered acoustic Doppler current profiler (ADCP) boats used for making discharge measurements. (Left photograph by Jeff Woodward, Environment Canada and right photograph by Geoffrey D. Cartano, U.S. Geological Survey.)

Tethered ADCP boats have become a common deployment method. Certain considerations need to be made when making tethered ADCP boat measurements. Tethered boats are used in a variety of settings, but primarily from the downstream side of bridges for convenience. Bridge piers can cause excessive turbulence during high streamflow, especially if debris accumulations are present on the piers and the piers are skewed to the flow. The effect of bridge pier-induced turbulence may be reduced by lengthening the tether to increase the distance between the bridge and the tethered boat. Attention should be paid to the cross section to ensure that there are no large eddies that could cause flow to be non-homogeneous. Possible alternatives to measuring off the downstream side of bridges include bank-operated cableways or having personnel on each bank with a rope attached to the platform, pulling it back and forth across the river.

Tethered Boat Manufacturers

Open Seas Instrumentation introduced this tethered boat in 2003. Currently, the USGS has no experience with this boat.

Unmanned, remote-control ADCP boats allow the deployment of ADCPs where deployment with a manned boat or tethered boat may not be feasible or ideal. Similar to (but smaller than) manned boats, a remote-control boat has self-contained motors and a remote-control system for maneuvering the boat across the river. Unlike the tethered boat, the remote-control boat has no rope (tether) restraints. Although remote-control boats have an increased risk of equipment loss because of potential loss of boat control, they provide the ability to launch a boat without a boat ramp and to collect data away from bridge effects (for example, upstream of a bridge) or at sites where no bridge or cableway is present. Currently remote-control boats are commercially available but extensive field experience with these boats is limited.

OceanScience, Inc.