Acoustic Doppler Current Profilers (ADCP’s) have represented a significant advance in the field of stream and river discharge measurement. A float-based ADCP measurement is typically much safer, faster and more accurate than traditional wading measurements with a current meter, or current meter readings taken with a bridge-board and current meter.
In many cases, in the absence of a conveniently located bridge, the challenge to a float-based ADCP river discharge measurement is the actual act of moving the ADCP across the stream/river channel. While the actual float-based ADCP measurement is relatively quick the act of setting up a system to move the ADCP across the water can occupy the greater portion of the total on-site time.
Wading measurements with a float-based ADCP extended away from the user so as to not cause disturbance can be effective but they are frowned upon by many agencies for reasons of personal safety. Other means of moving an ADCP across the channel include manual or powered, bank-operated cable-ways, remotely controlled, self-propelled boats, remote controlled, rope-traversing systems and remote controlled, steel cable traversing systems for existing, formerly “manned” cableways.
This post will focus on traversing systems for rope and steel cable, bank-operated cableways and remote controlled boats will be touched on in later posts.
Rope Traversing Systems
Remote controlled rope and cable traversing systems provide the advantage of moving the ADCP across the channel at a user-selectable, consistent speed which has been shown to improve ADCP discharge measurement. Rope-traversing systems are relatively inexpensive and consist of a handheld remote-control unit and a battery-powered, portable, rope/cable traversing unit. Some researchers have constructed their own units by scavenging parts from consumer electronics such as remote controlled cars, boats, etc. Rope traversing systems are also manufactured commercially by Hydrological Services and Ocean Science.
(Video of Hydrological Services Flying Fox Rope-Traversing System with RD Instruments RiverRay on the Boise River)
A major challenge to rope traversing systems on medium –sized rivers and canals is getting the rope across the channel and securing it on the other side. This often involves shuttling personnel to the opposite side of the channel to assist, which may involve its own challenges.
A good rope traversing system will have an intuitive, menu-driven controller, with an outdoor visible LCD that displays, at a minimum, the distance traveled and the speed-over-water. The speed over water should be selectable across a common range and something that can be locked in with a dial and latching toggle switch. The remotely controlled traversing unit should include a shaft-encoder or other means of measuring distance traveled so that the ADCP can start/stop at the same position on each pass. As it is almost impossible to not have some sag in the rope, the traveler should have enough power to travel up small inclines while dragging a payload equivalent to the weight of an ADCP.
Traversing Systems for Cableways with Steel Cables
In the U.S. It is estimated that the USGS alone operates approximately 1600 (manned) cableways (C. Russell Wagner, USGS Stream Gauging Cableways). Manned cableways are also operated by many other public and private entities.
As personnel safety concerns increase and infrastructure ages, many manned cableways are no longer suited for normal operation. Remotely controlled, steel-cable traversing systems can leverage the existing infrastructure at these sites with no additional site costs.
Steel-cable traversing systems include the same major components of a rope traversing system, with a few major differences. These systems are generally used on wider channels and require remote control devices with longer range. A traveler for a steel cable system needs to have drive and contact pulleys that are more robust than rope driven systems and can fit a range of different diameters of steel cable where rope traversing systems are typically designed for a select rope size or even a specific rope.
At existing cableways many cables are suspended at a higher elevation above the water, these traversing systems should have a hoist for raising and lowering the ADCP. This allows the ADCP to be attached at a safe and convenient location on the bank, remotely driven out over the water and then lowered to the appropriate point at the waters-edge. This hoist must be able to support the weight of the ADCP/float in addition to a stabilizing weight of ≈20-30#.
The ability to select a constant traveler speed, measure the distance traveled, zero this measurement at either bank and see the battery voltage are also important features of a quality traversing system. Some systems also include failsafe mechanisms so that the traveler automatically returns to the “home” bank in the event of loss of communication with the remote control or low battery voltage.
(Video of Hydrological Services Cable Fox, Cable-Traversing system with an RD Instruments Stream Pro on the Bear River)
Frontier Precision has experience with various traversing systems and ADCP’s and can consult with you on a system to meet your needs.
