D.2.3 Measuring from a Bank-Controlled Cableway

Conducting the measurement with a cableway is generally preferred to doing it from boats or from bridges with piers. The lengthy set up time required for boat measurements is eliminated as are areas of disturbed flow caused by bridge abutments and piers. Safety of personnel, and the possible need to employ a certified boat operator, are other factors that make cableways attractive.

Bank-controlled cableways employ a capstan drive arrangement that moves a traveler across the river and positions the meter vertically and horizontally. They also send a signal from the meter through the suspension cable to the operator on shore, for velocity measurement. Cableways with cable cars for personnel are outside the scope of this manual. See Section C-5 for a description of two kinds of bank-controlled cableways, and photographs.

Measuring with a Cableway

1. Record the size of weight and the position of the meter above the bottom of the weight or bottom feeler plate.
2. Locate the initial point. It will usually be located near the top of bank on the control side of the watercourse.
3. Position the meter over the initial point, and set the horizontal displacement at zero. Move the meter assembly directly over the nearest water edge and note distance from the initial point. Do not touch the horizontal reset lever during the course of the measurement.
4. Observe and record soundings to the nearest 2 cm at each vertical. Be careful not to over estimate depth by allowing the cable to extend beyond the point when the sounding weight initially touches bottom. The distribution of verticals must be in accordance with that outlined in Section B.2.4.
5. At each vertical, set meter to appropriate depths for velocity measurement. Observe and record the time and revolutions of the current meter.
6. At the end of the measurement, record the time of completion and make some notes to identify the edge of the channel. Record any pertinent information that may have had an effect on the measurement results.

Precautions and Tips

1. Marking the Initial Point. The initial point should be marked very clearly because all distances, observations of depth, and velocities must be referenced to this point. This information must be included in the field data book.
2. "Zeroing" the Meter. Soundings are usually made with the meter at the water surface, that is, with the bottom half of the bucket wheel or propeller submerged and the horizontal section of the tail assembly at the water surface. The distance between the meter and the bottom of the weight must be added to the soundings indicated on the reel counter to obtain the correct depth. The slight amount of drag on the meter and weight when the "meter is zero-ed" has a stabilizing effect that makes the process of sounding quicker and easier than when attempting to "zero" the bottom of the weight. There is also the convenience of not having to apply a correction each time the meter is positioned in the vertical.
3. Effects of Vertical Movement of the Meter. Some cables may undulate from the pulling motion required to move the traveler block from one vertical to the next, or from vigorous cranking movements when sounding with heavy weights. This motion must be allowed to subside before carrying out the depth and velocity observations. This is of particular importance when measuring velocities below 0.75 m/s, because the effects of vertical movement on the current meter are significant in this range.
4. Direction of Flow. The direction of flow is often not perpendicular to the metering section. Even worse, the flow may be inconsistent throughout the section, or the flow may change from vertical to vertical with the section, and vary with changes in stage. In these cases, measure the cosine value; make the appropriate correction for angular flow.