CHAPTER 13 - SPECIAL MEASUREMENT METHODS IN OPEN CHANNELS

10. Measurement by Floats

The approximate velocity of flow in a canal or stream and discharge may be determined by the use of floats (British Standards Institution, 1964). Because a number of other methods are usually easier and more accurate to use, this method should be used only when the other methods are impractical or impossible. A reach of canal, straight and uniform in cross section and grade and with a minimum of surface waves, should be chosen for this method. Surface velocity measurements should only be attempted on windless days to avoid wind-caused deflection of the floats. Even under the best conditions, surface floats are often diverted from a direct course between measuring stations because of surface disturbances and crosscurrents. Surface floats are immersed one-fourth or less of the flow depth. Rod floats are submerged more than one-fourth of the depth but do not touch the bottom.

Cross sections are established along the straight reach of the channel at a beginning, midpoint, and end. The cross sections should be located far enough apart so the time interval required for the float to travel from one cross section to another can be accurately measured. The midpoint cross section provides a check on the velocity measurements made between the beginning and end sections. The channel width across the sections should be divided into at least three, and preferably at least five, segments of equal width. The average depth of each segment must then be determined. The float must be released far enough upstream from the first cross section to attain stream velocity before reaching the cross section. The times at which the float passes each section should be observed by stopwatch and recorded. The procedure is repeated with floats in each of the segments across the canal, and several measurements should be made in each segment.

For flows in canals and reasonably smooth streams, the measured surface float velocities should be multiplied by the coefficients as listed below:

 Table 13-1. Coefficients to correct surface float velocities to mean channel velocities Average depth in reach (ft) Coefficient 1 2 3 4 5 6 9 12 15 >20 0.66 0.68 0.70 0.72 0.74 0.76 0.77 0.78 0.79 0.80

The corrected velocities should then be multiplied by the cross-sectional area of the corresponding stream segments to obtain the segment discharges. The sum of the segment discharges will be the total discharge.

A method used extensively in India to determine velocities in open channels makes use of rod or tube floats. This device consists of a square or round wooden rod with a width or diameter of 1 to 2 in, depending on the length. The rod is designed with a weighted end so it will float in a vertical position with the length of the immersed portion about 0.9 times the depth of the water. This method is based on the reasonable assumption that the velocity of a rod float extending from the water surface to very near the bottom of a channel will closely represent the mean velocity of the water. Streams are divided into segments as described for the float method, except that velocities in areas near the banks of the channel are not measured by the rod method but are assumed to be two-thirds or three-quarters of the mean velocity of adjacent segments.

The rod float method may be used in canals with straight stretches that are regular and uniform in cross section and grade. Where these conditions exist and the flow is free of cross currents and eddies, discharge measurements may be made with a high degree of accuracy.

The accuracy of float methods are limited by many factors, including a lack of preciseness in the coefficients, too few stream segments being used, appreciable changes in stream depth along the test reach, oblique currents, wind forces, and experimental errors in measuring time and distances. Often, a number of people are required to perform this technique and make observations. The course of the floats is difficult to control, and they can be easily retarded by dragging on submerged debris and on the sides and bottom of the channel.