Velocity measurements by floats and float diffusion.

Abstract

For many centuries, streams, rivers and oceans have been used for the disposal of all kinds of man-made wastes and pollutants, of which some are extremely harmful. Studies of the spreading of these effluents require the knowledge of the velocity distribution and current patterns. A practical and alternative way to fixed current meters is the use of tracers such as combined floats or drogues. The purpose of this laboratory investigation is to study the use of combined floats, consisting of a floating disc and a spherical weight, for velocity and discharge measurements and float diffusion. An experimental program was carried out in the hydraulics laboratory and it was found useful to investigate the effect of (i) length and width of the study section, (ii) float size and its depth of immersion and finally (iii) the velocity itself (the magnitude). Because of the limitations involved, three float sizes were selected for which only the disc diameter varied; the sphere diameter, however, was kept constant. When compared to the velocity measured by the Pitot tube, a combination of the smallest float submerged at channel depths above 0.6d in a relatively uniform small section (width and length) was found more appropriate and consequently more accurate. Float diffusion on the other hand was found to depend mainly on the study section length and the float size. Flow velocity was not found to be critical, especially for large floats, and the diffusion width seems to be much diminished when large floats are used.