A practical treatise on hydraulic and water-supply engineering: by Fanning J

Author:Fanning, J[ohn] T[homas] [from old catalog]
Language: eng
Format: epub
Tags: Water-supply engineering, Hydraulic engineering
Publisher: New York, D. Van Nostrand
Published: 1886-03-25T05:00:00+00:00

It is desirable that the float be controlled as fully as possible by the mid-depth velocity, where, in artiiicnal channels and deep streams, the film of most constant velocity is found. The reactions and eddies that continnnlly agitate all the particles that flow near the bottom and sides of the stream, and tlie wind pi-essure and motion along the surface, make the motions of all perimeter (so called) films very comjilex, and continually cause the parabolic velocity valui-*s in the central vertical plane to change betwt:»cn flatter and sharper cur\'(»s, or to straighten out and double up, hinged, as it were, upon a mid-depth i)oint; hence the bottom, side, and suiface velocities are liable to great irregularities, and these irregularities are projected to some extent through the whole bmly of the water. These eff'ects may be readily observed in a stream carrying fine quartz sand, upon a sunshiny day, if a position is taken so that the sunlight is reflected from the sand-grains to the eye. If in such cAse the eye and body is moved along with the current, the whole mass of water appears in violent agitation and the particles npjiear to move upward, downward, backward, forwai-d, and across, with writhing motions, illustrating the method by which the water tosses up and bears

FLOW OF WATER IN OPEN CHANNELS.

forward its load of sediment. In tlie midst of this agitation, the film having a velocity ueaivst to the mean resultant of onward progress is usually over the mid-channel of a straight course^ and near to, or a little below, the centre of depth. The suspended float that takes this mean velocity is mort* certain to give a reliahle velocity measure than that controlled by any other point of the stream section.

34;j. Miixiiiuiin Velocity Floats.—If it is desired to place the submerged float in the film of maximum velocity in artificial channels, then this may be sought over the mid-channel, and between the surface and one-third the depth, according to the cross-section of the stream and velocity of tlow. In a smooth rectangular section with depth equal to mdth, or with depth one-half width, it will probably be near one-third the depth, and higher as the de])th of stream is proportionately less, until depth is only one-fourth breadth, when it will have quite, or nearly, reached the surface.

The film of masimxun velocity may reacn the surface in trapezoidal canals when depth of stream is only one-third mean breadth. It is at one-fourth depth in the trapezoidal channel, Fig. 61, in which bottom breadth equals twice depth.

In shallow streams, the maximum velocity is at or near the surface.

344. Relative Velocities and Vohinieft clue to DiflTerent Depthn.—When the mean velocity has been reliably determined in a channel, or small stream, at some iven section, and for some particular depth, it is often desirable to construct a table of velocities and volumes of flow, for other depths in the same section, so that if a reading of depth is taken at any time from

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.