Online Encyclopedia

Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.

PROCESSES FOR GAUGING

Online Encyclopedia

Originally appearing in Volume V14, Page 85 of the 1911 Encyclopedia Britannica.

» Make a correction to this article.
» Add information or comments to this article.

Encyclopedia Home :: PRE-PYR

Spread the word:

del.icio.us it!

PROCESSES FOR GAUGING STREAMS § 146. Gauging by Observation of the Maximum. See also:

SURFACE

Surface Velocity.—The method of gauging which involves the least trouble is to deter-mine the surface velocity at the See also:

THREAD (0. Eng. praed, literally, that which is twisted, prawan, to twist, to throw, cf. " throwster," a silk-winder, Ger. drehen, to twist, turn, Du. draad, Ger. Draht, thread, wire)

thread of the stream, and to deduce from it the mean velocity of the whole See also:

CROSS

cross See also:

section. The maximum surface velocity may be determined by floats or by a current See also:

METER, ELECTRIC

meter. Unfortunately the ratio of the maximum surface to the mean velocity is extremely variable. Thus putting vo for the surface velocity at the thread of the stream, and vm for the mean velocity of the whole cross section, v,,,/vo has been found to have the following values:- v./v. De See also:

PRONY, GASPARD CLAIR FRANCOIS MARIE RICHE DE (1755-1839)

Prony, experiments on small wooden channels 0.8164 Experiments on the See also:

Seine o•62 Destrem and De Prony, experiments on the See also:

NEVA

Neva 0.78 Boileau, experiments on canals . . . . 0.82 Baumgartner, experiments on the See also:

GARONNE (Lat. Garumna)

Garonne 0.80 B1-linings (mean) 0.85 See also:

Cunningham, Solani See also:

AQUEDUCT (Lat. aqua, water, and ducere, to lead; Gr. apaymryeiov, apayeeywov, vlrovopos)

aqueduct 0.823 See also:

Ill! ;1 !i Various formulae, either empirical or based on some theory of the See also:

VERTICAL (from Lat. vertex, highest point)

vertical and See also:

HORIZONTAL

horizontal velocity curves, have been proposed for determining the ratio v,,,,/va. See also:

BAZIN

Bazin found from his experiments the empirical expression v,,, vo-25.4/ (mi); where m is the See also:

HYDRAULIC

hydraulic mean See also:

DEPTH

depth and i the slope of the stream. In the See also:

case of See also:

IRRIGATION (Lat. in, and rigare, to water or wet)

irrigation canals and See also:

rivers, it is often important to determine the See also:

DISCHARGE (adapted from the O. Fr. discharge, modern decharge, from a med. Lat. discargare, to unload, dis- and earn care, to load, cf. " charge ")

discharge either daily or at other intervals of See also:

time, while the depth and consequently the mean velocity is varying.

Cunningham (See also:

ROORKEE

Roorkee Prof. Papers, iv. 47), has shown that, for a given See also:

PART

part of such. a stream, where the See also:

bed is See also:

REGULAR

regular and of permanent section, a See also:

SIMPLE

simple See also:

formula may be found for the variation of the central surface velocity with the depth. When once the constants of this formula have been determined by measuring the central surface velocity and depth, in different conditions of the stream, the surface velocity can be obtained by simply observing the depth of the stream, and from this the mean velocity and discharge can be calculated. Let z be the depth of the stream, and vo the surface velocity, both measured at the thread of the stream. Then vo =cz; where c is a See also:

CONSTANT, BENJAMIN (1845-1902)

constant which for the Solani aqueduct had the values 1.9 to 2, the depths being 6 to 10 ft., and the velocities 32 to 41 ft. Without any See also:

ASSUMPTION, FEAST OF

assumption of a formula, however, the surface velocities, or still better the mean velocities, for different conditions of the stream may be plotted on a See also:

DIAGRAM

diagram in which the abscissae are depths and the ordinates velocities. The continuous See also:

CURVE (Lat. curvus, bent)

curve through points so found would then always give the velocity for any observed depth of the stream, without the need of making any new See also:

FLOAT (in O. Eng. floc and flota, in the verbal form f eotan; the Teutonic root is flut-, another form of flu-, seen in " flow," cf. " fleet "; the root is seen in Gr. a-M e, to sail, Lat. pluere, to rain; the Lat, fluere and fluctus, wave, is not connect

float or current meter observations. § 147. Mean Velocity determined by observing a See also:

SERIES (a Latin word from serere, to join)

Series of Surface Velocities.—The ratio of the mean velocity to the surface velocity in one See also:

LONGITUDINAL

longitudinal section is better ascertained than the ratio of the central surface velocity to the mean velocity of the whole cross section. Suppose the See also:

RIVER

river divided into a number of compartments by equidistant longitudinal planes, and the surface velocity observed in each compartment. From this the mean velocity in each compartment and the discharge can be calculated.

The sum of the partial discharges will be the See also:

TOTAL

total discharge of the stream. When wires or See also:

ROPES, JOHN CODMAN (1836–1899)

ropes can be stretched across the stream, the compartments can be marked out by tags attached to them. Suppose two such ropes stretched across the stream, and floats dropped in above the upper rope. By observing within which compartment the path of the float lies, and noting the time of transit between the ropes, the surface velocity in each compartment can be ascertained The mean velocity in each compartment is 0.85 to 0.91 of the surface velocity in that compartment. Putting k for this ratio, and vi, tv_ . . . for the observed velocities, in compartments of See also:

AREA

area SA, 51¢ . . . then the total discharge is Q = k (i21v1 +t12v2 + . . . ). If several floats are allowed See also:

toy pass over each compartment, the mean of all those corresponding to one compartment is to be taken as the surface velocity of that compartment. This method is very applicable in the case of large streams or rivers too wide to stretch a rope across. The paths of the floats are then ascertained in this way.

Let fig. 148 represent a portion of the river, which should be straight and See also:

FREE

free from obstructions. Suppose a See also:

BASE

base See also:

LINE

line AB measured parallel to the thread of the stream, and let the mean cross section of the stream be ascertained either by See also:

SOUNDING (for derivation see SOUND above)

sounding the terminal cross sections AE, BF, or by sounding a series of equidistant cross sections. The cross sections are taken at right angles to the base line. Observers are placed at A and B with theodolites or See also:

box sextants. The floats are dropped in from a See also:

BOAT (0. Eng. bdt; the true etymological connexion with Dutch and Ger. boot, Fr. bateau, Ital. battello presents great difficulties; Celtic forms are from O. Eng.)

boat above AE, and picked up by another boat below BF. An observer with a See also:

CHRONOGRAPH

chronograph or See also:

WATCH (in O. Eng. wcecce, a keeping guard or watching, from wacian, to guard, watch, wacan, to wake)

watch notes the time in which each float passes from AE to BF. The method of proceeding is this. The observer r F A sets his See also:

THEODOLITE

theodolite in the direc- tion AE, and gives a See also:

SIGNAL

signal to drop a float. B keeps his See also:

INSTRUMENT (Lat. instrumentum, from insincere, to build up, furnish, arrange, prepare)

instrument on the float as it comes down. At the moment the float arrives at C in the line AE, the observer at A calls out. B clamps his instrument and reads off the See also:

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle See also:

ABC, and the time observer begins to See also:

note the time of transit.

B now points his instrument in the direction BF, and A keeps the float on the cross See also:

WIRE (A.S. wir, a wire; cf. Swed. vire, to twist, M.H.G. wiere, a gold ornament, Lat. viriae, armlets, ultimately from the root wi, to twist, bind)

wire of his instrument. At the moment the float arrives at D in the line BF, the observer B calls out, A clamps his instrument and reads off the angle See also:

BAD, BCD

BAD, and the time observer notes the time of transit from C to D. Thus all the data are determined for plotting the path CD of the float and determining its velocity. By dropping in a series of floats, a number of surface velocities can he determined. When all these have been plotted, the river can bedivided into convenient compartments. The observations belonging to each compartment are then averaged, and the mean velocity and discharge calculated. It is obvious that, as the surface velocity is greatly altered by See also:

wind, experimente of this See also:

KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind should be made in very See also:

CALM

calm See also:

WEATHER (O. Eng. weder; the word is common to Teutonic languages; cf. Du. weder, Dan. veir, Icel. ve8r, and Ger. Wetter and Gewitter, storm; the root is wa- to blow, from which is derived " wind ")

weather. The ratio of the surface velocity to the mean velocity in the same vertical can be ascertained from the formulae for the vertical velocity curve already given (§ tot). Exner, in Erbkam's Zeitschrift for 1895, gave the following convenient formula. Let v be the mean and V the surface velocity in any given vertical longitudinal section, the depth of which is h v/V = (I +o.1478,/ h)/(1 -+•2216,/ h). If vertical velocity rods are used instead of See also:

COMMON

common floats, the mean velocity is directly determined for the vertical section in which the See also:

rod floats. No formula of reduction is then necessary.

The observed velocity has simply to be multiplied by the area of the compartment to which it belongs. § 148. Mean Velocity of the Stream from a Series of See also:

Mid Depth Velocities.—In the gaugings of the See also:

MISSISSIPPI

Mississippi it was found that the mid depth velocity differed by only a very small quantity from the mean velocity in the vertical section, and it was uninfluenced by wind. If therefore a series of mid depth velocities are determined by See also:

double floats or by a current meter, they may be taken to be the mean velocities of the compartments in which they occur, and no formula of reduction is necessary. If floats are used, the method is precisely the same as that described in the last See also:

PARAGRAPH

paragraph for surface floats. The paths of the double floats are observed and plotted, and the mean taken of those corresponding to each of the compartments into which the river is divided. The discharge is the sum of the products of the observed mean mid depth velocities and the areas of the compartments. § 149. P. P. Boileau's See also:

PROCESS

Process for Gauging Streams.—Let U be the mean velocity at a given section of a stream, V the maximum velocity, or that of the See also:

PRINCIPAL

principal filament, which is generally a little below the surface, W and w the greatest and least velocities at the surface. The distance of the principal filament from the surface is generally less than one-See also:

FOURTH

fourth of the depth of the stream; W is a little less than V; and U lies between W and w.

As the surface velocities See also:

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change continuously from the centre towards the sides there are at the surface two filaments having a velocity equal to U. The determination of the position of these filaments, which Boileau terms the gauging filaments, cannot be effected entirely by theory. But, for sections of a stream in which there are no abrupt changes of depth, their position can be very approximately assigned. Let A and I be the horizontal distances of the surface filament, having the velocity W, from the gauging filament, which has the velocity U, and from the See also:

BANK

bank on one See also:

side. Then A/l=c4,1 ((W+2w)/7(W-w)l, c being a numerical constant. From gaugings by See also:

HUMPHREYS, ANDREW ATKINSON (1810-1883)

Humphreys and See also:

Abbot, Bazin and See also:

Baumgarten, the values c=0.919, 0.922 and 0.925 are obtained. Boileau adopts as a mean value 0.922. Hence, if W and w are determined by float gauging or otherwise, A can be found, and then a single velocity observation at A ft. from the filament of maximum velocity gives, without need of any reduction, the mean velocity of the stream. More conveniently W, w, and U can be measured from a horizontal surface velocity curve, obtained from a series of float observations. § I5o. See also:

DIRECT

Direct Determination of the Mean Velocity by a Current Meter or See also:

DARCY, THOMAS DARCY, BARON (1467-1537)

Darcy See also:

GAUGE, or GAGE (Med. Lat. gauja, jaugia, Fr. jauge, perhaps connected with Fr. jale, a bowl, galon, gallon)

Gauge.—The only method of determining the mean velocity at a cross section of a stream which involves no assumption of the ratio of the mean velocity to other quantities is this—a See also:

PLANK

plank See also:

BRIDGE

bridge is fixed across the stream near its surface. From this, velocities are observed at a sufficient number of points in the cross section of the stream, evenly distributed over its area.

The mean of these is the true mean velocity of the stream. In Darcy and Bazin's experiments on small streams, the velocity was thus observed at 36 points in the cross section. When the stream is too large to See also:

FIX, THEODORE (180o-1846)

fix a bridge across it, the observations may be taken from a boat, or from a couple of boats with a gangway between them, anchored successively at a series of points across the width of the stream. The position of the boat for each series of observations is fixed by angular observations to a base line on See also:

SHORE

shore. § 151. A. R. Harlacher's Graphic Method of determining the Discharge from a Series of Current Meter Observations.—Let ABC (fig. 149) be the cross section of a river at which a See also:

COMPLETE

complete series of current meter observations have been taken. Let I., II., I7t , . . be the verticals at different points of which the velocities were mete tired. AF---B~ 111 ii E Suppose the depths at I., II., III (fig.

149), set off as vertical t out in this way. The upper figure shows the section of the river and the positions of the verticals at which the soundings and gaugings were taken. The See also:

LOWER

lower gives the curves of equal velocity, worked out from the current meter observations, by the aid of vertical velocity curves. The vertical See also:

scale in this figure is ten times as See also:

GREAT

great as in the other. The discharge calculated from the See also:

CONTOUR

contour curves is 14.1087 cubic metres per second. In the lower figure some other interesting curves are See also:

DRAWN

drawn. Thus, the uppermost dotted curve is the curve through points at which the maximum velocity was found; it shows that the maximum velocity was always a little below the surface, and at a greater depth at the centre than at the sides. The next curve shows the depth at which the mean velocity for each vertical was found. The next is the curve of equal velocity corresponding to the mean velocity of the stream; that is, it passes through points in the cross section where the velocity was identical with the mean velocity of the stream.

End of Article: PROCESSES FOR GAUGING

Additional information and Comments

There are no comments yet for this article.

» Add information or comments to this article.

Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.

Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.

Links to articles and home page are always encouraged.

[back]
PROCESS OF

[next]
PROCESSION PATH (Lat. ambitus templi)