thickness and freeboard must be *subtracted *from the

measured y to obtain the *correct *value of y.

be approximated by measuring the height of the use of

and

the stream of water above the top of the pipe (fig.

v = velocity at center of concentric rings of equal

F-8). Two types of flow must be recognized when deal-

area

ing with fountain flow. At low crest heights, the dis-

n = number of concentric rings

charge has the character of weir flow, while at high

crest heights the discharge has the character of jet

flow. Intermediate values result in erratic flow with

respect to the height of the crest H.

(1) Where the flow exhibits jet character, it can be

proximately by measuring a point on the arc of the

computed from

stream of water emerging from the pipe (fig. F-6),

(F-5)

using the following equation:

where

Q = flow, gallons, per minute

(F-4)

K = constant varying from 0.87 to 0.97 for pipes 2

where

to 6 inches in diameter and h = 6 to 24 inches

Q = flow, gallons per minute

D = inside pipe diameter, inches

A = area of stream of water at end of pipe in

H = vertical height of water jet, inches

square inches. If the pipe is not flowing full,

Where the flow exhibits weir character, it can be ap-

the value of A is the cross-sectional area of the

proximated by using the Francis Formula, Q = 3.33

water jet where it emerges from the pipe, The

Bh3/2, with B being the circumference of the pipe.

area of the stream can be obtained by multi-

(2) Some values of fountain flow for various nom-

plying the area of the pipe times the Effective

inal pipe sizes and heights of crest are given in table

Area Factor (EAF) in figure F-7 using the

F-1.

ratio of the freeboard to the inside diameter of

the pipe.

a. *Weirs. *Flow in open channels can be measured by

x = distance along axis of the discharge pipe

through which the stream of water moves

weirs constructed in the channel. Certain dimensional

from the end of the pipe to a point(s), inches

relations should be recognized in constructing a weir

to obtain the most accurate flow measurements as

y = distance perpendicular to the axis of the dis-

shown in figure F-9. The weir plate should be a non-

charge pipe through which the stream of

corrosive metal about 1/4 inch thick with the crest 1/8

water drops, measured from the top or surface

inch wide, and the downstream portion of the plate

of the stream of water to point(s), inches

beveled at 45 degrees. The crest should be smooth, and

It should be noted that the x and y distances are meas-

the plate should be mounted in a vertical plane perpen-

ured from the top of the stream of water; if y is meas-

dicular to the flow. The channel walls should be

ured in the field from the top of the pipe, the pipe

U. S. Army Corps of Engineers

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