The Oakley herald. [volume] (Oakley, Idaho) 1908-1961, 1912-05-03

the measurement OF comparatively
small heads of water Is the weir
measurement. The weir consists es-
sentially of thin notch Of specific
shape. which the water is caused to
flow over The amount of fow de
ponds upon the depth OF water flow
ng over the crest, as the bottom Of
the notch is called. The weir has
been used for the measurement OF
water for hundreds of years and the
method of its installation and the
discharge OF water can be measured
over it with an error of less than one
per cent, if care Is used.
There are several forms of weirs,
the name of each designat-
ing the shape of the notch. Cippo-
letti, an Italian engineer. evolved
and perfected the weir which bears
his name, many years ago, this be-
ting the weir that is now most gener-
ally used in the west.
Weir Box.-In order to maintain
the weir in proper and constant
position and to prevent leakage
around and under it, and in order
that all water that is to be measured
should be conducted over it, it is
usually necessary to construct some
sort of box or frame to hold the
type of this box which is recon-
mended for a one foot Cippoletti
weir is nine feet long, two and a half
feet deep and three feet wide inside
measurement, and with a one foot
weir will measure with accuracy
amounts ranging from ten to fifty
Idaho miners inches.
Where a weir box is built, it is nee
essary that it be made OF sufficient
size and depth in comparison to the
size of the weir notch to eliminate
all excess velocity of approach. II
the box is built too narrow or too
shallow it will add to the velocity of
approach to such an extent that cor
rect measurement can not be Sc
cured. 1t is but a mere matter of
convenience. however. to have q box
Of the exact length prescribed. for
that part of the box above the weir
may be omitted II a settling basin or
pool OF the same size constructed
in the ditch above the weir. Tho
weir boxes and pools which should
be constructed for weirs of larger
sizes should be In the same proper
ton with respect to the size OF the
weir as for the one foot The size
of weir box that is required In order
to eliminate velocity of approach has
been found. by experiment. to be ap-
proximately seven times the cross
section of the weir notch.
Discharge.-The discharge of weirs
has been accurately calculated by
various engineers who have carried
on hundreds of experiments. cover
ing years of time, and it is found
that under like conditions the same
depth always produces the same dis
charge over the same size of a weir.
The conditions given for the con
struction and installation of weirs
must be rigidly adhered to, however,
II accurate measurement is to be
made. The formula which has been
evolved and which gives the dis
charge Of accurately constructed
Cippoletti weirs is
O equals 1857 L Fl 80.
Where Q equals the number OF Gu
bic feet per second l. equals the
length of crest in feet and II equals
the depth of water on the crest in
Feet provided the same is measured
from point level with the crest and
up stream from 1t at distance equal
to the length of the crest
l. The weir box should be set with
with hn hNt -
ditch and should be level in all direc.
tons The weir itself should be *
actly level and perpendicular.
2. The channel leading to the weir
should be of uniform cross section. or
what Is still better should gradually
enlarge as the weir is approached.
The axis of the stream should pass
through the weir and perpendicular
to 1t; or. in other words. the weir
should be located at right angles to
the MIDDLE of the stream. The ad-
stream should be free from
internal cross currents or eddies. as
these have an influence upon the dis
charge.
3. The water should be brought as
nearly as possible to a state of rest
before 1t enters the weir. An excess
OF approach due to the We
locity OF the advancing current will af-
rect uncrease) the discharge more
than almost any other one thing.
This velocity can be reduced by wild
ening and deepening the box or pool
above the weir. 1t is calculated that
weirs three feet long with a depth of
water of twelve inches, should not
have a greater velocity of approach
han six inches per second, which
amount may be allowed to increase
very slightly where greater depths
over wider weirs are used. By con
'tructing the box or pool above the
weir with a cross section at least
seven times that of the weir notch. a
low velocity of approach
s usually secured.