The Blackfoot optimist. [volume] (Blackfoot, Idaho) 1907-1918, 1911-12-28

,nomical method of water measurement
hat has been devised for the measure ,
nent of comparatively small heads OF /
water is the weir measurement. The
weir consists essentially of thin !
notch OF a specific shape, which the
water is caused to flow over. The
amount of flow depends upon the depth
of water flowing over the crest, as
the bottom notch is called. The weir
has been used for the measurement
of water fer hundreds OF years and the
method OF it's installation and the dis
charge of the water over it have both
been worked Que so carefully that wat
en- can be measured over it with an err
ror of less than one per cent, if care
is used
There are several forms of weirs,
the name of each designating the
shape of the notch. Cippoletti, an It
allan engineer, evolved and perfected
the weir which bears his name, many
years ago, this being the weir that is
now most generally used in the West.
Other weirs namely, the rectangular
weirs, are constructed and operated
in much the same way the Sc me rules
being observed in the measurement,
practically the only difference existing
being, as has been staetd, in the shape
Of the notch and the formula used in
Cippoletti weir is trapezcidal in shape,
the computation of the discharge. They
having straight bottom or crest with
sides that slope outward from the YER
teal at the rate of ore horizontal to
four vertical. A one foot Cippoletti
weir has straight horizontal bottom
Or crest one foot in length with
width between the sides of Is inches
at height of 6 inches above the crest.
II the notch is made of wood the sides
and bottom should be beveled Oft on
the down stream side, leaving a sharp
thin edge on the up-slre- m side. This
is Necessary in order to secure
sharp clear-cut issuing stream which
1t is necessary to secure if accurate
measurement is to be obtained. 1t is
the upper side Of the weir board with
usually the best policy to line or face
a piece of galvanized iron from which
the proper size and shape notch L.
been cut. In this case the notch in
the board should be cut slightly larger
than that in the galvanized iron slop-
iag or beveling the edges in the same
manner as before. The use of the
galvanized iron or other metalic crest
is especially recommended as it not
only ilsures a sharp clear-cut issuing
stream upon which accurate measure
ment depends, but in addition, it also
furnishes a permanent ulch4 ngeable
crest for the water to flow through.
provided the necessary requirements
are observed in the construction ana
installation OF weir, the. amount of
water flowing over it depends upon bu
two factors-the length of crest and
the depth of the water on the crest,
The length OF crest being known, tie
depth OF water should be measured,
not on the crest, but from a point
level with the crest and up stream
from it distance equal to ITS length.
Weir Box.-ln order to main tain the
weir in proper and constant position
and to prevent leakage around or un-
der it, and in order that all water
that is to be measured should be con
Quoted over it, it is usually necessary
to construct some sort of box or fran,
to hold the weir in place. A common
FORM or type of this box which is rec
commended for a one foot Cippolettj
weir is feet long, 21a feet deep and
3 feet wide inside measurement. and
with one foot weir will measure wit
accuracy amounts ranging from 10 tc
80 Idaho miners inches.
Where a weir box is built, it is
necessary that it be made of sufficient
size and depth in comparison to the
size OF the weir notch to climate all
excess velocity of approach. If the
Dox IS built too narrow or too shallow
1t will add to the velocity of approach
TO such an extent that correct messur
ment can not be secured. 1t is but 3
mere matter of convenience, however
to have box of the exact length pre
scrlbed, fer that part of the box above
the weir may be omitted if settins
basin or pool of the same size is con
structed in the ditch above the weir
me boxes and pools should be con
structed fer weirs of larger size!
should be in the same proportion will
respect to the size of the weir as fo]
the one foot. The size of the wei,
e box that is required in order to elimi
nate velocity of approach has beef
found by experiment to be approxima,
~ seven times the cross section of tn
; weir notch. -
Discharge.-The discha rye of weir
. mas been accurately calculated by var
ious engineers who have carried o,
hundreds OF experiments covering We;
OF time, and it is found that under lii
conditions the same depth always prc
duces the same discharge over tn
I same size of weir. The condition
: given for the construction and instaI
: lation Of weirs must be rigidly adhere
to. however, if accurate measurement
Is to be made. The formula whic
has been evolved and which gives tn
discharge of accurately coastructe
: Cippoletti weirs is:
Q equals 1867 LII 80
Where @ Is the number of cubi
feet per second,
L the length of crest in feet
and H the depth of water on tn
| crest in feet.
provided the same is measured frol
, a point level with the crest and u
stream from it at a distance equal t
the length of the crest. In order t
r facilitate for error to a minimum. tai
les of discharge for weirs have bee
r compiled. These may be secured fro
my office or from any reliable book {