The Idaho Republican. [volume] (Blackfoot, Idaho) 1904-1932, 1911-12-22

water of IL inches, shoald not hav
|, a greater velocity of approach tina
. 6 inches per second, which amour
may be allowed to increase yer
slightly where greater depths owe
wider weirs are used. By construc,
ing the box or pool above the wei
with cross section at least seve
times that of the weir notch, a su
ficiently low velocity of approach i
usually secured.
The issuing stream of water mus
have full contraction; that is, it mus
be sharp and clear cut This
brought about by making the up
stream edge of the weir board a
nearly,a knife edge as possible, an,
by keeping the sides and bottom O
the box below the weir at a goot
distance from the falling stream 81
that the air may pass freely cleal
around and in under the issuing
stream. The water below the wei
should never be allowed to back III
to the level of the crest SO as to eX
clude the air from under the issu
ing stream else complete contraction
will not be secured and the discharge
will be affected If air can pass
freely underneath the issuing stream
the level of the water below the well
has no influence on the discharge
An observance of the following rules
will usually insure complete con-
traction.
a. The opening of the weir or the
weir notch must be made in a plane
surface perpendicular to the direc-
ton of the flow of water.
b. The opening itself must have a
sharp edge on the upstream side and
the walls should be beveled away on
the downstream side.
C. The distance of the crest of the
weir from the bottom of the box or
basin should be at least three times
the depth of the water on the crest.
d. The distance of the sides of the
weir notch from the sides of the box
or channel should be at least twice
as great as the depth of the water
flowing over the crest.
e. In order that the lateral con-
traction should not be disturbed the
length of the weir crest should be
approximately three times that of the
depth of water flowing over it.
8. The depth of water flowing over
the weir should not be less than 2
inches nor more than one-third of
the length of the crest, and it should
be measured where it will not be af-
fected by waves, wind or eddies, and
far enough up stream so that it will
not be influenced by the curvature
induced by the falling water. This
can not be done by measuring it ex-
actly over the crest. The distance
above the crest which is usually
found sufficient, is up stream from
The crest a distance equal to the
ength of the crest. A handy method
)f establishing point from which |
% measure the head and a method |
which is to be recommended, Is to
1rive heavy stake up stream from
he weir and at the side where it
IAN be easily reached This stake
should be driven in firmly so as not
o be easily changed and should be
aWed off squarely at point slight
y below the level of the crest which
an be determined by the aid of a
level and straight edge.
heavy spike or nail should then be
lriven vertically into the top of the
take and the top of it's head should
re exactly leveled with the crest by
he same method. The head of water
lowing over the weir can then be
ound by measuring down from the
urface of the water to the head of
he nail with an ordinary carpen-
er's rule or square. The head must
re measured within 1900 of the
ength of crest in order to limit the
rror to IL of one per cent. A very
ccurate measurement of the head
an only be made with the hook
uage which consists of long rod
in an upturned metallic
ook, the rod sliding up and down
n fixed support being actuated by !.
screw motion, while the head of
'ater Is read from vernier at
ached to the rod. The rod is run :
own until the point of the hook is !
Glow water and then raised slowly
ntil the point reaches the surface
over the hook.
This method eliminates the effect {
capillary attraction on the rule,
ut is not practical for farmers
eirs on account of the expense of in
If care is used, the head Z
an be measured accurately enough
'r all practical purposes with an or :
mary rule in the manner previously
escribed.
The most frequent cause of error
weir measurements Is excess We
'city of approach and this must be
uarded against by providing a good :
zed pool above the weirs which
lust be kept clean and free from {
ediment.
"Don'ts' in Regard to Weirs- q
1. Do not set a weir Immediately r
Glow curve in the ditch, for the i
lrve will cause the water to flow r
, the side of the crest. d
2. Do not set it Immediately be- 8
w or too close to a head gate where v
lie water has high velocity, as by t
, doing, it will cause too high We e
city of approach. t
3. Don't allow the water below Il
lie weir to back up even with the @
est as 1t will not allow complete e
IN ~ nf i'm and will ont douyu ba -,