The Richfield recorder. (Richfield, Idaho) 1909-1931, 1911-12-14

f0~~
= slightly where greater depths eve
! wider weirs are used. BY construct
ing the box or pool above the weir
5 with a cross section at least seven
times that of the weir notch, a suf
ficiently low velocity of approach is
usually secured.
The issuing stream of water must
have full contraction; that is, it must
be sharp and clear cut. This is
brought about by making the up
stream edge of the weir board as
nearly a knife edge as possible, and
by keeping the sides and bottom of
the box below the weir at a good
distance from the falling stream so
that the air may pass freely clear
around n in under the issuing
stream. The water below the weir
should never be allowed to back up
to the level of the crest so as to ex-
clude the air from under the issu-
ing stream else complete contraction
viii 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 weir
has nc influence on the discharge.
An observance of the following rules
viii 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
hncTn- nhArrI1- h. 90 D.-
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 I
approximately three times that of the I
depth of water flowing over it. I
g. The depth of water flowing over l
the weir should not be less than 2 q
inches nor more than one-third of
the length of the crest, and it should 1
be measured where it will not be af- d
fected by waves, wind or eddies, and I
far enough up stream so that it will L
be influenced by the curvature u
induced by the falling water. This
not be done by measuring it ex-
actly over the crest. The distance =
above the crest which is usually 4
found sufficient, is up stream from f.
crest a distance equal to the 4
ength of the crest. A handy method IL
If establishing a point from which h
o measure the head and a method q
which is to be recommended, is TO !
a heavy stake up stream from o
weir and at the side where it 4
an be easily reached. This stake
'hould be driven in firmly so as not :,
o be easily changed and should be n
off squareIy at a point slight %
y below the level of the crest which :"
be determined by the aid of a ~
arpenter's level and straight edge. -
heavy spike or nail should then be -
vertically into the top of the =
take and the top of its head should &c
re exactly leveled with the crest by y
he same method. The head of water y
over the weir car then be %
ound by measuring down from the sd
of the water to the head of =,
he nail with an ordinary carpen- =,
er's rule or square. The head must ~
e measured within 1900 of the " y
ength of crest in order to limit the ~
to IL of one per cent. A very -
ccdrate measurement of the head -
an only be made with the hook -
udge which consists of a long rod ~
terminating in an upturned metallic -.
ook, the rod sliding up and down -
n a fixed support being actuated by a.
screw motion, while the head of ~
Tater is read from a vernier at
iched to the rod. The rod is run Gu
own until the point of the hook is ie
Glow water n then raised slowly na
ntil the point reaches the surface al
Immediately over the hook. "
This method eliminates the effect VI
f capillary attraction on the rule, q
ut is not practical for farmers te
'eirs on account of the expense of in "a
If care is used, the head 8
an be measured accurately enough =
'r all practical purposes with an or ~
mary rule in the manne previously 49,
escribed. ue
The most frequent cause of error ~
weir measurements is excess We %
'city of approach and this must be W.
larded against by providing a good "a
zed pool above the weirs which -
lust be kept clean and free from y
'Don'ts" in Regard to Weirs- ev
1. Do not set a weir Immediately M
Glow a curve in the ditch, for the I'M
lrve will u the water to flow RIG
the side of the crest. dii
2. Do not set it Immediately be- 8
" or too close to a head gate where we
lie water has high velocity, as by thu
doing, 1t will cause too high a We en-
city of approach. to
3. Don't allow the water below in.
Ie weir to back up even with the on
as it will not allow complete ca,
attraction and will cut down the va
scharge. on
4. Do not set the weir any other an
ay than perpendicular and at right 2 !
giles to the fle=Lef IL e stsea - va
8. Do not attempt to use too small WI
weir. Put in a larger weir where vie
e water to be measured exceeds a PEI