Montpelier examiner. [volume] (Montpelier, Idaho) 1895-1937, 1912-03-15
Duty and Measurement
Xxx Of Water Xxx
BY DON n. BARK, U. s. Engineer,
Ia Charge Of irrigation lnvesti-
cations In Idaho.
Oregon Short Line Railroad Dem
onstration Train Lecture.
Continued from last weekJ

Continued from last week.
4. The issuing stream of water
must have full contraction; that is. II
must be sharp and clear cut. This
ia brought about by making the up
stream edge of the weir board as
nearly knife edge as possible. and
by keeping the sides and bottom oil
| the box below the weir at a good dis
lance from the falling stream SO that
the air may pass freely clear around
and in under the issuing stream. The
water below the weir should never bt
allowed to back up to the level OF
| the crest SO as to exclude the ali
| from under the issuing stream. else
complete contraction will not be Sc
cured and the discharge will be affect
ed. If air can pass freely underneath
the issuing stream the level of the
water below the weir has no influence
on the discharge. An observance OF
the following rules will usually in
sure complete contraction.
a. The opening of the weir or the
weir notch must be made in plane
surface perpendicular to the direction
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
Q. 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 1t.
5. The depth of water flowing over
the weir should not be less than two
inches nor more than one-third of the
| length of the crest, and 1t should be
measured where 1t will not be affect
cd by waves, wind or eddies, and far
enough up stream SO that 1t will not
be Influenced by the curvature In
duced by the falling water. This can
not be done by measuring 1t exactly
over the crest The distance above
the crest which is usually found suffi-
Gent, is up stream from the crest a
distance equal to the length OF the
crest.
The most frequent cause of error
in weir measurements is excess We
locity OF approach and this must be
guarded against by providing good
sized pool above the weirs which
must be kept clean and free from
sediment.
"Don'ts" In Regard to Weirs.
l. Do not set a weir Immediately
below a curve in the ditch, for the
curve will cause the water to flow to
the side of the crest.
Q. Do not set it Immediately below
or too close to head gate where the
water has high velocity. as by SO do
ing. 1t will cause too high velocity
Of approach.
3. Don't allow the water below the
weir to back up even with the crest
as 1t will not allow complete contrac-
toin and will cut down the discharge.
4. Do not set the weir any other
way than perpendicular and at right
angles to the flow of the stream.
8. Do not attempt to use too small
a weir. Put In a larger weir where
the water to be measured exceeds a
depth ON the crest of 1-3 the crest
length.
6. Do not allow the pool above the
weir to fill up with sediment, as the
resulting decrease in the cross sec
ton will increase the velocity OF ap-
proach.
/. The weir box should be set with
It's floor even with the bottom of the
Duty Of Water.
To know the proper amount Of ir-
rigation water that should be applied
to the different soils and crops In or
der to make the greatest possible
yield, has always been a serious prob
lem to all irrigators It has always
been known that in order to secure
the biggest possible crop 1t Is neces-
sary to apply different amounts to
different crops and to different soils
with the same crop, but just how
much this amount should be has never
been known only in an indefinite way.
The Irrigation Investigations de-
partment, which I represent in the
state of Idaho, has, In cooperation
with the state engineers ONCE, Deen
carrying on a series of experiments
for the past two years by means of
which 1t is hoped to determine this
very important factor. and we have
secured vast amount of data con-
corning 1t. These experiments have
all been carried out on comparatively
large areas and In a practical way.
It does not give any indication of
how much water is necessary for
maximum crop production on any
soil to merely measure the amount
that some farmer applies for the rea
son that the same or a larger crop
might have been produced with either
civino Him a chanee