The Caldwell tribune. [volume] (Caldwell, Idaho Territory [Idaho]) 1883-1928, 1912-01-05

or what is. still better, should grad
ually enlarge as the weir is approach
ed. The axis of the stream should
pass through the weir and perpen-
dicular to it; or, in other. words, the
weir should be located at right angles
to the middle of the stream. The ad-
vancing stream should be free from
internal cross currents or eddles, as
these have an influence upon the dis,
charge.
g. The water should be brought as
nearly as possible to state OF rest
before it enters the weir. An excess
OF velocity of approach due to the We
locity of the advancing current will
affect Gncrease) the: discharge more
than almost any other one thing; Ihls
velocity can be reduced. by widening
and deepening the box or pool above
the weir 1t is calculated that weir.
3 feet long with depth OF water ct
II inches, should not have a greater
velocity of approach than inches per
second, which amount may be allow
ed to increase very slightly where
greater depths over wider weirs are
used. By. constructing the box or
pool above the weir, with a cross sec
ton at least seven times that OF the
weir notch, sufficiently low velocity
of approach is usually secured.
+ 4. 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 near
ly knife edge. as, possible, and by.
keeping the. sides and bottom Of the
box below the weir at good els
lance from the falling stream SO mat
the air may pass freely clear around
and 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 exclude the air from.
under the, issuing stream else COM
plcte contraction will not be secnr, d.
ard tie discharge will he affected. If,
air can pass freely underneath the is
suing stream the level Of the wafer be-
low the weir has no influence on the
discharge An observance of the fol-
lowing rules will. usually insure. COM.
plete 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 ups stream 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 fow
ing over the crest.
e. In order that the lateral con-
traction should not be disturbed the
length of the weir crest should be ap-
proximately three times that Of the
water flowing over it.
; S. The depth 'water5Aowing 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 affected
by waves, wind or eddies; and far
enough up stream so that it will not
be influenced by, the curvature induc-
ed by the falling water. This can not
be done by measuring it exactly over
the crest. The distance above the
crest which is usually found sufficient,
is up stream from the crest distance
equal to the. length of, the crest. A
handy method Of establishing point
from which to measure the head and,
method which is to be recommend!
ed. is to drive a..t1eavy stake up stream;
from the weir and at the side where;
it can be easily reached. This stake
should be driven in firmly so as not to.
be easily changed and should be saw
ed off squarely at a point slightly bel
low the level. of the crest which cam
be determined by the aid Of carr
penter's level and straight edge. A;
heavy spike or nail: should then be!
driven vertically into the top of the
stake and the top of it's, head should
be exactly leveled with the crest BY
the same method. The head of water
flowing over the weir can then, be
found by measuring down from. the
surface. Of, the water to the head of
the nail with an ordinary carpenter s
rule or square. The head must be-
measured within 1900 of the length
of crest lin order to limit the error
to. Il of one per cent. A very accus
rate measurement of the head can only
be made with the hook guage which
consists of long rod terminating i1
an upturned metallic hook, the rod
sliding UP and down on a fixed supt
port being actuated by screw mo!
tion,. while. the. head of water is read
from a vernier. attached to the rod.
The rod is ruin down until the point
of the hook is below water and then
raised slowly until the: point reaches
the surface Immediately over the
hook.
This method eliminates the effect
OF capillary attraction on the rule, but
is not PRACTICAL for farmers weirs on
account OF the expense of installation,
1t care is used, the head can be meas
ured accurately enough for all practi-
cal purposes with an ordinary rule in
the manner previously described. !
The most frequent cause, of error
in weir measurements. is excess We.
locity. of approach. and this must be
guarded against by providing a good
sized pool above the weirs which must
be kept clean and free from sediment