PCWA-L 467.pdf - PCWA Middle Fork American River Project ...

66 MEASUREMENT OF STAGE AND DISCHARGE
Where levels from a reference mark show the elevation of the adjustable
index to be in error, the screw holding the index is loosened,
the index is raised or lowered to its proper position, and the index
screw is retightened. The elevation ofthe water surface in the stilling
well is next obtained by measuring with a steel tape to the water
surface from the index or other suitable point of known elevation.
Any further adjustment that is needed to make the gage height agree
with the water-surface elevation is made by adjusting the length of
tape at the float clamp. After all adjustments are complete, a record
should be made for future reference of the tape graduation at which
the tape enters the tape clamp. If the stage ofthe stream is changing,
the valves on the stilling-well intakes should be closed while adjustments
to the float-tape gage are being made in order to maintain a
constant water level in the stilling well.
ELECTRIC-TAPE GAGE
Change in the datum ofan electric-tape gage (p. 28) can only result
from movement of the structure supporting the gage, because the
position of the index of the gage is permanent with respect te the
instrument. An electric-tape gage may also read incorrectly (overregister)
if the tape and weight are insecurely clamped together and
slippage ofthe tape has occurred. Where levels from a reference mark
show the gage index to be in error, nothing can be done about moving
the index, but its new (and true) gage height is made a matter of
record. The original datum of the gage can be maintained, however,
by adjusting the effective length of the tape, that is, by changing the
length o£tape that is inserted and clamped in the gage weight (fig.12).
In other words, when the tape is unreeled so that the bottom of the
weight is, for example, 1.00 ft (0.3 m) below the index, the tape gage
should read 1.00 ft (0.3 m) less than the gage height of the index.
After the adjustment is completed, a record should be made, for future
reference, of the tape graduation at which the tape enters the weight.
Ifthe stage of the stream is changing rapidly at the time a reading is
to be taken in the stilling well, it is helpful to dose the valves on the
stilling-well intakes in order to obtain a constant water level in the
stilling well.
Use of the electric-tape gage as the inside reference gage is practically
a necessity where a small diameter stilling well is used in a cold
climate. In that situation oil must be added to prevent freezing, and
because the stilling well is too small to accommodate an oil tube (p.
48), the oil must be added in the well itself. Because the oil is lighter
than water, the surface of the floating oil in the stilling well will be
higher than the water level at the streamward end ofthe stilling-well