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

4. STAGE MEASUREMENT 53
REFERENCE AND AUXILIARY CAGES AT RECORDING GAGING STATIONS
Nonrecording gages, which were discussed on pages 24-32, are
used both as reference and auxiliary gages at recording streamgaging
stations. As used in this manual a reference gage is the gage
to which the recording instrument is set; it is the base gage for the
recording station. All other nonrecording gages at the recording station
are considered to be auxiliary gages. A detailed discussion follows.
Outsickgage-At bubble-gage stations a nonrecording gage, established
in close proximity to the buhble orifice, acts as the base or
reference gage for checking and resetting the gage height indicated
by the water-stage recorder.
At stations equipped with a stilling well for the operation of a
float-operated stage recorder, there is always the possibility that the
stage in the stilling well may not be representative ofthe stage ofthe
stream. For example, intakes can become plugged, floats can spring a
leak, or oil can leak out of wells or oil tubes. Consequently, a nonrecording
auxiliary gage is installed outside the stilling well so that
the water level of the stream can be determined directly for comparison
with the stage in the stilling well. It is not necessary that the two
observed stages agree precisely; hydraulic conditions at the station
may be such that precise agreement is not possible. Ifa reading ofthe
outside auxiliary gage indicates that an unsatisfactory record is
being obtained by the recording gage, the trouble is rectified immediately,
If immediate repairs are not feasible, or if there has been
an instrument failure, the outside auxiliary gage is used as a temporary
substitute for the recording gage. The outside auxiliary gage can
be read as needed by a local observer to continue the record of stage
until the malfunction of the recording station is rectified.
Staff or wire-weight gages are usually used as outside auxiliary
gages. Outside staff gages located in the pools near the gage structures
are visible in figures 5, 6, 22, 25, and 30; a wire-weight auxiliary
gage on the parapet wall of the bridge is visible in figure 1, and
another is seen in figure 27.
Inside gage-At gaging stations equipped with a stilling well a
nonrecording gage inside the structure is used to indicate the watersurface
elevation in the stilling well. Readings on this inside gage are
compared with readings on the outside auxiliary gage to assure that
the stilling-well intakes are functioning properly. If the intakes are
functioning properly, the inside gage is used for checking and reset·
ting the gage height indicated by the water-stage recorder. In short,
the inside gage is the base or reference gage for the station. Float- or
electric-tape gages, or vertical staff gages, are the inside reference
gages most commonly used in stilling wells.