Stage
Measurement
The stage of a
stream or lake is the height of the water surface above an
established datum plane. The water-surface elevation
referred to some arbitrary or predetermined gage datum is
called the gage height. Gage height is often used
interchangeably with the more general term stage,
although gage height is more appropriate when
used with a reading on a gage. Stage or gage height is
usually expressed in feet and hundredths of a foot.
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STILLING WELL INSTALLATION WITH WIRE
WEIGHT GAGE |
In stream
gaging, gage heights are used as independent variable in
stage-discharge relation to derive discharges.
Reliability of the discharge record is therefore dependent
on the reliability of the gage-height record as well as
the stage-discharge relation. Several different types of
stage measuring devices are available from the simple to
the most sophisticated technology with automatic digital
data recording.
Gage-height
records may be obtained by a water-stage recorder, by
systematic observation of a water-stage recorder, by
systematic observation of a non-recording gage, or by
noting only peak stages with a crest stage gage. For
U.S. Geological Survey
primary records at gaging stations, two basic types of
water level sensors are used: the float in a stilling
well, or by a gas purge (bubble gage) system which
transmits the pressure head of water in the stream to a
manometer or pressure transducer. The primary instrument
used by the USGS since the 1950's for stage measurement
has been the STACOM manometer with a bubbler pressure
system. However, due to the environmental and personal
safety hazards with the mercury used with this manometer,
these are no longer used and have been replaced by several
different instrument options which are described in this
section.
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PS-2 INSTALLATION |
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Recorders for these sensor elements can be
graphic, such as the Type F and A-71, or digital data
loggers such as the Aqualog series or other commercially
available loggers. Systems can be set up to also record
both graphically and digitally. Non-recording gages
generally used are staff, wire-weight, float-tape and
electric tape.
Non-recording gages are still in general use as auxiliary
gages at water-stage recorder installations to serve the
following purposes:
-
As
a reference gage to indicate the water-surface elevation
in the stream.
-
As
a reference gage to indicate the water-surface elevation
in the stilling well. Gage readings on the stream are
compared with the reference readings in the well to
determine whether stream stage is being obtained in the
well.
-
As
a temporary substitute for the recorder when the intakes
are plugged or there is equipment failure. The outside
reference gage can be read as needed by a local observer
to continue the record of stage during the malfunction.
Staff and wire
weight gages are normally used at non-recording gaging
stations. They are also used as the outside reference
gage at recording gaging stations. Float- and
electric-tape gages and the vertical staff gage are used
in stilling wells. Staff gages are read directly whereas
the other three types are read by measurement to the water
surface from a fixed point.
