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

4. STAGE MEASUREMENT 23
ferred to some arbitrary or predetermined gage datum is called tbe
"gage height." Gage height is often used interchangeably witb the
more general term I'stage," although gage height is more appropriate
when used to indicate a reading on a gage. Stage or gage beight is
usually expressed in feet and hundredths of a foot, or in meters and
bundredths or thousandths of a meter.
Records of gage heigbt are used with a stage-discharge relation in
computing records of stream discharge. The reliability of the discharge
record is therefore dependent on the reliability of the gageheight
record as well as on the accuracy of the stage-discharge relation.
Records of stream stage are also useful in themselves for such
purposes as the design of structures affected by stream elevation and
the planning of flood-plain use. The gage-height record of a lake or
reservoir provides, in addition to elevations, indexes of surface area
and volume of the water body.
A record of stage may be obtained by systematic observations of a
nonrecording gage or by means of a water-stage recorder. Specialpurpose
gages that do not give a complete record of stage are discussed
on pages 74-78. The advantages of the nonrecording gage are
the low initial cost and the ease ofinstallation, The disadvantages are
the need for an observer and the lack of accuracy of the estimated
continuous-stage graph drawn through the plotted points ofobserved
stage. For long-term operation the advantages of the recording gage
far outweigh those of the nonrecording gage, and therefore the use of
the nonrecording gage as a base gage is not recommended. However,
at a recording-gage station, one or more nonrecording gages should he
maintained as auxiliary gages for the operation of the station (p.
53-54). Telemetering systems are often used to transmit gage-beigbt
information to points distant from the gaging station (p. 54-59).
DATUM OF GAGE
The datum of tbe gage may be a recognized datum, such as mean
sea level, or an arbitrary datum plane chosen for convenience. An
arbitrary datum plane is selected for the convenience of using relatively
low numbers for gage heights. To eliminate the possibility of
minus values of gage height, the datum selected for operating purposes
is below the elevation of zero flow on a natural controL Where
an artificial control is used, the gage datum is usually set at the
elevation of zero flow.
As a general rule a permanent datum should be maintained so that
only one datum for the gage-height record is used for the life of the
station. An exception occurs at gage sites where excessive streambed
scour, after installation of the station, results in low-flow stages having
a negative gage height. In that situation a change in gage datum