Fundamentals of Engineering Design
5.1.6.1 Specific Gage Analysis
Perhaps one of the most useful tools available to the river engineer or geomorphologist for assessing
the historical stability of a river system is the specific gage record. According to Blench (1966):
There is no single sufficient test whether a channel is in-regime. However, for rivers,
the most powerful single test is to plot curves of "specific gage" against time; if the
curves neither rise nor fall consistently the channel is in-regime in the vicinity of the
gaging site for most practical purposes.
A specific gage record is simply a graph of stage for a specific discharge at a particular gaging location
plotted against time. A channel is considered to be in equilibrium if the specific gage record shows no
consistent increasing or decreasing trends over time, while an increasing or decreasing trend is indicative
of an aggradational or degradational condition, respectively (Figure 5.7).
Figure 5.7 Definition Sketch of Specific Gage Record
Figure 5.8 is a hypothetical curve to help illustrate the procedure for developing a specific gage
record. The first step in the development of a specific gage record is to establish the stage-discharge
relationship at the gage for the period of record being analyzed. The stage-discharge relationship is
generally depicted in the form of a stage-discharge rating curve which is a plot of the measured water
discharge versus the observed stage at the time of measurement. A rating curve is developed for each year
in the period of record. A regression curve is then fit to the data and plotted on the scatter plot. The
regression curve is often fit by "eye," but the use of a curve fitting technique is recommended in order
to provide a more consistent procedure that minimizes subjectivity. Since the specific gage record reflects
only observed data it is important that the regression line does not extend beyond the limits of the measured
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