Fundamentals of Engineering Design
data. For this reason there may be some years where the gage reading for very large or small discharges
may have to be omitted. In this case there will simply be a gap in the specific gage record for that year.
In some instances, there may be insufficient data to construct a rating curve for each year in the
1970 1971 1972
Figure 5.8 Development of Specific Gage Record
period of record. In these situations it may be necessary to combine the data from several years in order
to obtain a large enough data set to develop a reliable rating curve. In this case the rating curve would
reflect conditions over some time periods such as five or ten years.
Once the rating curves have been developed, the discharges to be used in the specific gage record
must be selected. This selection will depend, in a large part, on the objectives of the study. It is usually
advisable to select discharges that encompasses the entire range of observed flows. This is important
because the behavior of the low and high flows are often quite different. For this example discharges of
100 cfs, 500 cfs, and 1,000 cfs were selected. The stage for each of these flows at each time period
is determined from the regression curves in Figure 5.8. For example, the stage for 500 cfs was 26 feet in
1970, 29.8 feet in 1971, and 22.5 feet in 1972. This data is then plotted to produce the specific gage
record shown in Figure 5.8. Note that in 1971 there is no observed point on the specific gage record for
the 1,000 cfs flow because the rating curve only extended up to 800 cfs that year.
The development of a specific gage record is a simple, straightforward procedure. However, the
interpretation of specific gage records is more complex. The following paragraphs provide examples of
the use of specific gage records.
To utilize a specific gage record properly it is necessary to understand exactly what it is a specific
gage depicts. Specific gage records are often used to show aggradational and degradational trends in a
river. Aggradation and degradation are terms that are generally associated with the increase or decrease