F.2.4 Extending the Rating Curve

For relatively new stations with few discharge measurements, it is often desirable to extend the stage-discharge curve beyond the highest discharge measurement available. This can be accomplished with the use of double logarithmic plotting paper. Plot the full range of measured discharges against stage.

In most cases, this logarithmic plot of measurements will form a straight line in the high flow range. This makes it a useful tool in extending curves beyond the highest discharge measurement. The "curve" as determined in the log plot is then transferred to the standard stage discharge plot.

Discharges estimated by indirect methods may be used to confirm the high end of the curve extension developed by the log plot. Indirect measurement of discharge is determined after the high water event has passed and involves engineering surveys to determine the geometry of the channel, and the application of hydraulic formulae. The most commonly used procedure is the slope-area method, which is described in various engineering texts.

F.2.5 Curve Labelling

The stage-discharge curve must now be numbered and labelled. Use the following procedure:

(a) The first curve used in the first year of operation will usually be designated "Curve No. l." However, another number, such as 31, may be selected if desired, but always increase curve numbers in chronological order of when the curve was derived.

(b) Use a diagonal line from the curve to the notation.

(c) Label any succeeding curves as "Curve No. 2," "Curve No. 3," etc. (or "Curve No. 32," "Curve No. 33," etc.).

(d) Enter the dates for the period of use of each curve in the space provided within the sheet title block.

F.2.6 Stage-Discharge Tables

Having established a best-fit curve, complete with zero flow stage, the next step is to prepare a stage-discharge table from the curve (Figure F-3). The following procedure is used to compile a stage-discharge table from the stage-discharge curve:

1. Identify the number of the stage-discharge table as shown in Figure F-3, which must correspond to the stage-discharge curve number. The data processor's initials and the date the table was compiled are recorded as well.
2. When compiling the stage-discharge table, deviate as little as possible from the exact coordinates, as indicated by the curve. Express discharges to at least the same number of significant figures as required for daily discharges.
3. When the stage-discharge table is completed, plot the values on the curve sheet to ensure that the original delineation of the curve is consistent with the table.
4. In some cases, a new stage-discharge curve is exactly the same as a former curve through part of the range in stage. In preparing the new stage-discharge table for these areas, copy the data from the former table through the range of stage in which the new curve and the former curve are identical. Then compile the new table in the range of stage where the two curves diverge. The new table will cover the entire range of stage.
5. If a stage-discharge curve is extended above or below the original range, the same original number and date identification may be used. However, an explanatory note should be added on Form AQU-07, as well as the date when this extension was made. Note that this applies only if the curve is extended and not if it is revised.
6. Enter the dates in the space provided for the periods of use of each table.

F.2.7 Expanded Stage Discharge Table

For production of discharge data, it is recommended that the discharge figure be tabulated to at least every 0.002 m of gauge height. Figure F-4 illustrates the expanded stage-discharge table format. This example was produced by digitizing a manually drawn curve.