Rating curve, its uses and drawing method

Rating curve

A rating curve (stage-discharge curve) is the graph or equation showing the relation between the water level (stage) and the discharge at a river section.

Uses

1. Converts continuous stage records into discharge, giving daily, monthly and annual flows.
2. Used for flood forecasting and design of hydraulic structures (weirs, intakes, bridges).
3. Extends the discharge record to high flows by extrapolation, and detects changes in the channel (shift).

Drawing the rating curve

1. Choose a section with a stable control, install a gauge and fix the datum.
2. Measure discharge by current meter at different stages (low to flood), recording the stage each time.
3. Plot stage (vertical axis) against discharge (horizontal axis) on arithmetic paper and draw a smooth curve through the points.
4. Fit $Q = C(G-a)^b$: find $a$ (stage for zero discharge) by trial or by the three-point method, then plot $Q$ against $(G-a)$ on log-log paper. The points should lie on a straight line with slope $b$ and intercept $\log C$ (at $G-a=1$).
5. Check the fit by correlation, and extend the line beyond the highest measurement by extrapolation.
6. Verify regularly with new gaugings; revise the curve if points shift.