--- title: "Rating curve, permanent and shifting controls" subject: "Engineering Hydrology" chapter: 4 key: "semester_5/engineering_hydrology:rating-curve-controls" source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/rating-curve-controls site: BCT Engineering --- # Rating curve, permanent and shifting controls Engineering Hydrology · Chapter 4: Surface Runoff - Asked in: 2079 Baisakh, 3+3 marks - Video: [UNDERSTANDING STREAMFLOW AND RATING CURVES | Hydrology 101 Lesson 10](https://www.youtube.com/watch?v=rbpAvw2g05E) — Clear Creek Solutions - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/rating-curve-controls - Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-4 ## Question How are rating curves developed? Also discuss permanent and shifting controls with appropriate illustrations/figures. ## Short answer A rating curve is developed by measuring stage and discharge over a range, fitting Q = C(G-a)^b. A permanent control (rock ledge) gives a stable curve; a shifting control (sand bed) requires frequent updates and shift corrections due to scour or deposition. ## Answer #### Development of a rating curve 1. Select a gauging section with a stable control and install a gauge. 2. Measure discharge by current meter (or another method) at different stages covering low to high flow, and record the stage at the same time. 3. Plot stage $G$ (y-axis) against discharge $Q$ (x-axis) on arithmetic scale, and also on log-log paper after subtracting the stage for zero flow $a$. 4. Fit a smooth curve $Q = C(G-a)^b$, with $a$, $C$ and $b$ found by trial or regression. 5. Check by correlation, and extend the curve to flood stage by extrapolation (log-log straight line or conveyance method). 6. Check the rating regularly with new measurements, and revise it when the points depart from the curve. #### Control The **control** is a physical feature of the channel at or downstream of the gauge that governs the stage-discharge relation. **Permanent control:** the relation does not change with time. Examples: a rock ledge, a weir or a narrow rock gorge. The same stage always gives the same discharge, and a single rating curve is valid for years. ```text water ~~~~~~~~~\ level \___ <- rock ledge / weir (stable control) bed ____________/ \_______ ``` **Shifting control:** the bed or banks are made of sand or silt, so scour, deposition, vegetation growth or debris change the section and the curve shifts. A given stage then gives different discharges at different times. ```text Stage | / / / <- curves shift with time | / / / (scour: right; silting: left) | / / / +------------------- Q ``` For a shifting control, frequent discharge measurements are needed, and the rating curve is adjusted with time-varying shift corrections (a shift curve). Where the shift is caused by variable backwater, the stage-fall-discharge relation is used instead. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is a rating curve developed? A. Measure rainfall B. Use pan data C. Measure discharge at various stages with the stage, and fit Q = C(G-a)^b D. Guess Answer: C. Simultaneous stage and discharge data are plotted and a curve fitted. (Key point: Development) 2. What is a control? A. A channel feature at or downstream of the gauge governing the stage-discharge relation B. A gauge C. A bridge D. A rainfall station Answer: A. Examples include rock ledges, weirs, and riffles. (Key point: Control) 3. What characterises a permanent control? A. Sandy bed B. Needs frequent measurement C. The same stage always gives the same discharge for years D. Curve shifts with time Answer: C. Rock ledge or weir does not change, so one curve holds. (Key point: Permanent control) 4. How is a shifting control handled? A. One curve forever B. Frequent measurements and shift corrections to the curve C. Use rainfall D. Ignore it Answer: B. Sand or silt beds change with scour and deposition, so the curve is adjusted with time. 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