Shifting control in stage-discharge relation

Definition

A shifting control is a control (the channel feature that governs the stage-discharge relation) that changes with time, so the stage-discharge relation at the gauging section is not constant. The same stage gives different discharges at different times.

Causes of shifting control

1. Scour and deposition of the alluvial (sand, silt) bed by the flow, the commonest cause: aggradation lowers the discharge for a given stage, degradation raises it.
2. Growth and decay of aquatic vegetation or crops in the channel, which change roughness.
3. Moving bed forms (dunes, ripples) in sandy rivers.
4. Debris, boulders or ice jams at the control.
5. Changes in bank shape due to erosion or collapse.
6. Backwater effects from tributaries, downstream structures or tides.
7. Variable slope (hysteresis) between rising and falling floods; the rising stage gives more discharge than the falling stage for the same stage.
8. Human activity such as sand mining, bridge or weir construction, and dredging.

For such sections frequent discharge measurements are made and the shift is corrected with time.