Regime channels in irrigation engineering: Under what combined conditions will a channel naturally attain regime (stable) conditions without silting or scouring?

Introduction / Context:
Regime channels are those which, over time, adjust their section, slope, and hydraulic radius so that the sediment they carry neither deposits (siltation) nor erodes (scour). This concept is central to alluvial channel design in irrigation engineering, notably in Lacey’s theory and regime approaches.

Given Data / Assumptions:

• An alluvial channel carrying sediment in suspension.
• Bed and banks formed of incoherent (loose) alluvium.
• Exam-style conceptual conditions driving a stable (regime) state.

Concept / Approach:
For a regime to exist, the hydraulic and sediment-transport environment must be steady, allowing the channel to self-form to an equilibrium geometry. This requires: (1) constant discharge, (2) sediment characteristics (silt grade and silt charge) remaining constant, and (3) bed/bank material of the same general nature as the transported sediment so that the channel can reshape itself compatibly.

Step-by-Step Solution:
Identify key regime prerequisites: steady hydraulics and steady sediment supply.Confirm the bed/bank material is incoherent alluvium similar to the load.When these hold together for sufficient duration, the channel evolves to a geometry that is neither silting nor scouring, i.e., regime.

Verification / Alternative check:
Regime equations (e.g., Lacey) implicitly assume such steady conditions. If any of these drivers change materially, the channel re-adjusts, indicating non-regime behavior until a new equilibrium is reached.

Why Other Options Are Wrong:
Discharge alone constant: Not sufficient without stable silt properties and compatible bed/bank material.Uniform flow in incoherent alluvium alone: Still needs steady discharge and sediment properties.Constant silt grade/charge alone: Insufficient if discharge fluctuates significantly.

Common Pitfalls:

• Confusing “regime” with “uniform flow” in rigid channels; regime is an alluvial-channel concept.
• Ignoring the sediment side (grade and charge) when considering stability.

Final Answer:
All the above