Silting of irrigation channels: What is the principal (main) cause leading to silting up of a channel in the field?

Introduction / Context:
Silting up of canals reduces discharge capacity and increases maintenance costs. Identifying the primary cause helps in rehabilitation and proper design of slopes and section dimensions for alluvial channels.

Given Data / Assumptions:

• An irrigation channel conveying sediment-laden flow.
• Multiple potential causes of silt deposition are listed.
• We seek the principal cause in practice-oriented terms.

Concept / Approach:
Deposition occurs when sediment-carrying capacity reduces. The most direct hydraulic driver of reduced transport capacity is insufficient velocity, which is often due to an inadequate bed slope. A slope that is too mild lowers velocity and shear, encouraging deposition along the bed and at transitions.

Step-by-Step Solution:
Sediment transport capacity ∝ flow velocity and shear stress.If slope is too mild, velocity falls below the threshold required to keep the sediment in suspension/bedload.Silt settles out, causing progressive silting of the channel.

Verification / Alternative check:
Even with good regulators and outlets, a persistently mild slope sustains low velocities and favors deposition. Conversely, correcting the slope (or increasing fall/bed drop) rapidly improves transport capacity, often reversing siltation trends after desilting.

Why Other Options Are Wrong:
Non-regime section: Contributes but is a broader long-term state; the immediate hydraulic driver is slope/velocity.Defective head regulator/outlets: These create local issues, but not typically the primary, system-wide cause.

Common Pitfalls:

• Attributing chronic silting solely to regulator settings while ignoring inadequate bed gradient.
• Overlooking the role of seasonal discharge variability when diagnosing slope issues.

Final Answer:
Inadequate bed slope (too mild a gradient)