Head regulator silt control: King’s vanes used at distributary heads are provided with which typical geometric and construction characteristics?

Introduction / Context:
At distributary head regulators, King’s vanes (guide walls/vanes) are installed to streamline approach flow, reduce cross currents, and minimize excessive silt entry into the offtake. Their geometry and placement are empirical, based on experience and model studies.

Given Data / Assumptions:

• Main canal with offtake distributary at an angle.
• Objective: improve approach hydraulics and silt exclusion.
• Typical dimensions and spacing used in practice.

Concept / Approach:
King’s vanes are low-height walls that align streamlines toward the offtake, reducing transverse components that carry silt. The construction (R.C.C. or steel plates), height relative to flow depth, spacing as multiples of height, and extension beyond a 2:1 guide line are all characteristic features used to achieve the objective.

Step-by-Step Solution:
1) Provide robust vanes (R.C.C./steel) of adequate thickness (~8 cm).2) Set vane height to about 1/3–1/4 of canal depth so they influence surface/subsurface flow without acting as barriers.3) Space vanes at multiples of their height to create progressive alignment.4) Extend vanes ~1 m beyond a line at 2:1 to-axis to complete flow guidance toward the head regulator.

Verification / Alternative check:
Physical and numerical models show improved intake conditions and reduced silt ingress with properly proportioned vanes. Field retrofits often follow similar proportions.

Why Other Options Are Wrong:

• Choosing any one feature alone misses the integrated geometric arrangement needed for efficacy.

Common Pitfalls:

• Oversized vanes causing stagnation zones and sediment deposition at the head.
• Too small or widely spaced vanes yielding negligible guidance.

Final Answer:
All of the above.