Raw-water intake design: The entry opening is screened to exclude debris, and its sill elevation is placed at what typical level relative to the source bed to minimise sediment intake while ensuring submergence?

Introduction / Context:
Intake structures must limit debris and sediment ingress while maintaining reliable submergence. Correct vertical placement of the intake opening reduces sediment load and avoids drawing surface scum.

Given Data / Assumptions:

• Screened intake in a river/lake/reservoir.
• Typical engineering rule-of-thumb clearances from bed and surface.
• Goal is to minimise sediment uptake and floating debris.

Concept / Approach:

Sediment concentrations are highest close to the bed; floating debris accumulates near the surface. Placing the intake several metres above the bed, commonly around 2.5 m (subject to local hydraulics), strikes a balance between submergence and sediment avoidance.

Step-by-Step Solution:

Verification / Alternative check:

Design guides present intake sill elevations based on depth, velocity, and sediment regimes, often converging on a few metres above the bed unless special conditions apply.

Why Other Options Are Wrong:

• (a) At water surface would entrain floating matter and air.
• (b) At bed maximises sediment entry.
• (d) and (e) ignore hydraulics and sediment transport principles.

Common Pitfalls:

• Ignoring seasonal depth variations and setting sill too close to bed.
• Underestimating the need for multiple-level offtakes in reservoirs.

Final Answer:

Approximately 2.5 m above the bed of the source.