Barrage layout concept: In a river barrage, the crest level is normally kept how, relative to gate size, to control pond level and pass floods economically?

Introduction / Context:
A barrage is a gated diversion structure that maintains pond level for canal offtakes and allows efficient flood passage. The choice of crest level and gate size directly influences afflux, energy dissipation, and economy.

Given Data / Assumptions:

• River with variable discharges including floods.
• Gated structure (barrage), not a fixed weir.
• Objective: control headwater level with minimal afflux during high flows.

Concept / Approach:
Keeping a low crest reduces permanent obstruction; large gates provide adjustable opening, enabling low afflux during flood and controlled pond level during lean periods. This flexibility is the principal benefit of a barrage over a high fixed weir.

Step-by-Step Solution:
Select low crest to minimize constant head and reduce sediment deposition upstream.Use large gates to regulate pond level and pass floods efficiently.Therefore: low crest with large gates is the standard arrangement.

Verification / Alternative check:
Case studies of major barrages (e.g., on alluvial rivers) consistently show gated spans above a relatively low crest profile with stilling basins for energy dissipation.

Why Other Options Are Wrong:

• High crest with large/no gates: Increases afflux and siltation; less flexible.
• Low crest with no gates: Behaves like a low fixed weir; poor control.
• Medium crest with small gates: Restricts capacity, increases upstream levels.

Common Pitfalls:
Confusing barrages with weirs; overlooking sediment management and afflux limits when raising crest levels.

Final Answer:
Low crest with large gates