Dam Spillways — Weir Profile Commonly Used Which type of weir profile is most commonly adopted for dam spillways to match the underside of the free overfall (design nappe) and pass large discharges efficiently?

Introduction:
Spillways must pass flood flows safely while minimizing cavitation and structural loads. The crest shape that conforms to the underside of the ideal nappe for a sharp-crested overflow is called the ogee profile and is widely used in dam engineering.

Given Data / Assumptions:

• High-head spillway on a dam.
• Free overflow with adequate aeration.
• Crest designed for the project design head.

Concept / Approach:
An ogee weir has an S-shaped crest approximating the lower nappe of a sharp-crested weir at the design head. This alignment reduces flow separation, minimizes negative pressures, and improves discharge capacity compared to rectangular or broad-crested forms for the same head.

Step-by-Step Solution:
Identify the functional need: efficient, safe spillway discharge.Match to crest profile that follows the design nappe: ogee.Conclude that ogee weirs are the standard for dam spillways.

Verification / Alternative check:
Dam design manuals and case studies show ogee crests as the prevalent choice for gravity and arch dams under free overflow conditions.

Why Other Options Are Wrong:
Narrow or broad-crested weirs are used in canals and measurement; submerged weirs operate under tailwater control and are not typical spillway crests.

Common Pitfalls:
Using an ogee crest without considering aeration or cavitation protection; failing to adjust the crest for different design heads.

Final Answer:
Ogee weir