Terminology of sharp-crested weirs In open-channel hydraulics, the upper edge or surface of a weir over which water flows is known as:

Introduction / Context:
Correct terminology for weir components is essential for interpreting design manuals and test data. The part of the weir that the water just passes over controls head–discharge calibration and is central to weir classification.

Given Data / Assumptions:

• A sharp-crested weir (rectangular or V-notch) under free, fully aerated flow.
• Standard definitions used in hydraulic structures.

Concept / Approach:
The sharp upstream edge of a weir that water passes over is called the crest or sill. After passing over the crest, the sheet of water that springs clear is called the nappe. Confusing crest (a fixed structural edge) with nappe (the flowing sheet of water) is a common mistake.

Step-by-Step Distinction:

Verification / Alternative check:
Textbook definitions consistently use crest (or sill) for the top edge and nappe for the water sheet; field drawings label the crest elevation as the control datum for head measurement.

Why Other Options Are Wrong:
“Nappe” is the water sheet, not the structural edge. “Vein” is an outdated synonym for jet/stream; not the structural part. “None of these” and “apron” (downstream floor) are incorrect here.

Common Pitfalls:
Measuring head too near the crest; proper head measurement point is usually placed upstream to avoid velocity-of-approach effects.

Final Answer:
sill (crest)