Weirs – terminology for the overflow edge In open-channel flow measurement, the top edge of a weir over which the water flows is called the:

Introduction / Context:
Accurate terminology is important when citing discharge formulas and calibration data for hydraulic structures. Weirs are sharp-edged or broad-crested barriers used to measure or control flow in open channels.

Given Data / Assumptions:

• Free overfall occurs; air can circulate beneath the nappe for sharp crests.
• Standard weir configurations: rectangular, triangular (V-notch), trapezoidal (Cipolletti), and broad-crested.
• No special geometry beyond naming is required.

Concept / Approach:
The overflow edge is formally called the crest (also termed the sill in some civil-hydraulics texts). Water that passes over forms a sheet known as the nappe. Misnaming crest as nappe or as an orifice leads to misuse of equations meant for free-surface versus pressurized openings.

Step-by-Step Solution:
Step 1: Identify the physical feature: the upper edge of the barrier.Step 2: Recall standard term: crest (sill).Step 3: Distinguish it from the nappe (the flowing sheet) and from an orifice (a closed, pressurized opening).Step 4: Choose “sill or crest.”

Verification / Alternative check:
Design handbooks label the elevation of the overflow edge as the crest level; freeboard and head are referenced to this crest elevation.

Why Other Options Are Wrong:

• Nappe or vein: refers to the sheet of water, not the edge.
• Orifice: applies to closed-conduit openings below a water surface.
• Control sill only in dams: semantics aside, “crest” is the standard term for the weir top edge.
• Toe: denotes the downstream foot of a structure, not the overflow edge.

Common Pitfalls:
Using orifice discharge equations for a sharp-crested weir because of terminology confusion.

Final Answer:
sill or crest