Weirs and Notches — Name of the Flowing Sheet In open-channel measurement, the thin sheet of water that flows over the crest of a notch or a weir is commonly known as:

Introduction:
Weirs and notches are standard devices to measure discharge in open channels. The flowing sheet over the crest has a specific name because its behavior (aeration, contraction) affects the head–discharge relationship and calibration.

Given Data / Assumptions:

• Sharp-crested notch or weir under free, fully aerated overflow.
• Steady approach conditions; head measured above the crest.
• Standard terminology in hydraulics texts.

Concept / Approach:
The thin, free-falling sheet that separates from the crest is called the nappe (also termed the vein). Its aeration and shape determine whether the standard formulas (with the 3/2 head exponent) apply without submergence corrections.

Step-by-Step Solution:
Identify crest (sill) as the physical top edge of the weir.Recognize the free-falling sheet of water as the nappe/vein.Therefore the correct term for the sheet is ‘‘nappe or vein’’.

Verification / Alternative check:
Diagrams for sharp-crested weirs label the thin falling sheet explicitly as the nappe; laboratory observations confirm this terminology.

Why Other Options Are Wrong:
Sill or crest refers to the edge/structure, not the water sheet.
Orifice refers to flow through a small opening with enclosed boundaries, not free overfall.

Common Pitfalls:
Confusing the crest (structure) with the nappe (flow); ignoring the need for aeration to maintain a clean, separated nappe.

Final Answer:
nappe or vein