Weirs and nappes terminology: when no air is entrained below the nappe and the jet adheres to the downstream face, what is the nappe called?

Introduction / Context:
Over weirs, the sheet of water that flows over the crest is called the nappe. Its behaviour depends on ventilation and pressure beneath the nappe, which influences discharge coefficients and downstream conditions.

Given Data / Assumptions:

• Sharp-crested or broad-crested weir with visible nappe.
• No air space under the nappe; the jet adheres to the downstream face.
• Atmospheric conditions elsewhere, steady flow.

Concept / Approach:

If the space beneath the nappe is freely ventilated, the pressure under the nappe is atmospheric and the nappe separates cleanly (free nappe). When ventilation is insufficient, sub-atmospheric pressure forms, drawing the nappe toward the downstream face; the jet then adheres, producing a clinging (or depressed) nappe depending on the exact profile. The required technical term for adherent behaviour is clinging nappe.

Step-by-Step Solution:

Verification / Alternative check:

Textbook definitions classify nappes as free (ventilated) and clinging/depressed (unventilated). Adherence distinguishes clinging from a separated free jet.

Why Other Options Are Wrong:

(a) Free nappe requires ventilation and detachment. (b) Depressed nappe describes lower-than-atmospheric pressure causing contraction but without full adherence; the prompt explicitly says it adheres, so clinging is precise. (d) Ventilated nappe contradicts the condition. (e) Submerged nappe is when tailwater submerges the crest.

Common Pitfalls:

Using “free” whenever the jet is visible; ignoring the role of ventilation under the nappe.

Final Answer:

clinging nappe