Weirs – behavior of nappe at the ends of a rectangular crest As water flows over a rectangular weir (crest/sill), what happens to the jet stream along the ends of the sill due to side-edge effects?

Introduction:
Side contractions are a hallmark of sharp-crested rectangular weirs that do not span the entire channel width. The nappe narrows near the side edges, reducing the effective width and requiring a correction when computing discharge. This question asks for the qualitative change at the ends of the sill.

Given Data / Assumptions:

• Free (not submerged) rectangular weir with end contractions present.
• Side walls are away from the nappe edges so lateral contraction can occur.
• Sharp crest; typical heads where standard formulas apply.

Concept / Approach:

Because flow lines near the side edges bend around the corners, the jet contracts laterally at each end, forming a reduced effective width. Empirical weir equations incorporate this via an “end contraction” correction, often expressed as L_e = L − k * n * H, where n is the number of contracting ends (usually 2) and k is an empirical factor of order unity.

Step-by-Step Solution:

Verification / Alternative check:

Laboratory observations show visible narrowing and a drop in discharge relative to a suppressed weir (no side contraction), verifying the contraction phenomenon.

Why Other Options Are Wrong:

Expands/does not change: Opposite of observed end-contraction effect.Oscillation/thickening at center: Not the primary behavior explained by edge contraction.

Common Pitfalls:

Confusing end contraction with velocity-of-approach correction; they are separate effects.

Final Answer:

contracts