Weirs and End Contractions – Suppressed vs contracted crest When the end contractions of a rectangular weir are suppressed (i.e., the nappe is allowed to touch the side walls), the number of end contractions n used in the standard discharge formula is taken as what value?

Introduction:
Rectangular weirs are categorized as suppressed or contracted depending on whether the nappe contracts at the side walls. In discharge calculations, a correction using the count of end contractions n is applied to the effective width. This question checks whether the learner knows that in a fully suppressed weir, the side contractions are absent and thus n is taken as zero in the standard formula.

Given Data / Assumptions:

• Rectangular sharp-crested weir under free flow conditions.
• “Suppressed” means the weir crest spans the full channel width and the nappe touches the side walls.
• Standard empirical discharge correlations for small to moderate heads are assumed.

Concept / Approach:

The practical discharge expression for a rectangular weir includes side contraction adjustments. For a contracted weir, the effective width b_e is reduced from the physical width b by a term proportional to n * h, where h is the head over crest and n is the number of end contractions (0, 1, or 2). A “suppressed” weir has no lateral contraction, therefore n = 0, and the effective width equals the channel width. This increases discharge relative to an otherwise similar contracted weir at the same head.

Step-by-Step Solution:

Verification / Alternative check:

Laboratory correlations and design manuals consistently treat suppressed rectangular weirs with n = 0 because the side nappe is constrained by the channel walls and cannot contract laterally.

Why Other Options Are Wrong:

Disagree: Contradicts the standard definition of a suppressed weir.n = 1 for one side suppressed: That setting corresponds to exactly one contracted side, which is not “fully suppressed”.n = 2 for fully suppressed: Reverses the meaning; n = 2 is for two end contractions (both sides free).n depends only on head: Side contraction depends on geometry, not only on head.

Common Pitfalls:

Confusing “suppressed” with “submerged” or thinking the contraction count changes with head. Suppression refers to geometry (absence of side contraction), so n = 0 is a structural condition.

Final Answer:

Agree