Weirs – definition of a narrow-crested weir A sharp-edged weir is called a narrow-crested weir if the width of its crest is __________ half the head of water above the crest (measured upstream):

Introduction:
Weir classification influences discharge equations and head–discharge calibration. This question tests the criterion that distinguishes narrow-crested weirs from broad-crested ones based on crest width relative to upstream head.

Given Data / Assumptions:

• Steady open-channel flow over a sharp-edged obstruction (weir).
• Head measured as water level above crest.
• Negligible approach velocity correction for conceptual classification.

Concept / Approach:
If the crest width is small compared with the head, flow accelerates over a short distance and behaves like flow over a sharp edge (narrow-crested). If the crest is wide compared with the head, the flow attains near-critical conditions on the crest (broad-crested behavior).

Step-by-Step Solution:
1) Define criterion: crest width b compared to head H.2) Narrow-crested if b < 0.5 * H (rule-of-thumb threshold used in many texts).3) Therefore the blank should read “less than”.

Verification / Alternative check:
Empirical discharge coefficients for narrow vs broad crests reflect different flow regimes; lab observations corroborate the width–head criterion.

Why Other Options Are Wrong:

• Equal to / more than: move classification toward broad-crested behavior.
• Indeterminate: classification is standard based on b/H ratio.

Common Pitfalls:
Confusing “narrow-crested” with “sharp-crested” terminology; ignoring approach velocity effects (important for discharge calculation but not for this definition).

Final Answer:
less than