Section 4: Soil and Water Conservation Engineering
Flow Measurement: Weirs, Flumes & Orifices
Subtopic: Suppressed vs contracted weirs
Question Statement:
According to Francis formula for a sharp-crested rectangular weir with $n$ end contractions, the effective crest length $L'$ is related to nominal length $L$ and head $H$ by:
Options:
(A) $L' = L - 0.1 n H$
(B) $L' = L + 0.1 n H$
(C) $L' = L - 0.2 n H^2$
(D) $L' = L \sqrt{1 - 0.1 n H}$
Answer Key:
A
Step-by-Step Solution & Explanation
Francis' empirical weir formula accounts for end contraction by reducing the effective crest length by $0.1 H$ for each end contraction:
$L' = L - 0.1 n H$
For a weir with 2 end contractions ($n = 2$), $L' = L - 0.2 H$.