Question

A submerged weir is one in which the water level on the down stream side of the weir is:

• A. Just at the crest level
• B. Below the crest level
• C. Above crest level
• D. At same elevation as water surface on upstream

Answer 1

The correct option is C Above crest level

1. When the water level on the downstream of the weir is above the crest level of weir, then the weir is said to be submerged weir.
2. During floods often weirs constructed across river become submerged.
3. Submerged weirs have larger discharging capacity as compared to freely discharging weirs.
4. $\text{The discharge over submerged weir may be obtained by dividing it into two part}{Q}_1\text{and}{Q}_2.$

$\text{Where,}{Q}_1\text{and}{Q}_2\text{are the discharge through the free and the drowned portion respectively.}$
$Q_1=\frac{2}{3}{C}_{d_1}L\sqrt{2g}(H_1-H_2{)}^{\frac{3}{2}}$

$Q_2=C_{d_2}L\times H_2\sqrt{2g(H_1-H_2)}$

$\text{Where,}{H}_1\text{and}{H}_2\text{are respectively the heads on the upstream and downstream of weir.}$

L = Length of weir

$C_{d_1}\text{and}{C}_{d_2}\text{are coefficient of discharge of freely and drowned portion.}$