Bissoy.com
Doctors Medicines MCQs Q&A

When the crest length of the weir equals the width of the channel, on which water flows, the number of end contractions 'n' in Francis formula will be

When the crest length of the weir equals the width of the channel, on which water flows, the number of end contractions 'n' in Francis formula will be Correct Answer zero

Explanation:

Weirs are commonly used for the measurement of open channel flow rate.

Suppressed weir: A rectangular weir whose notch or opening sides are coincident with the sides of the approach channel is known as a Suppressed weir. In this type of Weir crest length (l) is equal to the width of the channel.

If the crest length of the weir is less than the width of the channel, it is known as a Contracted weir.

Francis formula:

When the length of the weir is less than the width of the stream, we find there will be a lateral contraction at each end such a weir is called a contracted weir.

According to Francis each lateral contraction (also called end contraction) is equal to 0.1 H.

If the actual length of the weir is L, then the effective length of the weir will be (L – 0.2 H).

In some cases there may be intermediate obstacles like piers over the weir. In such a case if l is the length of the weir after making deductions for the widths of the obstacles and if there are n lateral contractions the effective length of the weir will be (L – 0.1 nH).

Q =  1.84 (L- nH) H3/2

If the length of the weir is exactly equal to the width of the approaching stream, there will not be any end contractions(n= 0). Such a weir is called a suppressed weir, and for such a weir Francis formula simplifies to

Q = 1.84 LH3/2

Related Questions

A weir is said to be narrow-crested weir, if the width of the crest of the weir is __________ half the height of water above the weir crest.
A weir is said to be broad crested weir, if the width of the crest of the weir is ________ half the height of water above the weir crest.
A weir is said to be narrow-crested, if the width of the crest of the weir is Where H = height of water above weir crest
The head of water over a rectangular weir is 38 cm. The length of the crest of the weir end contraction suppressed is 1.3 m. Find the discharge using the Francis formula.
The head of water over a rectangular weir is 28 cm. The length of the crest of the weir end contraction suppressed is 1.27 m. Find the discharge using the Francis formula.
The head of water over a rectangular weir is 26 cm. The length of the crest of the weir end contraction suppressed is 1.25 m. Find the discharge using the Francis formula.
P. A weir is a hydraulic structure where majority or the entire ponding is achieved by a raised crest and smaller part or nil part by the shutters. Q. A weir is a hydraulic structure that is made by constructing the width of the channel only. R. A barrage is a hydraulic structure that is made by creating a choking condition only. S. A barrage is a hydraulic structure where majority or the entire ponding is achieved by a shutter and smaller part or nil part by the raised crest. Identify the correct statements in the above.
The discharge over a rectangular weir, considering the velocity of approach, is (where H1 = H + Ha)
(H1 = Total height of water above the weir, H = Height of water over the crest of the weir and Ha = Height of water due to velocity of approach)
According to Francis formula, the discharge over a rectangular weir is (where n = Number of end contractions)
Consider the following statements with respect to weir under discussion: 1. Its design corresponds to soft sandy foundation. 2. The difference in weir crest and downstream river bed may not exceed 3. When water passes over it, the longitudinal location of the formation of a hydraulic jump is variable. This weir is of the type