Understanding Weir with End Contraction

A weir is a barrier built across a river or channel to change the flow characteristics of the water. It is often used to measure the flow rate or raise the water level. Weirs can be categorized based on various features, including whether they have end contractions. End contraction refers to the phenomenon where the flowing water (the nappe) contracts or narrows laterally as it passes over the weir crest, away from the sides of the channel. This contraction happens because the water particles approaching the weir from the sides have a velocity component parallel to the weir crest towards the centre.

Weir Width vs. Channel Width with End Contraction

When a weir is installed in a channel, its physical width can be equal to or less than the channel width.

• If the weir's sides are flush with the channel sides, there is no end contraction. This is a weir without end contraction, sometimes called a suppressed weir. The effective width for flow calculation is equal to the channel width.
• If the weir's sides are set inwards, away from the channel sides, or if the channel sides stop abruptly at the weir, the water flow contracts as it passes over the weir crest. This causes end contraction.

For a weir with end contraction, the effective width of the weir over which the flow occurs is reduced due to the lateral contraction of the nappe. The physical width of the weir structure itself is typically less than the channel width to allow for this contraction effect at the sides. Therefore, for a weir designed to have end contraction, the width of the weir structure is made less than the width of the channel it is placed in. This allows the water to contract inwards from the sides of the channel as it flows over the weir crest. Consider the options provided:

• Option 1: less than the width of channel
• Option 2: more than the width of channel
• Option 3: equal to width of channel
• Option 4: half the width of channel

Based on the definition and behavior of a weir with end contraction, its physical width must be less than the channel width to facilitate the contraction of the water flow at the ends of the weir crest. The phrase "weir with end contraction" implies that the design allows for this contraction to occur effectively, which is achieved by having the weir structure itself be narrower than the channel.