Elementary theory of Notches and Weirs: A notch is an opening in the side of a measuring tank or reservoir extending above the free surface. A weir is a notch. A notch or a weir is a convenient device for the measurement of discharge in an open channel. A notch or a weir is an obstruction provided in a channel that. 18 Nov Weirs act like miniature dams, blocking the flow of water, and In cases with small discharge, triangular or v-notch weirs are typically used.

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This is a weir whose shape is so designed that the discharge over the weir is proportional to the head of water over the crest. This is because, for all heads the ratio of the head to the wetted length or crest is constant. Water surface downstream of the weir should be at least 0.

In the case of a broad crested weir, there is a support for the falling nappe over the crest in the direction of flow. There are many designs of weir, but commonly water flows freely over the top of the weir crest before cascading down to a lower level.

Let the fall in water level be dh in a small interval of time dt. We know for a rectangular weir with the two end contractions, the discharge is given by —. A notch or a weir is a convenient device for the measurement of discharge in an open channel. A triangular notch has certain advantages over the rectangular notch when used as a gauging device in a hydraulic laboratory.

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According to Francis each lateral contraction also called end contraction is equal to 0. Let I be the length of the weir. As the water flows over the weir, it reaches a uniform depth of flow h, over the crest. Because a weir impounds water behind it and alters the flow regime of the river, it can have an effect on the local ecology. Narrow crested weir Broad crested weir 4. They can be classified into waste weirs and gauging weirs based on the purpose for which they are provided.


Water flow measurement in open channels using weirs and venturi flumes – Part 1: Velocity Of Approach Sometimes, a weir is provided in a stream or a river to measure the flow of water.

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Ecological Engineering, Let it be required to find the time taken for the head of water over the weir to fall from a value H 1 to a value H 2. Definition A structure, used to dam up a stream or river, over which the water flows, is called a weir. The equation for flow through trapezoidal notch is obtained from the equations for rectangular weirw V-notches. Available from Global Engineering Documents at http: If the length of the weir is exactly equal to the width of the approaching stream, there will not be any end contractions.

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They are commonly seen in locations where a river has multiple users who may need nocthes bypass the structure. Weisr using this site, you agree to the Terms of Use and Privacy Policy. Such a weir is called an ogee weir. On larger rivers, a weir can also alter the flow characteristics of ad waterway to the point that vessels are able to navigate areas previously inaccessible due to extreme currents or eddies. Time of Emptying a Reservoir by a Rectangular Weir: In many countries, it is now a requirement by law to build fish ladders into the design of a weir that ensures that fish can bypass the barrier and access upstream habitat.


This is one of the most common types of weir found worldwide.

V Notch Weir Discharge Calculator and Equations

In this way it reaches a maximum rise of 0. If the pressure below the nappe is negative, it is called a depressed nappe. It is in a way, a large orifice having no upper edge. Let the heads on the weirs be H and h. A gauging weir is meant for gauging the rate of flow of water.

We may first calculate the discharge ignoring the velocity of approach. Flow measurement weirs must be well maintained if they are to remain accurate. An anicut is a masonry dam provided across a river for the wers of raising the water level on the upstream side to a sufficient extent in the dry season, so that the water can be carried by gravitation to places where it otherwise could not reach.

Weirs are said to be geometrically similar when the ratio of their corresponding linear dimensions remains the same. The triangular V-notch and trapezoidal weirs are of order one.

Classification or Types of Notches and Weirs | Fluid Mechanics

Typically the reduced river velocity upstream can lead to increased siltation deposition of fine particles of silt and clay on the river bottom that reduces the water oxygen content and smothers invertebrate habitat and fish spawning sites. According to the nature of discharge: American Society for Testing and Materials. Now a more accurate value of the discharge is calculated from the above equation. Time of Emptying a Reservoir by a Triangular Notch: Accordingly, the crest of an overflow spillway on a large notcges may therefore be referred to as a weir.