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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The discharge of the nappe, in this case, depends upon the amount of ventilation and the negative pressure. Our equations are shown below. Notfhes is a weir whose shape is so designed that the discharge over the weir is proportional to the head of water over the crest. Ecological Engineering, 91, The notch angle must be between 20 o and o 0.

As in the case of orifice, the actual discharge through a notch or weir can be found by multiplying the theoretical discharge by a coefficient of discharge to allow for energy losses and the contraction of the cross-section of the stream at the bottom and sides. If the bulk of the weir is thicker than 0. Sometimes masonry structures are built on the top of the raised weir in which openings fitted with sluice gates are provided.

A weir is a notch on a large scale, used, for example, to measure the flow of a river, and may be sharp edged or have a substantial breadth in the direction of flow.

Flow measurement weirs must be well maintained if they are to remain accurate. Let v a be the velocity of approach. A notch weir is any weir where the physical barrier is significantly higher than the water level except for a specific notch often V-shaped cut into the panel. A dam is usually specifically designed to impound water behind a wall, whilst a weir is designed to alter the river flow characteristics.


Weirs for this purpose are commonly found upstream of towns and villages and can either be automated or manually operated.

Irrigation in the Pacific Northwest

High-order polynomial weirs are providing wider range of Head-Discharge relationships, and hence better control of the flow at outlets of lakes, ponds, and reservoirs. We know, in the case of a sharp crested weir the nappe as it leaves the crest springs or rises slightly at the lower surface.

All of the references show similar curves for C and k vs. Weirs Dams by type Rivers Hydraulic engineering.

Classification or Types of Notches and Weirs | Fluid Mechanics

If you compare it to the graphs shown in the references, it looks nearly identical which implies that motches fits are very good. V-Notch Triangular Weir Calculator.

The sheet of water which springs free from the crest is called the nappe. If these conditions are not met, it can make flow measurement complicated, inaccurate or even impossible.

Such an arrangement is helpful to regulate the full supply level or the flood level in the river as well as in the channels which amd off from the river. Ventilation Of Rectangular Weirs It has been observed that whenever water is flowing over a rectangular weir, having no end contractions, the nappe i. Weirs constructed for this purpose are especially common on the River Thames wrirs, and most are situated near each of the river’s 45 locks.

Let I be the length of the weir. Consider a weir provided across a river. In certain cases flash boards are provided over the crest of the weir to regulate the full supply level in the river and the channel during the dry season.

The energy created by the change in height of the water can then be used to power waterwheels and power sawmills, grinding wheels and other equipment. Anv a weir is called an ogee weir. Retrieved 20 May The total discharge Q consists of components Q 1 and Q 2. Retrieved from ” https: Variation of Viscosity of Fluid: The crest of the weir is horizontal.


The only difference between a notch and a weir is that the notch of a small size and the weir is of a bigger one. Let at any instant the head of water over the weir be h. A common distinction between dams and weirs is that water flows over the top crest of a weir or underneath it for at least some of its weigs.

Notches and Weirs

Thus in an ogee weir, the solid boundary of the weir exactly coincides with wrirs bottom surface of the nappe of the sharp crested wiers under the designed head. Since the geometry of the top of the weir is known and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow.

A weir is also a notch but it is made on a large scale. Archived copy as title Articles lacking in-text citations from August All articles lacking in-text citations. A notch is an opening in the side of a measuring tank or reservoir extending above the free surface. If the sheet wfirs water springs free as it leaves the crest, the weir is called a sharp crestel weir.

The rate of flow can be determined by measuring the height of the upstream water level. Let H be the head of water over the weir. Let their lengths be L and I. Archived from the original on Norches such a case, the water, approaching the weir, has got some velocity, known as velocity of approach.