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WEIRS - (flow measuring)

WEIR

• its a barrier across the flowing fluid designed to alter the flow characteristics.

• it is used to prevent flooding, measure discharge, and render the river navigable.

• it take forms of obstructions smaller than the conventional dams.

• Since the geometry of top of the weir is known and all water flows over the weir, depth of water behind the weir can be converted to a rate of flow.

• discharge can be summarized as;

Q=CLHn

• When used for flow measurement it is important that the weir crest is free of rust or impurities.

• Roughness of any form will cause the weir to discharge more water than indicated by the standard discharge equations or tables.

TYPES

1.LABYRINTH WEIR

It uses a trapezoidal shaped weir geometry to increase the weir length. They are versatile structures and can be modified to many applications.

2. BROAD CRESTED WEIR

It is a flat crested structure, with long crest compared to flow thicknes. When the crest is broad, the streamlines become parallel to the crest invert and the pressure distribution above the crest is hydrostatic.

3. SHARP CRESTED WEIR

It allows the water to flow cleanly away from weir. Also known as FAYOUM weir. These weirs are typically .25 inch (6.4 mm) or thinner metal plates. They are in many shapes and styles like rectangular, v notch and cipoletti weirs.... It can exhibit accuracies to about +/-2%.

4. COMPOUND WEIR

For accurate flow measurement over a wider range of flow rates, a compound weir combines two or more types -typically a v-notch weir with a rectangular weir. These weirs are intented to measure no more than 35% of the pipe's open channel flow capacity.

5. V-NOTCH WEIR

Is a triangular channel section, used to measure small discharge values. The upper edge of the section is always above the water level, so the channel is always triangular simplifying calculation. V-notch weirs are preferred for low discharges as head above the weir crest is more sensitive to changes in flow compared to rectangular weirs.

V-notch weirs typically achieve accuracies of 2-5% . V-notch weirs are sized between 22-1/20 and 1200, with 22-1/20,300,450,600,900 and 1200 the common increments- although the free flow discharge equations can be developed from one universal equation for v-notch weirs from 250 to 1200 in size.

DRAW BACKS• Typically increases the oxygen content of

the water which brings a detrimental effect on the ecology of the local water bodies.

• Even though the water around weirs can often appear relatively calm, they can be extremely dangerous places to boat, swim, etc..as the circulation patterns on the downstream side typically known as hydraulic jump, can submerge a person indefinitely.

• It can become a point where the garbage and other wastes accumulates.

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