DISCHARGE OVER A BROAD-CRESTED WEIR GOVERNMENT ENGINEERING COLLEGE, DAHOD CIVIL ENGINEERING DEPARTMENT ACTIVE LEARNING ASSIGMENT DISCHARGE OVER A BROAD-CRESTED WEIR FLUID MECHANICS (2130602) PRESENTED BY : PATEL ASHVIN D. (130180106076) GUIDED BY:- PROF. P.M. BARIA

1.Introduction The broad-crested weir is open –channel floe measurement device which combines hydraulic characteristics of both weirs and flumes   Sometimes the “ ramp flume ’’ is used in referring to broad-crested weirs As with related open-channel measurement devices, the broad-crested weir has an upstream converging section, a throat section, and a downstream diverging section

The broad-crested weir can be calibrated for submerged flow conditions ; however, it is desirable , to entire range of discharge under which it is intended to function   When operating under free –flow conditions, critical flow will occur over the crest (sill), the downstream condition do not affect the calibration The broad-crested weir can be calibrated in the field or laboratory; however, a major advantage of the structure is that it can be accurately calibrated based on theoretical equation without the need for independent laboratory measurements Downstream of the structure the depth will not be affected; so, the required head loss is mail=infested (in one way ) as an increase in the upstream depth

2.The broad crested weir:   Board crested weirs are robust structures that are general constructed from reinforced concrete and which usually span the full width of the channel. They are used to measure the discharge of rivers, and are much more suited for this purpose than the relatively flimsy sharp crested weirs. Additionally by virtue of being a critical depth meter, the board crested weir has the advantage that it operates effectively with higher downstream water levels than a sharp crested weir. Only rectangular broad crested weirs will be considered, although there are a variety of possible shapes: triangular, trapezoidal and round crested all being quite common. If a standard shape is used then there is a large body of literature available relating to their design, operation, calibration and coefficient of discharge ( see BS3680 ). However, if a unique design is adopted, then it will have to be calibrated either in the field by river gauging or by means of a scaled-down model in the laboratory.

Fig1.Broad crested weir

If , 2b is more than H, the weir is called a broad-crested weir. This is a weir having very broad crest ( sill ) so that the flow of water over crest may be compared to the flow of water in a channel. Applying Bernoulli’s equation at A and B, O + O + H = O + h +

condition for maximum Maximum discharge will be, Qmax = 1.705 Cd .l . H3/2

2.1 Advantages and disadvantages : The design and construction of the structure is simple, thus it can be relatively inexpensive to install A theoretical calibration based on post-construction dimensions can be obtained, and the accuracy of the calibration is such that the discharge error is less that two percent As with other open – conditions, a staff gauge which is marked in discharge units can be placed upstream; this allows a direct reading of the discharge without the need for tables, curves, or calculators The head loss across the structure is usually small, and it can be installed in channels with flat sloprs without greatly affecting existing upstream flow depths

Disadvantages For water supplies with sediments, there will be deposition upstream of the structure The upstream water depth will be some what higher than it was without the structure Farmer and other water user tend to oppose the installation of this structure because they believe that it significantly reduces the channel flow capacity

EXAMPLE-1 : A broad crested weir 10 m long has velocity of water 5 m/sec. determine the head of water on upstream side of weir , if the head on the downstream side of weir is 20 cm. also find discharge over the weir. On the downstream side of weir is 20 cm. also find discharge over the weir. Take cd = 0.62 Solution: V= 5 m/sec h = 20 cm = 0.2 m L = 10 m cd = 0.62 Discharge Q = Cd . L . h . H = 0.62 × 10 × 0.2 × 5 = 6.2 m3 / sec

V = V2 = 2g ( H – h ) 52 = 2 × 9.8 × ( H - 0.20 ) 25 = 19.62 (H – 0.20 ) 1.274 = H – 0.20 H = 1.474 m

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