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

2. 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

3. 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

4. 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.

5. Fig1.Broad crested weir

6. 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 +

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

8. 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

9. 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

10. 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
= m3 / sec

11. V =
V2 = 2g ( H – h )
52 = 2 × 9.8 × ( H )
25 = (H – 0.20 )
1.274 = H – 0.20
H = m

12. THANK YOU !