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BFC21103 Hydraulics

Assignment No. 5 Q1. Explain the functions of these hydraulic structures:

a. sluice gate

b. weir

c. spillway

d. energy dissipator structures Solution: a. sluice gate is used to control and measure flow rate in open channel such as in

irrigation canal, river, tidal inlet, and reservoir. (1) b. weir is an overflow structure built across an open channel (such as river) to

measure the flow rate. (1) `

y2 y1

yo

V1

Vo

vena contracta

H

P

sharp-crested weir

Q

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c. spillway is built in a reservoir to allow the flow of excess water in the reservoir to downstream as to ensure the safety of the reservoir. (1)

d. energy dissipator structures are built downstream of a spillway to reduce the velocity and energy of the flow. (1)

Q2. A 1.5 m-wide sluice gate with an opening of 10 cm is discharging water at a rate of 3

m/s. Calculate the flow discharge of the flow if the flow depth behind the sluice gate is 3 m and the downstream flow depth is 2 m. Coefficient of discharge Cd = 0.585.

Solution: Given yo = 3 m, y2 = 2 m, b = 1.5 m, a = 0.1 m, V1 = 3 m/s, and Cd = 0.585 Velocity of flow through sluice gate

( )[ ]2111 2 yygV o =

g

Vyy o 2

21

1 =

81.92

33

2

1 =y

541.21 =y m (1) Underflow influence 459.0541.231 ==yyo m < y2 = 2 m y1 is being submerged by y2 (1)

spillway

reservoir

chute block baffle block end sill spillway

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Thus,

( )2d 2 yygbaCQ o = ( )2381.92585.01.05.1 =Q

3887.0=Q m3/s (1) Q3. A trapezoidal weir with side slope 1:4 and width 0.5 m is used to measure flow rate of a

river. Compute the discharge if the head above the weir is 25 cm and Cd = 0.675. Solution: Trapezoidal weir with side slope 1:4 Cippoletti weir Given L = 0.5 m, H1 = 0.25 m, and Cd = 0.675 Discharge over a Cippoletti weir

23

1d 23

2LHgCQ =

23

25.05.081.92675.03

2 =Q

1246.0=Q m3/s (1)