This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Dam Outlet”.
1. The bar screens used to cover the dam outlets to prevent entry of debris, or ice into the spillway conduits are called __________________
a) gate controlled ports
b) projecting collars
c) trash racks
Explanation: The trash racks are made from steel bars at a spacing of about 5 to 15 cm c/c in both the directions. The floating objects like debris, ice get collected on these racks and can be removed by manual labor as well as automatic power-driven system. The spacing depends upon the maximum size of debris required to be excluded.
2. Projecting collars are provided on sides of a rectangular tunnel of length L made through an earthen dam to increase the seepage path. Their projection length (X) and numbers (N) are decided so as to provide increased seepage path equal to 2NX where 2NX should generally be ______________________
a) greater than L/4
b) greater than L/3
c) greater than L/2
d) greater than L
Explanation: The seepage is reduced by increasing the length of the seepage path by at least 25%. The increase in seepage path must generally be greater than L/4.
3. The outlet provided in a dam body to release water for the downstream water demand is known as ______________________
Explanation: A sluiceway is a pipe or a tunnel that passes through the body of the dam or through some hillside at one end of the dam and discharges into the stream below. The sluiceways are preferred to place outside the limits of the embankments in case of spillways.
4. A 2 m diameter sluiceway at RL 300 m is provided through a concrete overflow dam section to release water to the downstream where tail water level is not more than 275 m. The discharge through this outlet when the water level in the reservoir is at FRL of 330 m is of the order of_________________
a) 20 cumecs
b) 40 cumecs
c) 50 cumecs
d) 80 cumecs
Explanation: The discharge passing through the dam outlet is given by using the equation-
Q = Cd. A. (2gHL)1/2 where Cd is the coefficient of discharge = 0.62 for free over-fall, H = Water head over centre line of sluice = 330 – 300 = 30 m and A = π/4 x 4 = 3.14 m2
Q = 0.62 x 3.14 x (2 x 9.81 x 30)1/2 = 47.23 cumecs.
5. A 4 m diameter tunnel has been constructed through an earthen dam with a bell mouth entry. The water levels on the U/s and D/s sides of the dam are at RL 226 m and 210 m respectively. What will be the discharge through this outlet?
a) 64 cumecs
b) 104 cumecs
c) 165 cumecs
d) 216 cumecs
Explanation: The discharge through the outlet is given by –
Q = Cd. A. (2gHL)1/2 where HL = 226 – 210 = 16 m
Q = 0.8 x (π/4 x 16) x (2 x 9.81 x 16) = 164 cumecs.
6. Which of the following entrance of the sluiceway is most superior?
a) A square-edged entrance
b) A Bell-mouthed entrance
c) A rectangular entrance
d) A circular entrance
Explanation: As compared to Bell-mouthed or other entrance, a square edge entrance is likely to cause more separation of flow and more danger of consequent cavitation. The shape of the bell-mouthed entrance is generally elliptical and the extra cost involved I shaping this entrance is usually justified except for small projects under low heads.
7. What is the correct Douma’s equation for rectangular tunnels or conduits?
a) X2 + 10.4 Y2 = d2
b) 4X2 + 10.4 Y2 = d2
c) 4X2 + 44.4 Y2 = d2
d) X2 – 10.4 Y2 = d2
Explanation: Douma has suggested the following equation –
i. For circular conduits: 4X2 + 44.4 Y2 = d2 where d is the diameter of the circular conduit
ii. For rectangular tunnels or conduits: X2 + 10.4 Y2 = d2 where d is the width or height of the conduit depending on whether the sides or top and bottom curves are being designed. Where X and Y are the coordinates of any point on the curve and d is the width or height of the conduit.
8. What is the value of head loss for the fully open gate and butterfly valves?
a) 0.5 V2 / 2g
b) V2 / 2g
c) 0.2 V2 / 2g
d) 0.04 V2 / 2g
Explanation: The head loss depends upon the type of gate and valves. For fully open gates and butterfly valves, a loss of about 0.2 V2 / 2g (where V is the flow velocity through the conduit) may be taken and is taken as nil for ring follower gates.
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