# Irrigation Engineering Questions and Answers – Seepage Theories – Design of Vertical Drop Weir on Bligh’s Theory

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This set of Irrigation Engineering Question Paper focuses on “Seepage Theories – Design of Vertical Drop Weir on Bligh’s Theory”.

1. What is the formula for the upstream apron length according to Bligh?
a) 2.208 (CH)1/2
b) 2.208 (H/13)1/2
c) 2.208 (H)1/2
d) 2.208 (C/H)1/2

Explanation: The upstream apron length according to Bligh is 2.208 (CH)1/2, where C is the creep coefficient and H is the head of water against the weir. The downstream apron length is given by 2.208 (H/13)1/2 which should be sufficient to withstand uplift pressures below it.

2. What is the empirical formula given by Bligh for the design of the top width of the weir wall?
a) B’ = H / (G-1)
b) B’ = H / (G-1)1/2
c) B’ = (H + height of weir)/G
d) B’ = H / (G+1)

Explanation: According to Bligh, the top width of weir wall (B’) is given by H / (G-1)1/2 where H is head of water over the weir wall at the time of maximum flood and G is the specific gravity of floor material. The top width generally varies from 1.5 m to 1.8 m.

3. What is the correct calculation for balance floor length?
a) Total length – (the downstream length + twice the cut-off length)
b) Total length + (the downstream length + twice the cut-off length)
c) The downstream length – twice the cut-off length
d) The downstream length + twice the cut-off length

Explanation: In order to prevent the scour from reaching the pucca floor, upstream and downstream aprons are provided and the length of these aprons are put forward by Bligh’s formulas. The cut-off is provided by Khosla’s theory and then the balance floor length is provided under the crest and on the upstream side.

4. The ordinate of the uplift pressure at a point is found to be 2.8 m while designing the downstream floor thickness of a weir. If the relative density of concrete is 2.4 then the minimum thickness of the floor to be provided for resisting uplift pressure without accounting safety factor is _______
a) 1.16 m
b) 2 m
c) 0.8 m
d) 2.8 m

Explanation: D/s floor thickness is given by (Neglecting the factor of safety value = 4/3)
t = h / (G-1) where h = 2.8 m and G = 2.4
t = 2.8 / 1.4 = 2 m.

5. The minimum thickness (t) of the downstream floor required in the design of weir can be expressed by the equation ___________
a) h / (G+1)
b) (h-t) / (G-1)
c) h / (G-1)
d) 1.33 h /(G-1)

Explanation: As per Bligh’s theory, the thickness of D/s floor is expressed as h/(G-1) where h = ordinate of the H.G line above the top of the floor and G is the specific gravity of the floor material. To allow a suitable F.O.S this value is generally increased by 33%.

6. The width provided at the site for the river to flow and also the length of the weir is ____________
a) waterway
b) launching apron
c) top width
d) bed mean width

Explanation: Waterway is provided between the abutments of a weir for river water to flow. It is also the length of the weir and may be calculated from Lacey’s regime perimeter formula. The weir length is generally increased to avoid the risk of overtopping of the training works.

7. BY increasing the length of the floor on the upstream of the weir axis?
a) the uplift pressure will increase below the floor downstream of the weir
b) the uplift pressure will decrease below the floor downstream of the weir
c) the uplift pressure will remain the same in both cases
d) the uplift pressure may increase or decrease

Explanation: The value of upstream cutoffs has a considerable effect on decreasing the uplift pressure especially from a value of 1 m to 2 m. The provision of sheet pile at the U/s end and sufficient thickness of floor resists the uplift pressure.

8. Calculate the length of the downstream floor for a vertical drop weir having crest shutters, if the total maximum head loss is 2.5 m. Assume the value of Bligh’s constant as 12.
a) 11.62 m
b) 11.80 m
c) 12 m
d) 10 m

Explanation: Total max. head loss (HL) = 2.5 m and C = 12
As per Bligh, the length of D/s floor with crest shutters is given by –
L2 = 2.21 C (HL/13)1/2 = 11.62 m.

9. Calculate the top width of the weir wall (B’) if the head of the water over the weir wall at the time of the maximum flood is 2 m. Take the specific gravity of the floor material as 2.4.
a) 1.7 m
b) 2 m
c) 2.4 m
d) 1.3 m

Explanation: As per Bligh, the top width of the weir wall (B’) is given by B’ = H / (G-1)1/2.
Given G = 2.4 and H = 2 m
B’ = 2 / (2.4-1)1/2 = 1.69 m.

10. Calculate the length of upstream talus if the length of the downstream loose stone talus is 20 m.
a) 20 m
b) 10 m
c) 40 m
d) 15 m

Explanation: For designing the proper weir portion, the length of upstream talus (L4) is kept equal to half the length of D/s talus.
Given the length of loose D/s talus = 20 m
The length of upstream talus = 20/2 = 10 m.

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