This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Culverts Construction – High Flood Discharge Computation”.
1. What is the design frequency discharge for road bridges and culverts?
a) 1 in 100 years
b) 1 in 50 years
c) 1 in 75 years
d) 1 in 25 years
Explanation: Rail bridges may be designed for 1 in 100 year’s frequency discharge whereas road bridges and culverts are normally designed for 1 in 50-year discharge. The various empirical formulas and the rational formula are used for calculating the same.
2. The existing width of the stream is contracted at the bridge site by _________
a) U/s wing walls
b) D/s wing walls
c) Canal wings
d) Drain wings
Explanation: It is done by U/s wing walls or training works so as to make it equal to regime width and the bridge may then be spanned across it. This contraction of the stream up to regime width leads to the economy since the bridge length is reduced.
3. For purely rigid streams, the foundation of the bridge substructure is taken above the bed.
Explanation: From theoretical considerations, there will be no scouring and the foundation of the bridge sub-structure will have to be taken below the bed by a nominal amount not by scour considerations. It is done so as to provide sufficient grip on them.
4. Consider the following statements for quasi-alluvial streams.
i. The bed is not pitched and sides are banked with stable soils or pitching
ii. The bridge span length is equal to the length of water line between banks measured along HFL
iii. The stream is not contracted
Which of the following statements are correct?
a) i and ii
b) ii and iii
c) i and iii
d) i,ii and iii
Explanation: The stream is contracted so as to keep span length (L) lesser than the regime width (W) for the quasi-alluvial stream. The normal scour depth (D) is given by-
D = D’ (W’/L)0.61 where W’ = the natural unobstructed width of the stream and D’ = the scour depth in the quasi-alluvial stream having L=W’.
5. The relation between the scour depth (R) and Regime scour depth (Re) for an alluvial river having regime width (W) and contracted width (B) is given by ______
a) R = Re (W/B)0.61
b) R = Re (B/W)0.61
c) R = Re (W/B)0.33
d) R = Re (W/B)0.33
Explanation: For purely alluvial streams, the Lacey’s scour depth (R) is given by R = 1.35 (q2/f)1/3 where q is the discharge intensity per unit width of the stream at the bridge site.
The value of Lacey’s regime scour depth (Re) is given by R = Re (W/B)0.61.
6. The Lacey’s scour depth R given by equation R = 0.473 (Q/f)1/3 is applicable when the river width is equal to ___________
a) actual river width
b) regime width
c) any contracted width
d) span length
Explanation: For a purely alluvial stream having a span length (L) equal to regime width (W), Lacey’s regime scour depth is given by R’ = 0.473 (Q/f)1/3.
Silt factor (f) is obtained by the equation f = 1.76 d1/2 where d is the average size of particle in mm.
7. Culverts are all those road bridges whose spans are up to ________
a) 6 m
b) 8 m
c) 10 m
d) 12 m
Explanation: When the bridge span is less than 8 m, it is called Culvert Bridge. The minor bridge has a span length in between 8 m and 30 m and Major bridge have a span range of about 30 to 120 m. Long span bridges have a span length of more than 120 m.
8. A road bridge is proposed to be constructed across an alluvial stream with the estimated design discharge as 2500 m3/s. It is decided to adopt a total effective waterway equal to 300 m for this bridge as against the regime width of 237.5 m due to afflux considerations. If the HFL at the site is at RL 214 m, then the RL of the open foundation can be suggested as _______
a) 206.6 m
b) 208.5 m
c) 207.2 m
d) 210.2 m
Explanation: Using Lacey’s regime formula for alluvial stream –
R = 1.35 (q2/f)1/3 = 1.35 (2500/300)1/3 = 5.55 m
The foundation may be taken down below HFL by 1.33R or R + 1.2 m (whichever is more).
i. 1.33R = 7.38 m
ii. R + 1.2 = 6.75 m (<7.38 m)
Hence, foundation level = 214 – 7.38 = 206.6 m.
9. Calculate the value of maximum scour depth for a single span structure having normal scour depth 5m.
a) 5 m
b) 7.5 m
c) 2.5 m
d) 10 m
Explanation: The factor of safety for single-span structure on a straight reach of the stream is generally taken as 1.5 and is taken as 2.0 for multi-span structures.
Maximum scour depth = 1.5 x 5 = 7.5 m.
10. The average grain size of the bed material is 0.16 mm for medium silt, Lacey’s silt factor is likely to be _______
Explanation: Lacey’s silt factor (f) is given by f = 1.76 d1/2 where d is the average particle size in mm.
F = 1.76 (0.16)1/2
F = 0.704.
11. The waterway for a bridge on the river in alluvial plain having a dominant discharge of 3600 cumecs should be ________ m.
a) 250 m
b) 285 m
c) 300 m
d) 225 m
Explanation: The linear waterway of a bridge across a purely alluvial stream is equal to its regime width as given by Lacey.
W = L = 4.75 Q1/2 = 4.75 x 60 = 285 m.
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