Irrigation Engineering Multiple Choice Questions on “Small Road Bridges Construction – Depth of Bridge Foundation”.

1. Greater is the afflux, lesser will be the available clearance for the same deck level.

a) True

b) False

Answer: a

Clarification: The water level on the upstream side rise up when a bridge is constructed across a contracted stream. This maximum rise in water level at the site is afflux and clearance is nothing but freeboard. Hence, the greater is the afflux the lesser will be the freeboard for the same deck level.

2. Greater is the reduction in the linear waterway, the lesser is the afflux.

a) True

b) False

Answer: b

Clarification: The downstream depth is not affected by the bridge as it is governed by the hydraulic characteristics of the d/s channel. Hence, greater is the reduction in the linear waterway, greater is the afflux. It is assumed that the upstream depth before the bridge construction is the same that prevails even after the bridge construction.

3. The proposed masonry arch bridge having the total height of its abutments or piers from top to bottom as H can be proposed to have economical spans each equal to ______

a) 2/3 H

b) 2H

c) 1.2 H

d) 1.5 H

Answer: b

Clarification: As per the guidelines, the economical length of one span for masonry arch bridge is equal to 2H where H is the total height between the bottoms of the foundations to its top. The clear span length is equal to 1.5H for RCC slab bridges.

4. A bridge culvert is to be constructed across a proposed lined canal. The afflux at this site for a contracted canal width can be computed by __________

a) Unwin’s formula

b) Broad-crested weir formula

c) Orifice formula

d) Molesworth formula

Answer: d

Clarification: The afflux for non-erodible beds may be easily computed by using the Molesworth empirical formula given as-

h = [V^{2}/17.88 + 0.01524] [(A/a)^{2} – 1]

Where, h = afflux in meters, V = velocity in an unobstructed stream in m/sec, A = unobstructed sectional area of the stream in sq.m and a = obstructed reduced c/s area of the stream at the bridge site.

5. While going to the Tirupati Tirumala Devasthanam from Madras, highway road is found to be lowered at several places over which natural drainage flow during the rainy season. Such dips in the roads are called ___________________

a) freeways

b) road dips

c) causeways

d) broadways

Answer: c

Clarification: The arrangement where there is a lowering or dipping of the road to the ground level at frequent intervals and thus allowing the water to flow across the road is called causeways. Such a dip or causeway is provided on the longitudinal profile of the road. This is feasible in less important towns.

6. Which of the following formula is valid when the downstream depth is more than 80% of the upstream depth?

a) Unwin’s formula

b) Broad-crested weir formula

c) Orifice formula

d) Molesworth formula

Answer: c

Clarification: The weir formula is not valid when the afflux is less than 1/4 of the downstream depth i.e. the downstream depth is more than 80% of the upstream depth. In such a case, the discharge is calculated by the Orifice formula which is given by-

Q = C_{o} (2g) ^{1/2}. L. ϒ_{d}. [h + (1+e)V_{a}^{2} / 2g] where, C_{o} = coefficient for the orifice formula, ϒ_{d} = downstream depth, V_{a}^{2} / 2g = head due to velocity of approach.

7. What is the value of the coefficient of discharge for the wide bridge opening with floors?

a) 0.90

b) 0.94

c) 0.96

d) 0.98

Answer: c

Clarification: The value of Cd varies with different type of bridge openings.

TYPE OF BRIDGE OPENING | VALUE OF Cd |
---|---|

1. Narrow bridge with or without floors | 0.94 |

2. Wide bridge opening with floors | 0.98 |

3. Wide bridge opening with no bed floors | 0.96 |

8. Calculate the economic length of one span for RCC Slab Bridge if the total height of abutment or pier from the bottom of its foundation to its top is 6m.

a) 6 m

b) 4 m

c) 9 m

d) 3 m

Answer: c

Clarification: As per guidelines, the economical length of one span for RCC slab bridges is equal to 1.5H where H is the total height of abutment or pier from the bottom of its foundation to the top.

Clear span length = 1.5 x 6 = 9 m.

9. In Broad Crested Weir formula, the discharge is dependent on ________________

a) upstream water depth only

b) downstream water depth only

c) both upstream and downstream water depth

d) independent of both U/s and D/s water depth

Answer: a

Clarification: When the afflux is less than 1/4 D/s depth, there is no effect on the discharge or on the U/s depth due to the formation of the standing wave. The discharge is only dependent upon U/s depth and is independent of D/s depth. It is applicable only when afflux is more than 1/4 of D/s.

10. Calculate the longitudinal braking force (P) for four-lane bridges designed for class 70R loading.

a) 14 tonnes

b) 21 tonnes

c) 17.5 tonnes

d) 35 tonnes

Answer: b

Clarification: The longitudinal braking force is equal to the 20% of the first trainload plus 10% of the load of the succeeding trains in class 70R loading i.e. 70 tonnes.

For four lane bridges- P = 0.3 x 70 = 21 tonnes. (20% + 5% of the loads in lanes in excess of two)

11. The greater is the heading up of water at the inlet end, the greater will be the flow velocity in case of culvert across some flat terrain.

a) True

b) False

Answer: a

Clarification: In such cases, the design is based on an increased velocity of flow through the culvert. The water is allowed to head up at the inlet end of the culvert to create that velocity. The greater is the heading up of water, the greater is the head causing flow. Hence, the greater will be the flow velocity and lesser is the opening area.

12. The foundations of bridges on erodible beds are taken down below the HFL by an amount equal to _____________

a) 4/3 times the maximum scour depth

b) 2/3 times the maximum scour depth

c) 1.5 times the maximum scour depth

d) 2 times the maximum scour depth

Answer: a

Clarification: The value is taken down below HFL by 4/3 times the maximum scour depth or Max scour depth + 2 (for arched bridges) and Max scour depth + 1.2 for other bridges (whichever is more). The formula is applicable where there is no bed floor is provided under the structure.