# A road bridge is proposed to be constructed across an alluvial stream with the estimated design discharge as 2500 m^3/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 _______

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A road bridge is proposed to be constructed across an alluvial stream with the estimated design discharge as 2500 m^3/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

This question was addressed to me in an interview.

This intriguing question comes from Culverts Construction in chapter Culverts and Small Road Bridges Construction of Irrigation Engineering

by (102k points)
Correct answer is (a) 206.6 m

Best explanation: Using Lacey’s regime formula for alluvial stream –

R = 1.35 (q^2/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.

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