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 rational formula are used for calculating same.

It is done by U/s wing walls or training works so as to make it equal to regime width and bridge may n be spanned across it. This contraction of stream up to regime width leads to economy since bridge length is reduced.

From oretical considerations, re will be no scouring and foundation of bridge sub-structure will have to be taken below bed by a nominal amount not by scour considerations. It is done so as to provide sufficient grip on m.

The stream is contracted so as to keep span length (L) lesser than regime width (W) for quasi-alluvial stream. The normal scour depth (D) is given by-
D = D? (W?/L)^0.61 where W? = natural unobstructed width of stream and D? = scour depth in quasi-alluvial stream having L=W?.

For purely alluvial streams, Lacey?s scour depth (R) is given by R = 1.35 (q²/f)^1/3 where q is discharge intensity per unit width of stream at bridge site.
The value of Lacey?s regime scour depth (R_e) is given by R = R_e (W/B)^0.61.

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 equation f = 1.76 d^1/2 where d is average size of particle in mm.

When 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.

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.

The factor of safety for single-span structure on a straight reach of 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.

Lacey?s silt factor (f) is given by f = 1.76 d^1/2 where d is average particle size in mm.
F = 1.76 (0.16)^1/2
F = 0.704.