The maximum scour depth d_{im} for the condition of flow at noses of piers is
Explanation:
Maximum Scour depth:
When the above practical method is not possible, the mean depth of scouring may be computed by the given equation for natural streams in alluvial beds.
d_{sm }= 1.34 where
d_{sm }= Mean depth of Scour below HFL in meters
According to IRC code of practice for the design of substructure and foundation of bridges gives clear guidelines regarding the scour depth for the condition of flow at noses of piers and that is equal to 2.00d
Where d = depth of scouring according to lacey's theory.
\(d = 0.475\left ( \frac{^{Q}}{f} \right )^{\frac{1}{3}}\)
Additional scour depth is given from the IRS Code of practice for the design of sub-structures and foundations of bridges.
According to IRS,
Nature of the River |
Depth of Scour |
1.In a straight reach |
1.25 d |
2.At the moderate bend Conditions e.g along the apron of Guide bund |
1.5 d |
3. At a severe bend |
1.75d |
4.At a right angle bend or at nose Of piers |
2.00d |
5.In severe swirls e.g against mole Head of a guide bund |
2.5 to 2.75d |
Additional Information
The maximum depth of Scour D below the HFL is to be taken as below.
dsmax = 2.0dsm for pier
= 1.27dsm with approach retained or lowest bed level whichever is deeper for abutment
= 2.0dsm for scouring all around for abutment
dsmax = 2.0dsm in the right-angled bend.
= 1.75dsm at the severe bend.
= 1.5dsm at the moderate bend
= 1.27dsm in a straight reach
The minimum depth of foundation below HFL is kept at 1.33 D for erodible strata.