FACTOR FOR OBTAINING THE MAXIMUM SCOUR DEPTH.

BRIDGE FOUNDATIONS :-- The depth of the bridge foundations are determined by considerations of the safe bearing capacity of the soil after taking into account the effect of scour .The foundations should be taken below the scour level and to the level at which there is minimum variation in the moisture content of the sub Soil. Scour occurs when the bed velocity of the stream exceeds the velocity which can move the particles of the bed materials and the velocity varies with the gradient,the hydraulic mean depth and the character of the bed and banks and depends more on the depth than on the gradient.A river has to adjust it's velocity to what it's bed and banks can stand by changing its section .The prevailing velocities of the winter supplies in rivers may be as low and increases in the floods ,as greater scour follows increased velocity,the tendency is for the deeper parts of the section to become deeper still .The velocity of a falling river is a greater than that of a rising river .When the velocity is retarded silt is dropped and when the velocity is increased ,silt is picked up.Scour is worst when the river is falling.Depth of scour is ascertained as for as possible by actual sorroundings at the proposed site of the bridge during or immediately after a flood before the scour holes have had time to silt up .Due allowance should be made to increase in the scour resulting from obstructions (Contraction) at the bridge which will increase the velocity and hence the scour .The scouring action of the current is not uniform all along the bed width it is not so even in straight reaches ,there is deeper scour than normal at the piers or other obstructions and also at the bends ,therefore the maximum scour depth has to be determined for which the Laceys theory may be followed for the design purposes.The normal scour depth should be multiplied by the following factors for obtaining the maximum scour depth,in a straight reach of the stream when the bridge has no piers obstructing the flow 1.27 d,at a moderate bend 1.50 d. At a severe bend 1.75 d  at bad sites on curves or where the diagonal current exists or the bridge is a multi span structure 2d at the right angled bends 2d at noses of piers 2d at upstream noses of guide bunds 2.75 d .The above rules applies when no bed floor is provided and the stream is free to scour as it may .