Q. What do you mean by Depth of Scour?

The maximum scour depth in a stream can be ascertained when ever possible by actual sounding at or near the site proposed for the bridge during or immediately after a flood. Before the scour holes have had time to silt up due to allowance should be made in the observed depth for increase in scour resulting from-

1. The designed discharge being greater than the flood discharge during which the scour was observed.
2. The increase in velocity due to the obstruction in flow caused by construction of the bridge.

Where the above practical method of determining scour is not possible the following theoretical method may be used as a guide when dealing with natural streams is alluvial beds.      

D = 0.473 [ Q / F ]^? 

Where D = Depth of scour below HFL for regime conditions in a stable channel in meters.

           Q = Design discharge in cumecs.  

           F= Silt factor = 1.76 √m , m is the mean diameter of particles in mm.