The well need to embedded or sunk below the maximum scour level to a required depth in order that the resistance from the sides of well is able to withstand the lateral forces acting on the well. The depth of the bottom portion of well from the scour level is called the grip length. While the selection of depth of foundation we require to consider the grip length and bearing capacity of soil strata. The maximum and minimum base pressures during the drastic or critical loading conditions have to remain under the permissible range.
In order to determine the scour depth in a stream we can analyze the actual scouring at or near the proposed location of bridge while the flooding or immediately after flooding. However, the maximum scouring can be greater than the analyzed scour because the design discharge is higher than the flood discharge for which the soundings have been performed. Additionally, there occurs increase in water flow velocity because of the obstruction of flow from the bridge constructed. Also some extra allowance needs to be kept in the measured scour due to the proximity of piers.
When the actual sounding is difficult to be performed then Laceys formula can be utilized to calculate the normal depth of scour in case of the alluvial soils.
d = 0.473 (Q/f)^1/3
Where, d = normal scour depth, measured below high flood level (m)
Q = design discharge (m3/sec)
f =silt factor, calculated from relation; f = 1.76?dm
Where, dm=mean size of particle in mm
The regime width can be computed as, w =4.75?Q
If the actual ware way (L) is less than the regime width, the actual scour depth (d) is given by,
d = d9W/L)^0.61
You may also use the table of maximum scour depth recommended by experts kept in the experimented codes book.