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 Lacey�s 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.