Abutment Scour - Approach Section Covered with Vegetation or Armored Bed

The same analogy may be used in estimating abutment scour for a channel where the upstream bed or flood plain is covered with vegetation and no sediment is transported regardless the magnitude of the shear stress on the upstream bed. In such a case, the following equation should be used to compute the abutment scour depth.

y2/yc = kf(kv)0.857 	
The value of the contraction scour, yc, can be determined by using either Neill's or Laursen's clear water scour equations and can be calculated as

yc=q2/Vc
where:
q2 is the unit width discharge at bridge, Section 2.
Vc is the critical velocity, and
y2 is the flow depth at the abutment toe after scour has occurred.

The value of kf is dependent on the intensity of the spiral flow in the approach flow, and can be calculated by using the Froude number of the approach flow (See details).

The value of kv is related to the contraction ratio of the approach flow (See details).