Abutment shape factor, Left and Right Over-banks
Abutment scour is reduced by a streamlined shape that facilitates a smooth transition of the flow and a corresponding reduction in turbulence.
Two common examples of streamlined abutment shapes are vertical wall abutments with flared wing-walls and abutments placed on spill-through slopes.
The effectiveness of the abutment shape in reducing scour depends on two factors: (1) the horizontal length, X1, of the streamlined portion of the
abutment or spill through slope and (2) the total horizontal abutment and approach road length, X2, that is within the effective flow width of the approach flow.
Please refer to the Figure below for an illustration of the X1 and X2 values. As indicated in the Figure, measure or estimate X1 and X2 for spill-through
slopes on the ABSCOUR cross-section; not on the actual cross-section:
- The X1 value for a flared wing-wall is the horizontal distance perpendicular to the flow from the abutment face to the end of the wing-wall
- The X1 value for a spill-through slope is the horizontal distance perpendicular to the flow between the abutment toe (on the ABSCOUR cross-section)
and the location of the water surface line on the spill-through slope. (In some cases, the water surface may extend back to the abutment.)
- A vertical wall abutment without wing-walls or with a 90 degree wing-wall is not a streamlined shape and has an X1 value of zero.
The shape factor, Kt, is a function of the ratio of X1/X2. Equations 1-29 and 1-30 in the Users Manual compute the value of Kt. Kt is used in Equation 1-28 to
compute the final abutment scour, taking into account the abutment shape.

Figure 2-12 Abutment Shape Factor
(Measure or Estimate the X1 and X2 Values for Spill-Through Slopes Using the ABSCOUR Cross-Sections)