Figure 7.8. Schematic representation of abutment scour.
The horizontal vortex at the toe of the abutment is very similar to the horseshoe vortex that
forms at piers, and the vertical vortex that forms at the downstream end is similar to the wake
vortex that forms downstream of a pier, or that forms downstream of any flow separation. A
large amount of laboratory research has been conducted to determine the depth and location
of the scour hole that develops for the horizontal (so called horseshoe) vortex that occurs at
the upstream end of the abutment. From this research, numerous abutment scour equations
have been developed to predict this scour depth. However, very little field data exist to verify
the equations. In fact, many of the equations, as explained in Section 7.8.2, are spurious
correlations for the field case.
Only since 1992 has research been conducted to develop equations for the field case.
Maryland SHA has developed an abutment scour program titled ABSCOUR (Chang and
Davis 1999) based on Laursen's long contraction scour equations. Sturm (1999a, b)
developed equations based on his research at Georgia Institute of Technology.
Kouchakzadeh and Townsend (1999) and Trivino and Richardson (2000) present equations
based on momentum transfer for the determination of abutment scour depths. These
equations have had limited testing of computed vs. measured abutment scour and will not be
given.
The wake vortex at the downstream end of the abutment also causes abutment failures.
Sometimes the abutment does not fail but only the approach embankment is eroded.
Research and the development of methods to determine the erosion from the wake vortex
has not been conducted. Methods to protect abutments and the embankments against wake
vortex erosion are given in HEC-23 (Lagasse et al. 2001b).
In summary, engineering judgment is required in designing foundations for
abutments. As a minimum, abutment foundations should be designed assuming no
ground support (lateral or vertical) as a result of soil loss from long-term degradation
and contraction scour. The abutment should be protected from local scour from both
7.22
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