Abutment scour depends on the interaction of the flow obstructed by the abutment and
roadway approach and the flow in the main channel at the abutment. The discharge
returned to the main channel at the abutment is not simply a function of the abutment and
roadway length in the field case. Richardson and Richardson (1993) noted that abutment
scour depth depends on abutment shape, discharge in the main channel at the abutment,
discharge intercepted by the abutment and returned to the main channel at the abutment,
sediment characteristics, cross-sectional shape of the main channel at the abutment
(especially the depth of flow in the main channel and depth of the overbank flow at the
abutment), and alignment. In addition, field conditions may have tree-lined or vegetated
banks, low velocities, and shallow depths upstream of the abutment. Most of the early
laboratory research failed to replicate these field conditions.
7.8.3 Abutment Site Conditions
Abutments can be set back from the natural stream bank, placed at the bankline or, in some
cases, actually set into the channel itself. Common designs include stub abutments placed
on spill through slopes, and vertical wall abutments, with or without wingwalls. Scour at
abutments can be live-bed or clear-water scour. The bridge and approach road can cross
the stream and floodplain at a skew angle and this will have an effect on flow conditions at
the abutment. Finally, there can be varying amounts of overbank flow intercepted by the
approaches to the bridge and returned to the stream at the abutment. More severe abutment
scour will occur when the majority of overbank flow returns to the bridge opening directly
upstream of the bridge crossing. Less severe abutment scour will occur when overbank
flows gradually return to the main channel upstream of the bridge crossing.
The skew angle for an abutment (embankment) is depicted in Figure 7.10. For an abutment
angled downstream, the scour depth is decreased whereas the scour depth is increased for
an abutment angled upstream. An equation for adjusting abutment scour depth for
embankment skew is given in Section 7.8.5.
θ
Figure 7.10. Orientation of embankment angle θ to the flow.
7.24
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