7.8.4 Abutment Shape
without wing walls, and (3) vertical-wall abutments with wing walls (Figure 7.11). These
shapes can all have varying angles to the flow. As shown in Table 7.4, depth of scour is
approximately double for vertical-wall abutments as compared with spill-through abutments
for very short sections of the abutment and approach road. As the length of the abutment
and approach road in the floodplain increase, the effect of the spill-through slope is
decreased. For long approach road sections on the floodplain, this coefficient will approach
a value of 1.0. Similarly, scour for vertical wall abutments with wingwalls on short abutment
sections is reduced to 82 percent of the scour of vertical wall abutments without wingwalls.
As the length of the abutment and approach road in the floodplain increase, the effect of the
wingwall is decreased. For long approach road sections in the floodplain, this coefficient will
approach a value of 1.0.
Figure 7.11. Abutment shape.
Table 7.4. Abutment Shape Coefficients for Short Abutment Sections.
Description
K1
Vertical-wall abutment
1.00
Vertical-wall abutment with wing walls
0.82
Spill-through abutment
0.55
7.8.5 Froehlich's Live-Bed Scour Equation
To determine the potential depth of scour at existing bridges and to aid in the design of
foundations and placement of rock riprap or guide banks at new bridges, Froehlich's (1989)
live-bed scour equation or an equation from HIRE (Richardson et al. 1995) can be used.
Froehlich analyzed by regression analysis, 170 live-bed scour measurements in laboratory
flumes to obtain the following equation:
7.25
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