the upstream and downstream vortexes using riprap and/or guide banks. Guidelines
for the design of riprap and guide banks are given in HEC-23 (Lagasse et al. 2001b).
To protect the abutment and approach roadway from scour by the wake vortex several
SHAs use a 15-meter (50-ft) guide bank extending from the downstream corner of the
with riprap.
In the following sections, two equations from HEC-18 are presented for use in estimating
scour depths as a guide in designing abutment foundations. The methods can be used for
either clear-water or live-bed scour.
7.8.2 Commentary on Abutment Scour Equations
Until recently, the equations in the literature were developed using the abutment and
roadway approach length as one of the variables. This approach results in excessively
conservative estimates of scour depth. Richardson and Richardson (1993) pointed this out
in a discussion of Melville's (1992) paper and in a 1999 paper (Richardson and Lagasse
1999, p. 457). They stated.
"The reason the equations in the literature predict excessively conservative
abutment scour depths for the field situation is that, in the laboratory flume,
the discharge intercepted by the abutment is directly related to the abutment
length; whereas, in the field, this is rarely the case."
Figure 7.9 illustrates the difference. Thus, equations for predicting abutment scour would be
more applicable to field condition if they included the discharge intercepted by the
embankment rather than embankment length.
(a)
(b)
Figure 7.9. Comparison of laboratory flow characteristics to field flow conditions.
7.23
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