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.

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.

Figure 7.9. Comparison of laboratory flow characteristics to field flow conditions.

7.23

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