Figure 10.32. Plot of total scour for example problem.
It is important to evaluate the results of the scour computations carefully. For example,
although the total scour plot indicates that the total scour at the overbank piers is less than
for the channel piers, this does not indicate that the foundations for the overbank piers can
be set at a higher elevation. Due to the possibility of channel and thalweg shifting identified
with the Level 1 analysis, all of the piers should be set to account for the maximum total
scour. Also, the computed contraction scour is distributed uniformly across the channel in
Figure 10.32. However, in reality this may not occur. With the flow from the overbank area
returning to the channel, the contraction scour could be deeper at both abutments. The use
of guide banks would distribute the contraction scour more uniformly across the channel.
This would make a strong case for guide banks in addition to the protection they would
provide to the abutments. The WSPRO stream tube velocities could be used to distribute the
scour depths across this section.
The plot of the total scour also indicates that there is a possibility of overlapping scour holes
between the sixth pier and right abutment. Also, it is not clear from where the right abutment
scour should be measured, since the abutment is located at the channel bank. Both of these
uncertainties should be avoided for replacement and new bridges whenever possible.
Consequently, it would be advisable to set the right abutment back from the main channel.
This would also tend to reduce the magnitude of contraction scour in the main channel.
The possibility of lateral migration of the channel could also have an adverse effect on the
magnitude of the pier scour. This is because lateral migration will most likely skew the flow
to the piers. This problem can be minimized by using circular piers. An alternative approach
would be to install guide banks to align the flow through the bridge opening.
The plot of the scour prism in Figure 10.32 should be replotted to show the potential for the
scour to occur at any location in the bridge opening. This is shown in Figure 10.33.