Download FINAL APPENDIX A WITH PICTURES C REVISED

compute the scour cross-section on this basis. This section will be referred to
as Model B; compare the scour cross-sections for Model A and Model B;
select the most reasonable answer
PROBLEM: For a bridge location on a sharp bend, contraction/bend scour
may be unequally distributed with most of the scour occurring on the outside
of the bend.
EXAMPLE SOLUTION: (1) use the ABSCOUR program to compute the area
of contraction scour. (2) pro-rate more of the scour on the outside of the bend,
keeping the scour area constant. .
Other guidance on plotting the scour cross-section on the bridge plans
1.
For vertical wall abutments, plot values of y2 and y2a under the bridge,
measuring down from the water surface at the downstream side of the bridge.
2
Where the abutment scour is deeper than the channel scour, use an angle of 30
degrees to define the sides of the scour hole. Use a nominal value of 5 feet to
determine the width of the bottom of scour hole.
3
Where the abutment scour depths are at a higher elevation than the channel
contraction scour, use a smooth curve to define the transition area.
4
The user will need to determine the total scour at each foundation element, taking
into account the following factors:
o
o
o
o
o
Contraction scour
Abutment scour
Local pier scour
Lateral channel movement
Degradation
The current policy of the Office of Structures is to make a judgment on how to
best consider the total effect of these different aspects of scour on a case by case
basis as discussed in Chapter 11.
B.
ABSCOUR PROGRAM LOGIC
The following discussion is provided for insight into the logic used by the program in
computing flow distribution and velocity distribution at the bridge.
A current limitation of the HEC-RAS program used to model flow through a bridge is
that it provides for the distribution of flow under the bridge based on conveyance
calculations. This approach does not reflect the three dimensional flow patterns actually
observed in the field at bridge contractions. To obtain reasonable estimates of scour
depth, it is necessary to account for the high local flow velocities and turbulence near the
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