The computation of scour at bridges within HEC-RAS is based upon the methods outlined in Hydraulic Engineering Circular No. 18 (HEC No. 18, FHWA, 2001). Before performing a scour analysis with the HEC-RAS software, the engineer should thoroughly review the procedures outlined in that report. This chapter presents the methods and equations for computing contraction scour and local scour at piers and abutments. Most of the material in this chapter was taken directly from the HEC No. 18 publication (FHWA, 2001).
Note
HEC-RAS has not been updated to the Federal Highways latest procedures documented in HEC No. 18, Evaluating Scour at Bridges (FHWA, April 2012). Therefore some differences may arise in computed results for certain flow regimes.
For information on how to enter bridge scour data into HEC-RAS, to perform the bridge scour computations, and to view the bridge scour results, see "Modeling Ice-covered Rivers" of the HEC-RAS user's manual.
General Modeling Guidelines
In order to perform a bridge scour analysis, the user must first develop a hydraulic model of the river reach containing the bridge to be analyzed. This model should include several cross sections downstream from the bridge, such that any user defined downstream boundary condition does not affect the hydraulic results inside and just upstream of the bridge. The model should also include several cross sections upstream of the bridge, in order to evaluate the long-term effects of the bridge on the water surface profile upstream.
The hydraulic modeling of the bridge should be based on the procedures outlined in "Modeling Bridges" of this manual. If observed data are available, the model should be calibrated to the fullest extent possible. Once the hydraulic model has been calibrated (if observed data are available), the modeler can enter the design events to be used for the scour analysis. In general, the design event for a scour analysis is usually the 100 year (1 percent chance) event. In addition to this event, it is recommended that a 500 year (0.2 percent chance) event also be used to evaluate the bridge foundation under a super-flood condition.
After performing the water surface profile calculations for the design events, the bridge scour can then be evaluated. The total scour at a highway crossing is comprised of three components: long-term aggradation or degradation; contraction scour; and local scour at piers and abutments. The scour computations in the HEC-RAS software allow the user to compute contraction scour and local scour at piers and abutments. The current version of the HEC-RAS software does not allow the user to evaluate long-term aggradation and degradation. Long term aggradation and degradation should be evaluated before performing the bridge scour analysis. Procedures for performing this type of analysis are outlined in the HEC No. 18 report, and are beyond the scope of this discussion. The remaining discussions in this chapter are limited to the computation of contraction scour and local pier and abutment scour.
