HEC-RAS bridge model w/ "scour" hole

[buildman01]

We are currently working on a hydraulic model for a bridge replacement. The existing and proposed bridges are both single-span, with abutments located outside of the stream banks. Our initial topographic survey of the area indicated that there was a "hole" in the streambed on the upstream edge of the existing bridge. Our initial assumption was that it was caused by scour; however, our scour analysis indicated that aggregation would most likely occur here, rather than scour. Therefore, we are assuming that the hole was caused by something else (possibly livestock). Based on our scour calculations, we ran the hydraulic model ignoring the hole. However, the reviewer has indicated that we should not be ignoring it. My questions are:

1. What is the correct way to model this situation, with or without the hole? Are there any references that address this?
2. Assuming that we have to indeed model it this way, what is the best way to model it? The problem lies in that because of the location of the hole relative to the bridge, so it is difficult to just add cross sections at the transition points.

Any help would be greatly appreciated. Thanks!

 

[pinkpig]

If your goal is to model bridge hydraulics say to acquire stage and flow relationship for various existing and proposed conditions, I don't think ingoring the hole will have any substantial effect on your results.

 

[buildman01]

Yes, that is our goal. From a regulatory standpoint, we are required to check backwater and freeboard for the existing and proposed bridges.

 

[francesca]

Add the hole and mark it permanently ineffective flow?