A free span bridge with abutments outside of the channel needs to be analyzed for scour with the assumption that lateral channel migration could expose the abutments to high velocity flows. I am using the HEC-RAS Hydraulic Design Function for this analysis. What is the best way to go about this? I would think you could alter the bridge cross sections to model what you think may be the extent of lateral migration, but is there a way to objectively determine potential lateral migration of the channel? Or is it all subjective? Another way of evaluating scour comes to mind after reading this in HEC-18: "Serious consideration should be given to placing footings/foundations located on floodplains at elevations the same as those located in the main channel." What if the free span structure were modeled with a pier in the middle, the same shape and size as the abutment? This would give me the scour for the hypothetical pier that is modeling an abutment exposed by lateral channel migration. I would then use this scour value to design the depth of my abutments. This latter approach seems to be conservative and defensible. Any comments?
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Scour at buried abutments
- Thread starter SLOCivil
- Start date
OHbridgeguy
Structural
SLOCivil,
One thing to note is that the HEC-RAS scour function does not take into account the latest changes to HEC-18 so it may be best to do a hand calc for your scour. It isn't really a very long calc but you want to make sure that the proper provisions are used. I recommend you look at HEC-20 for guidance on the lateral stream migration question, though your approach is in general correct. If you have historic data on stream location it can help to determine future lateral migration. You can also use countermeasures (see HEC-23) to help prevent the migration if it's an issue.
Note that the scour equations for piers and abutments are VERY different. I wouldn't recommend using a pier equation on the abutment unless you are expecting the roadway behind your bridge to get completely washed out. The biggest effect the lateral migration will have is to increase the blocked flow length and volume behind the abutment and roadway embankment. Increasing that in the HIRE or NCHRP approaches should take care of your concern and in general increase the scour potential. Also note that the NCHRP approach combines the contraction and abutment scour, while the HIRE approach requires contraction scour to be assessed seperately and added.
A nice tool for doing the scour analysis is the FHWA Hydraulic Toolbox. It's freeware and I highly recommend it. Hope this helps.
One thing to note is that the HEC-RAS scour function does not take into account the latest changes to HEC-18 so it may be best to do a hand calc for your scour. It isn't really a very long calc but you want to make sure that the proper provisions are used. I recommend you look at HEC-20 for guidance on the lateral stream migration question, though your approach is in general correct. If you have historic data on stream location it can help to determine future lateral migration. You can also use countermeasures (see HEC-23) to help prevent the migration if it's an issue.
Note that the scour equations for piers and abutments are VERY different. I wouldn't recommend using a pier equation on the abutment unless you are expecting the roadway behind your bridge to get completely washed out. The biggest effect the lateral migration will have is to increase the blocked flow length and volume behind the abutment and roadway embankment. Increasing that in the HIRE or NCHRP approaches should take care of your concern and in general increase the scour potential. Also note that the NCHRP approach combines the contraction and abutment scour, while the HIRE approach requires contraction scour to be assessed seperately and added.
A nice tool for doing the scour analysis is the FHWA Hydraulic Toolbox. It's freeware and I highly recommend it. Hope this helps.
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