hi all:

I do several ASCE webinars and am thinking of adding another one but want to gauge the interest.

I have seen many mis-applications of HEC-RAS when performing bridge scour studies. I have compiled 6 so far of the most common mistakes (I will use a total of 8) that I have encountered and would like to present then in a 90 minute webinar that ASCE could market and generate revenues. I am open to suggestions for more problems that you may have seen and am willing to swap out my proposed problems if deemed that the suggested one are more important.

The topics are:

- Scour depth for piers should be using hydraulics at the thalweg and applied to all piers within the area of anticipated lateral movement of the main channel. If you assume that the main channel may move laterally, the worst case is if the thalweg moves to each of the piers - therefore the hydraulics at the thalweg (and resulting scour) should be applied to all piers and the scour depth should be applied to the thalweg elevation to determine the scour ELEVATION at each pier. The way HEC-RAS displays pier scour is not appropriate.
- Pier groups that are close together (within 5 pier diameters, per HEC-18) must be model as one pier using the group projected width and if skewed, take into consideration as a group and input to the skew variable in the scour menu.
- If debris is modeled on the piers for hydraulic considerations, must also add to the pier width for pier scour.
- Froehlich's pier/abutment scour already has safety factor (essentially 100%) in it so do not have to add a safety factor to the scour results.
- Don't double count pier skew. For elongated piers, enter width of pier (not projected width) and add the skew to the scour input. Enter the projected width for the bridge pier width for the hydraulics. If the situation is as per item 2 above, use the proper projected width of the pier group for the pier width in the hydraulics.
- Double check if approach and exit cross sections that are automatically assigned for bridge modeling are at the proper location. The proper ones to assign are cross sections 1 and 4 of the traditional cross sections used in HEC-RAS bridge modeling. If there are cross sections between 1 and 2 and between 3 and 4, HEC-RAS assigns the closest downstream cross sections from 2 and closest upstream cross sections from 3. This affects the contraction and abutment scour computations.
- Suggestions?
- Suggestions?

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David Williams Ph.D., P.E., PH, D.WRE, F.EWRI, F.ASCE

David T. Williams and Assoc.

Commerce City CO

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