Abutment Scour In HEC-RAS

[mwbrand14]

I'm working in HEC-RAS on a scour research project. My issue is that the values I am getting for abutment scour are artificially high. From what I understand, many DOT's ignore HEC-RAS's abutment scour outputs because in almost all cases HEC-RAS grossly overestimates how deep the scour will be. I have tried adjusting the deck in order to line the bottom of the channel to reduce the vertical scour, but HEC-RAS does not allow you to do that. Anyone who has more experience in HEC-RAS ever dealt with this before? If so, how did you solve the problems you ran into with overestimation and come to a reasonable solution?

 

[pinkpig]

Talk to your state DOT, I know a lot of DOTs do not allow to use HEC-RAS/HEC-18 abutment scour results for design.

 

[mwbrand14]

Really? I was unaware of that. I am working in Vermont, where they use HEC-RAS to get abutment scour for a rough estimate. From the impression they gave me it seemed that many state DOT's used this protocol. Would you mind me asking what other state DOT's do in their estimates for flow design? Do they use the 2-D programs instead? Thanks for your response!

 

[pinkpig]

as long as your regulating agency accept abutment scour result of HEC-RAS, it is okay for you.

there are DOTs not allowing HEC-RAS/HEC-18 results since the result is always too off. An example is Louisiana LADOTD -check out LADOTD hydraulic manual of 2011 Page 11-8. Another one is TxDOT. In TxDOT geotech manual Page 5-15, it reads that "Do not calculate abutment scour because none of the equations to date yield acceptable results. Protect abutments against potential scour through use of a flexible revetment, where possible."

TxDOT even uses a reduction factor of 0.5 to HEC-18 pier scour calculation under cohesive materials condition since it is know that HEC-18 results are way too conservative especially for cohesive soil.