bridge scour analysis (pier, footing, piles)by HEC-RAS

[pinkpig]

In order to correctly model or calculate bridge scour, do I need to consider the effect of underground (under riverbed) footing and piles?

I am using HEC-RAS to do my work, but it can only model the above ground part of the piers. Underground footing and piles are not even recorded in HEC-RAS.

So my questions are:

1. Under what scenarios do I need to consider the effects from underground footings and piles for scour analysis?
2. Apparently HEC-RAS is not suitable to model underground footings and piles, so what's the best alternative?

Thanks.

 

[gbam]

You will need to use some judgement. Check the long term bed change, degradation or aggradation. Make sure the footing is not exposed over the long term. If it is redo you hydraulic model to incorporate the exposed footing, etc... Also you will need to evaluate whether the footing and piles are exposed during a local scour event based on the pier/abutment scour. If they are you will need to evaluate the increase in scour from that point down. Hope that helps.

 

[francesca]

Read HEC-18.

http://www.fhwa.dot.gov/engineering/hydraulics/pubs/hif12003.pdf

HEC-RAS evaluates the worst-case scour considering erodible material. States typically have regulations regarding how to evaluate scour and ensuring that scour protection is in place to the determined depth. Scour is a function of pier geometry and the vortices that form around piers, moreso than the shape of the pile cap or pier foundation. Since most catastrophic bridge failures are scour-related, most scour analyses and policies are conservative. Also because it is hard to inspect scour under constant flow and under live bed conditions where the scour hole is filled during the downward limb of the flood event.

The main restriction on the accuracy of the HEC-RAS results is the limited knowledge of the subsurface information. That is why the HEC-RAS scour depth is given to a qualified geotechnical engineer, along with boring data and other subsurface information, to design scour protection and pile depths. HEC-RAS is not the be all and end all of scour design, it is merely an input.