Bridge Scour (sampling) 1

[SMIAH]

I need to assess the depth of scour anticipated after (during) an extreme flood (e.g. 500 years). I thought about using HEC RAS in order to estimate the scour potential.

The hydraulic analysis has already demonstrated that the bridge does not cause any major constriction of the flow for everything under the 500-year.

The bridge's footings are relatively exposed and local scour is already observed. In addition, the bridge is located at the end of a sharp bend and natural scouring is also already observed near the outside bank of the river.

My question is related to the estimation of the mean size fraction of bed material.

To do so, should I collect samples of the bed material from the river near the footings (and concrete walls) only? Or should I do so in the center of the river as well? Would it be better to collect 3 or more sample in the cross section at the upstream face of the bridge and calculate a mean D50?

It looks like the scour problem (because there' is already one) is a result from a combination of natural and local scour. I find it hard to "predict" what is going to happen if (when) an extreme flood hits.

 

[cvg]

I have seen where sediment sizes were averaged and then scour was calculated. I have also seen where scour was calculated using different methods or different sediment sizes or different cross sections and then the scour depth was averaged. Neither of these seems to address the fact that maximum scour will occur where the soil is finest or where the hydraulic conditions are most extreme. I would determine D50 for each sample. Do the analysis for the worst case sample and location. I think it is appropriate to take samples both near the footings and also in other locations in the river. The good thing is that you are actually taking samples at the bridge, as I so often see scour analysis done with just a few samples and possibly none very close to the bridge.

 

[SMIAH]

What would you do regarding the natural scour related to the fact that the bridge is located on bend (i.e. local lowering of the deepest part of the channel)?

How to threat this in RAS (if necessary)?
What scour protection or channel works to recommend (if any)?

I guess that I will do as you've mentioned and I will perform an analysis using the worst-case scenario (min. D50).

 

[beej67]

If the bridge has already undergone scour, then those scour processes have likely changed the d50 size of the material in or near those scour holes, as flow advects smaller particles downstream as bed or suspended load, and leaves some larger ones.

I don't know if that helps you or gives you an even bigger headache, but I felt it worth mentioning.

If you have the ability to collect samples from both the existing scour locations and a representative cross section upstream, I'd be very interested to see whether there were any major differences.



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[SMIAH]

Stream is quite meandering and the bridge is located at the outside of bend in the river.

I guess that I could collect 2-3 representative sample in a representative cross section upstream of the bend. And do the same thing at the bridge location. I'm quite sure from a visit on the field (might worth doing it though) that the D50 at the bridge will be higher than just upstream of that bend.

Will this increase the head ache.

 

[SMIAH]

A picture is worth a thousand words.

Taken in low-flow from the bridge just at the end of the bend.
Before the scour hole.

 

[cvg]

I generally add low flow incisement to get the total scour depth and assume the low flow channel could move from one side to the other. In this case, it looks like it would stay on one side rather than shift.

recommended scour protection or channel improvements depends on as-built conditions as well as on calculated scour. Improvements might include slope protection of the abutments, armoring the transition from the stream into the bridge, riprap around the bridge piers, or some sort of grade control structure at the bridge. You should discuss this with the county bridge engineer to decide what they prefer to maintain.