Scour - shocking results

[QCANADA]

The bridge has overbank flow forced back in the river channel by the bridge approach embankments. Based on the Live-bed scour in the contracted bridge opening, the maximum scour depth is ... 15 feet.

The D50 is 1.5 mm and the flow rate is almost 10,000 fps passing through an opening of 100 feet.

I know that the equation is conservative according to some reference (Richardson and Davis, 1995) because it assumes a long contraction where uniform flow can develop. I just think that this maximum scour depth is not... reasonable.

Something I should do before sending the results to the Structural Engineer?

 

[bimr]

You might consider double checking your calculations. Is that really 10,000 fps (6,800 miles per hour) that you refer to?

 

[EngWade]

Ditto, bimr.

 

[QCANADA]

Sorry! 10,000 cfs hehe

 

[francesca]

Sorry, 15 ft of live bed scour is not a lot. Monitoring after the storm won't help you to see that, either, because the scour will be filled back in. Your velocity with 10,000 cfs through 100 ft opening is going to be huge. With a 10 ft depth the average would be 10 fps, so significantly higher in the center of the channel. When I saw this thread title I thought "Oh, I bet someone has calculated an embankment scour of 100 ft."

 

[QCANADA]

The bridge is cutting a floodplain and there's a significant increase of velocity between the approach and the bridge.

I just thought it was overly conservative but you bring good point by saying that the scour will be filled back in.

I'm just keeping it back before sending it to Structural Engineer. I know that he never faced something like this before.

100 feet!

 

[cvg]

an experienced bridge designer certainly will not flinch at 15 feet of scour. However, in order to protect the banks and the abutment you will need to spend considerable money in channel improvements. It might be worthwhile to make the bridge opening a little wider than 100 feet. I would suggest you work with the designer, not hold anything back.

 

[gbam]

Another scour source you need to think about is the long term impact that your bridge will have on the riverine system. You may want look into a sediment transport model pre and post structure.

 

[francesca]

Live bed implies that the scour occurs on the up leg of the flood hydrograph and sediment is deposited again on the down leg. The sediment that is re-deposited is non-structural, so scour protection needs to extend beyond the depth. There is a simple check to see if live bed or clear water scour is likely. Aesthetically, live bed looks better, but structurally they're the same.

The embankment scour equations are outrageously conservative. Here we specify embankment scour protection (very successful) and ignore the scour data for embankment scour. Pier and contraction scour we take very seriously, and we assume channel migration and armor piers the same on the overbanks as the channel.

We also have strict rules regarding how much backwater is allowable: 0.14 ft is allowable without having to purchase flood easements. I would expect substantial backwater from cramming 10,000 cfs through a 100-ft wide opening... not to mention the 500-year storm.

 

[QCANADA]

Thanks, for the information.

We do ignore embankment scour as well.

Just to mention: that flow rate of 10,000 fps is the 500-year storm (well 1.7 x 100-year).