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Bridge piers for small 2D cell size

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I am curious about 2D modeling of bridge piers when the 2D cell size is small enough to appropriately model the individual piers. I have been using a grid cell of 1m or less. There are many bridge crossings in this stretch of river, and most of the piers are about the same width as my grid cells so I can model the piers as blocked cells using a Z polygon or Z line. The results seem to accurately capture changes in velocity patterns adjacent to the piers. I've attached an image of results through one bridge. My question is this: is it necessary to add additional form losses if the piers are modeled as blocked cells, or will this accurately estimate losses around the piers? (Assume all flow is below the low chord). If additional losses are necessary, is there an appropriate way to estimate these? Presumably this would be smaller than the form loss used for a percent blockage (which wouldn't account for these velocity patterns). 

I am using a 1D HEC-RAS model as well, but the 1D bridge modeling is producing some contradictory results depending on which bridge modeling method is used (either energy or momentum equations). I was hoping the Tuflow model results could provide some support for one method or the other. 

-Frances 

 

BridgePiers.JPG

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Morning Frances,

 

Bill Syme and Phil Ryan have a very useful presentation on different modelling techniques for piers.

Its located here in our publications library.

The first one addressed is the technique you've used of blocking out the pier cells.

In the absence of calibration data or anecdotal information for verification of a larger head drop than you're currently seeing, I would not apply an additional form loss. Any additional form loss could be used to take 3D effects (like mixing or vertical effects) or sub-grid scale features. However, I would expect these to pale to insignificance compared to the losses from the piers themselves, which look to be well captured by the blockage you've applied.

Discussion and debate welcome!

Regards,

 

 

Rachel

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