Floodline downstream of a bridge 1

[cht13er]

I have a strange issue. As shown on the attached sketch, a small creek is flooding over a road (approx 2').

Above/upstream of the bridge, the creek's banks completely contain the flow - but when it hits the bridge flows are forced around the concrete walls on either side of the road. Downstream of the bridge, flows are once again able to be conveyed in the creek with no flooding above the banks.


I've highlighted two areas in pink immediately upstream and downstream of the bridge. Upstream of the bridge I can see how there could not be any flooding - there is essentially a drop from teh top of the creek's bank to the road.

But downstream of the road -- how does water immediately get sucked into the creek again? Should that dotted floodline (which I'm saying exists even though HEC-RAS currently says it does not) be 1:1 back to the creek ... or should it follow the quickest way back to the creek (e.g. "water drop" tool in CAD)...?

Thanks for your thoughts!
Chris

 

[cht13er]

May have been useful to provide the attachment.

 

[Ryb01]

If the flows in the creek do happen to overtop the road, it is possible for the flows to be directed immediately bank to within the top of banks on the downstream side of the bridge.

However, before being sold initially on the modelling results, I would re-check the coding of the bridge as per the HEC-RAS design manual. Specifically the cross section locations and roadway geometry.

If the roadway is overtopped, you should ensure your bridge modelling routine is set to wier/pressure flow and code appropiately. The grading in close proximity to the bridge, will also determine whether the flow could actually be directed back into the watercourse. Hope this helps.

 

[gbam]

As Ryb01 suggests, double check your modeling. HECRAS is a one dimensional model and if discharge can be contained within the downstream channel it will be reflected as doing that. This does not mean it will actually happen in that manner; you will need to use some other tool or judgement and had computations to show it different.