Impact of Downstream Conveyance

[Anonymous]

Hi,

I’m evaluating a small bridge that crosses a stream which feeds a major river conveyance. The river conveyance is roughly 700 feet downstream of the bridge. The peak flows for the conveyance are 15 to 20 times greater than the peak flows for the stream. Is it possible to effectively model the small stream without a complete HEC-RAS model of the downstream conveyance? If so, how would you verify what impacts the downstream conveyance might have on the stream? Our stream topo currently includes runs 500 feet downstream of the bridge (200 feet short of the major river).

Thank you!

[Chris G.]

Typically in that situation, I’ve determined a concurrent flood level for the big stream and use that as the downstream boundary condition for your model, to evaluate flooding at the small bridge. If evaluating the small bridge for scour, I suggest using normal depth at the downstream end of your small stream. It’s good to do a sensitivity analysis on the downstream boundary to see what effect (if any) it has on your small bridge.

[Anonymous]

Thank you!

[Author]

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