scour calculations at multiopening structures

[ecabo]

How can I calculate scour at a multiopening structure? It’s a three bridges- multiopening structure.
I’ve been triyng to calculate it as a single structure (the same bridge), and changing the main channel station for every bridge, in order to obtain accurate velocities and scour calculations for every bridge.
Previously, I’ve compared the results of the two models: the one with the bridge as a multi- opening and the one with the bridge as a single structure. They were very similar.
Is it correct?

[Chris G.]

Have you tried running with Velocity Distribution turned on? That will allow you to get a good representation of velocity at various locations across your multi-opening structure.

[Anonymous]

Hello, thanks for your answer.
Yes, I’ve this option on. The problem is that Hec Ras does’n make the velocity distribution calculations inside the multi-opening estructure.
I think the only way to calculate this is entering the data manually, I will take the maximum average velocity from both upstream and downstream, which Hec Ras does calculate.
I will need to enter water surface depths for every pier manually (Hec Ras shows the water surface elevation at Multi Opening Output table) , too, because Hec Ras doesn’t do it either.
What do you think abut it?

[GoSparky]

When you do pier scour, you use the velocity at the first cross-section upstream of the bridge, not BR U or BR D. You should be able to see the velocity distribution at that cross-section and get the flow velocity approaching each pier. Or, if the channel can shift, it’s best to use the maximum velocity “slice” from the upstream cross-section for your pier scour calcs.

[David W]

I assume you have left and right abutments and piers in each opening. If the openings are hydraulically “isolated” from each other, you may want to put each opening into separate parallel reaches that separate upstream of the openings and reconnect downstream of the openings. This will allow scour computations in each opening. Also note that if one or more openings scour more than the others, it may “capture” the other openings’ water and increase the Q in the favored openings, thus causing more scour than predicted by your method since it assumes a constant flow distribution into each opening for the flood event.

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