inneffective flow areas at bridge cross sections

[Johns]

This is an issue I have been considering for a while:

When defining innefective areas, the default is that when stage rises above elevation (of innefective area) they are no longer included in conveyance calculations. To change this you can set them to 'permanent'.

Which setting do I want for bridge cross-sections?

I feel, intuitively, that even if bridge is over topped there still should be innefective areas upstream of the bridge (in default setting as soon as the bridge is overtopped inneffective areas dissapear - right?), especially if the bridge opening, or culvert, is relatively small. And therefore have set these inneffective areas to permanent. I have just read something that suggests they should not be premanent as 'When overtopping occurs, a portion of the total flow bypasses the bridge opening and is conveyed in the overbank area'.

permanent, or non-permanent?

help

John

[crystalct1]

I would say you are right. They are permanently prohibited to convey flow in those stages less than the roadway. The water will just sit. The water above will flow, but the bottom water will not. Only time that maybe conveyance would happen would be in the case of a perched bridge, where the road dips down pretty low out in the floodplain and water begins to convey over the roadway like a couple of side channels.