Bridge Modeling vs Culvert Modeling for bottomles culverts...

[lbgeng]

In cases where there is stream grading (especially in cases where there are low flow channels in the stream) occuring what would be the reasons for using Bridge modeling approach instead of culvert modeling for a 3 sided bottomless structure? I'm noticing that design criteria is being created based on overall flowrate and not stream conditions. In other words if you hit a certain threshold of discharge for the design storm then you are forced to design the structure a certain way.

Wouldn't the culvert options give you better data input for the stream, roughness coefficients, roadway input, actual culvert input?

The Bridge modeling approach forces you to make the culvert length the same as the roadway width even though that might not be the case. Plus wouldn't the concrete in the upper part of the arches have an effect on WSE as does the stream grading conditions and protections that are going to be used?

I just curious how y'all are modeling these structures.

 

[francesca]

Around these parts, anything 20' or greater span is a bridge and smaller structures are analyzed in HY-8. I haven't looked much into it in the Hydraulic Reference Manual, but when I modeled culverts before I got vastly different results to stage-storage hydrograph analyses. That was with pressure flow conditions, though.

 

[gbam]

the way I see it bridge modeling is best for low flow conditions as it factors in the xscc shape within/under the bridge. Once the inlet becomes submerged then there should be no/little difference in the hydraulics. As to the length, in HECRAS you can set the length of the bridge seperately from the road width using distances to upstream xsec.