Cross section locations for bridges w/o constriction

[Igloo_79]

Hi – I am modelling some bridges using a topographic survey, most of which have decks without any lateral constriction on the channel width. Thus, there is no contraction or expansion reach. In this case, is it necessary to have the 2 extra cross sections upstream and downstream (in addition to the 2 CSs bounding the bridge itself)?

Also, if you have a survey of the upstream bridge face/channel section only, can HEC-RAS project/interpolate a section for the downstream face? Or do you need a separate channel survey at the downstream section. Thanks.

[cameron]

It is generally best to have a survey cross-section on both the upstream and downstream of a bridge, but could make a copy of the upstream cross-section and set it at the downstream end of the bridge. You would probably need to lower the elevations on the copied cross-section though.

If there is no constriction you don’t need to add ineffective areas, but you would still need multiple cross-sections upstream and downstream of a bridge to model it correctly.

[Igloo_79]

Thanks for your reply. Perhaps I should’ve phrased my query differently:

I understand that sections 1 & 4 (figure below) are required US and DS of the bridge – however, if there is no expansion or contraction length (i.e. there is no lateral obstruction to flow), then the placement of these sections is much more subjective.

I’d appreciate any feedback on how to locate these sections. For instance, can they be a significant distance away (e.g. 100m) if there are not significant changes in channel profile?

[Anonymous]

My understanding is that the guidance for cross-section locations is intended to account for the expansion and contraction of flow through the bridge opening. If there is no expansion/contraction of flow, then perhaps the guidance is not applicable. That said, I don’t see any down side to following the guidance – it generally doesn’t hurt to apply more cross-sections to a model.

The distance that the cross-sections can be spaced is dependent on the reach characteristics, so I can’t say if 100m is too far.

[cameron]

There are some general guidance for cross-section spacing such as Samuel’s Equation or 5 times the channel width.

But as Keith stated, it is reach/model dependent.

[Author]

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