RAS 2D bridge/no bridge water surface discrepancy

[Zain]

Hello

Whenver i model a bridge in RAS 2D, there is always discrepancy in the water surface profile. If I compare water surface profiles for a “bridge” and “no bridge” scenario for a condition in which water never touches the low chord, I see a drop in water surface profile exactly at the bridge which generates this back water effect. Considering the fact that water is not touching the low chord, water surface for both bridge and NO BRIDGE scenarios should be the same but this isn’t the case. I noticed this first time in v6.1 and i thought maybe its fixed in v6.3.1 but i still see this discrepancy.

Does anyone else has noticed this? Is there a way to address it?

Thanks

[Scott Miller]

When a bridge is included in geometry, whether 1D or 2D, the geometry preprocessor calculates a set of curves that relate flow rate past the bridge as a function of headwater and tailwater. It is different from modeling the crossing with 2D equations. The water surface ought to be comparable between bridge and no-bridge scenarios, but it likely won’t be the same.

My preference, if the water surface never reaches the lower rung of the bridge, is to model reaches with bridges as open channel. In design of crossings in jurisdictions I work in, the bridge would have to be one or two feet higher than the 100-year flow. Which clearance depends on the scale of the flow. Piers and abutments can be modeled as topography.

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