Ineffective Flow Boundaries at end sections

[Anonymous]

I know that it’s important to have the furthest upstream and downstream channel cross sections set at fully expanded flow if possible, but very often the project survey we get does not extend more than 1000 feet up and downstream of the bridge. Usually I can get the upstream side to fully expand by 1000 feet, which is important for bridge scour calculations, but the downstream side gives me the most trouble. If I have ineffective flow boundaries at the furthest downstream cross section projected along the recommended expansion ratio, is that still okay?

[Anonymous]

I would read the 6 cross section importance for bridges in the RAS manual. 1000 ft US or DS is too large of a distance from the face of the bridge. IMO

In an instance like this i would not want flow expanding DS 1000ft using 0.3 and 0.5 coefficients unless you have visual representation of the stream acting this way. Usually i interpolate US and DS to get a closer distance to the bridge following the RAS recommendations for the 6 cross sections of a bridge.

Hence the standard saved table “Six XS Bridge” in RAS

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