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09/02/2011 16:14:12

I'm currently building a model with a number of bridges that are proving a bit problematic to get running due to locailsed areas of super-critical flow. I'm tending towards using the USBPR bridge unit where possible as the project is overseas and the bridge are likely to be similar in type to the US bridges on which the method was derived (rather than single arch bridges calibrated to UK data). For some of the bridges where Fr approaches/exceeds 1.0 at baseflow conditions, I'm finding I either have to: A) Increases initial flow significantly, or B) Adjust some bed levels significantly to smooth out the long section to get the model to run. I've also tried modelling some of the bridges as Bernoulli loss units which seems a bit more stable and requires less bed adjustment. However, I note that both methods are technically invalid for super-critical flows as they assume sub-critical. Use of an orifice unit is not really possible as the bridge openings are too complicated to approximate to a rectangle. So the question is, what is the best method for modelling a bridge where super-critical flow can occur. Any thoughts/opinions?

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