Momentum Modeling at Bridges

[Anonymous]

I am looking for more guidance about when to use the momentum bridge modeling approach and hoping that people on this forum have some experience. I have a few different models (typically of very channelized, urban rivers) and many of my bridges show a much higher upstream water surface using the momentum equation compared to the energy equation. So far, the only instruction I have found is to include momentum when there are piers. Are there times when the momentum equation predicts a water surface that is too high? We have found that a 2D model of the area computes a water surface much lower than the HEC-RAS momentum equation but are unsure of why.

I have also played around with turning on and off the friction and weight components. For some bridges this makes no difference, but for others it can make a big impact. Is there information about when it is appropriate to use both friction and weight vs one or the other?

[Anonymous]

A bridge with piers across a concrete channel with a high velocity stream is probably the situation where the momentum equation works best. (The momentum method can definitely have problems with natural streams especially if the shape of the channel/cross section changes a lot from upstream to downstream of the bridge).
The usual reason for using the momentum method is that the energy method does not capture the losses associated with the high velocity impacting the pier. The energy method captures the losses from the contraction/expansion of the flow. If there is no embankment, then the amount of contraction might be small. The energy method will also capture friction losses. But it does not directly account for the pier impact.
If the stream has a significant velocity, it wouldn’t suprise me that you are getting greater losses with momentum than energy. (If the 2D model does not account for impact losses then the same logic would also apply.) On the one hand, this might be a more accurate answer. But on the other, I agree that the momentum method can sometimes get crazy high answers and a lot of caution is recommended.

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