Tuesday, March 23, 2010

Bridge on a Spillway

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.

I just got an interesting question. How do I model a bridge that has a spillway just downstream of it? Here’s an example of what I’m talking about:

water flowing over the spillway at the Bitan Bridge, Xindian, 8 Aug 2007

As I see it, there are a three ways to approach this. You could go conventional, and model both a bridge and an inline weir and try to squeeze in a couple of cross sections in between. This is probably the easiest, but not always possible if the bridge is on top of, or very close to the weir.

Another approach is to model the bridge as a bridge, and the weir with a series of closely spaced cross sections. This can be problematic if you have a high drop over the spillway with low tailwater elevations. And the cross sections will have to be very tightly spaced, to prevent over-estimation of energy loss over the weir. This is not recommended for vertical drop structures.

A third approach would be to model both the bridge and the weir together as an inline structure. The bridge opening can be simulated using a gate (or series of gates). The space between the gates simulates the piers. The gate invert is the top of the weir. The gate height then simulates the distance from the top of the weir to the bottom chord of the bridge. The upper chord of the bridge can be entered in as the top of dam. Make sure to leave the gate wide open for the entire simulation. Also, a nice advantage is that each gate can have it’s own weir discharge coefficient.

image

Any other suggestions out there?

2 comments:

  1. Another approach that could work is to not model the bridge as a bridge and just use blocked obstructions to model the piers. In my opinion, this would be appropriate if you did not expect the water surface profile to touch the low chord during a flood event.

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  2. Nice ideas however, keep in mind that using any solution other than modeling the bridge in a conventional way means that you will not be able to carryout scour analysis automatically.

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