Friday, December 12, 2008

Failing Bridges

Written by Chris Goodell, P.E., D. WRE | WEST Consultants

In large flood studies, it's common for bridges to be impacted or overtopped by flood waters. A bridge holding back a lot of water, suddenly failing, could send an evern greater surge of water downstream. Some states require that bridges be failed during a dam breach simulation, if they are overtopped by a certain amount.

So, how do we fail bridges in HEC-RAS? There is no direct way, like we have with inline structures. However, by getting a little creative you can still simulate a failing bridge.

Try modeling the bridge as an inline structure. Youll have to model the bridge opening as a "gate", or series of gates (the piers could be modeled as the space between gates). Keep the gates fully open throughout the simulation and make sure that you calibrate the new inline structure to match the existing bridge structure. Tweak coefficients, adjust gate openings, whatever it takes to simulate the bridge over a wide range of flows. Then put in a breach like you would at a dam and you'll be able to fail it based on overtopping, overtopping with a duration, or at a specific time in the simulation.

This figure shows the original bridge.
And here is the same bridge modeled as an inline structure...
You might also try splitting the reach at the bridge. This works well if you expect to have a failure on the bridge approach embankment, as opposed to the bridge structure itself. Split the reach at the bridge into 2 parallel reaches. You'll have a divergence junction upstream and a convergence junction downstream of the bridge. Model the bridge deck and opening on one reach, and the embankment on the other. Then fail the embankment side.

Another option would be to place a "fake" lateral structure just upstream of the bridge. Connect it to the cross section just downstream of the bridge. In this example, you'll want to use the upper chord of the bridge for the lateral structure's crest profile. Keep the bridge structure (openings, piers, abutments, etc.) in the model as a bridge, but make the high chord of the deck and approach sufficiently high so that it never overtops during the simulation. We want the lateral structure to handle the overtopping flow and the original bridge to handle the pressure flow. Then put the breach data in the lateral structure and fail it based on your bridge failure overtopping criteria. This one is also good if you expect the failure to occur in the embankment.

Personally, I like the first option the best. It's easy to set up and easy to calibrate. Plus it looks more "real" than the other two.

Anyone have any other ideas for failing bridges in RAS???