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Bridge Instabilities

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Good afternoon.


I have a 1D/2D model that is unstable but I am unsure how to move forward.


The model consists of a 2d_zsh which is lowering the bed level between 2 known points.


2d_BC set as SX

2d_BC set as CN

1d_nwk set as B with a length of 12.98 us/ds inverts and 0.1 Form loss

intersecting the 1d_nwk there is a 1D_bg CS with cross section data in


downstream of this bridge is a toc operated sluice gate.


Three out of for simulations have worked fine, the final one failed due to instabilities. I have halved the timestep but it is still being a pain. The flow through the bridge is oscillating massively.


In this general area I am getting the message " Warning 2991 - negative V Depth" I am also getting negative U depth.


I don't understand why this simulation is not running, as the only difference to the other sims is the tidal input.(which is smaller than the other sims) The actual model is the same.


thanks for any/all help. let me know if you require more information.


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You didn't state any details about grid size, cross-section spacing and where they are, so I'll give you a generic answer.


We just recently did some training and covered something similar. An old bridge embankment was impinging majorly on the floodplain flow in the 2D outside my 1D channel and I was having problems with stability elswhere in the model. It turns out that I didn't have enough 1D cross-sections near my bridge embankment causing large water level differences between my 1D and 2D. As a result I was getting huge circulation upstream and downstream of my bridge causing instabilities elsewhere in the model. I suggest that you look at adding at least one (if not two) cross-sections upstream and downstream of the bridge to help the model sort out the water level differences between the 1D and 2D without generating large circulation currents in the 2D.




Franciscus Maas

Senior Hydraulic Engineer

Opus International Consultants Ltd



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