Using HEC-RAS to model complex urban drainage systems

[Balarka]

Hi All – I have a few years of experience using HEC-RAS to model relatively straightforward river systems using both steady state and unsteady state analysis. I was recently asked to help with getting a HEC-RAS model running for a complex urban drainage system (the image above shows the extent of the model).

The model consists of a complex interconnected network of open channels with numerous culverts and bridges, as well as three pump stations. We were able to get a steady state run working (without the pumps), but the unsteady state model crashes at almost the first computation. We are now in the process of working through the model geometry to identify any unusual geometry that might be contributing to the model instability.

My immediate impression is that this is not an application that HEC-RAS was really designed for and that an alternative modeling package, such as XP-SWMM or PC-SWMM would have been a much better choice for this problem. I have found XP-SWMM to be very stable when modelling complex urban drainage systems, while I have found HEC-RAS to be very sensitive in certain situations.

I am just wondering whether anyone else out there might be willing to comment or provide any feedback on this issue. I believe that HEC-RAS is theoretically capable of modelling this situation, but I am concerned that it may take a considerable amount of time to get the model running and I would like to suggest to the model builder that he considers importing the HEC-RAS model into PC-SWMM or XP-SWMM, as an insurance policy in the event that the HEC-RAS model is not successful.

[cameron]

It can be done in either xpswmm or HEC-RAS or many other models. HEC-RAS is free where other software has costs associated with it. It sounds like your problem may have to do with initial conditions or not having enough flow going through the system. You could try adding a minimum flow for reaches, but be careful not to go to large as it will add up at the downstream end.

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