Hydraulic open channel modeling question HEC-RAS? SWMM?

[max0000]

I've got a tricky hydraulic open channel modeling problem (see the attached rough sketch). Essentially I have a large open channel that carries the 10- and 50-year storms, then a large swale parallel to the open channel to help convey the 100-year storm. However, the swale cannot extend past our property, so we are looking at putting in a very large pipe to move the water from the swale into the main channel. I have a HEC-RAS model from the county for the main channel, but I'm guessing HEC-RAS cannot model the dead-end swale + large culvert. Would SWMM work for this? Or do you have any other ideas for how I could model the water surface elevations? The overall goal of the project is to mitigate for a big mound of dirt next to the main channel. Widening / improving the channel is not really an option.

 

[gbam]

I disagree, one can create a junction DS of the pipe outlet and have the swale as a tributary. One question is; what happens in the 100-yr? Does the main channel have capacity to convey the 100-year flow? Will the combined flows create overflow out of the main channel downstream of the junction.

Another option would be to calculate the losses at the pipe externally then hard code a starting condition.

 

[beej67]

If it were me, I'd do SWMM for the 10 and 50 and HEC-RAS for the 100. In the SWMM model I'd have the large swale as one node (or a trapezoidal reach with the geometry buried in the link instead of the node) with a culvert tied in to the main channel, being sure to allow backwater effects to propagate your model. In the HEC-RAS model I'd model the thing as one combined channel, neglecting the pipe, with sections both upstream and downstream of the pipe connection.

The SWMM model would be useful in being sure that your swale doesn't overflow at more frequent storms, the HEC-RAS model would be useful at showing the effects of the floodplain.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Ekenny_G]

You can use SWMM or HEC-HMS to model the system as described by others. The main channel and swale can be modeled as a REACH and the pipe will be modeled as an outlet structure discharging into the main channel (Reach).
During a large storm, the pipe will be submerged and flow overtopping, so HEC-RAS seem ideal for the 100-year. You will just make the Large swale your "main channel" and flow path thru the culvert and then back to the main channel. Add cross sections upstream and downstream of the pipe culvert.