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modeling swales

modeling swales

modeling swales

I am a new user of Hydrocad, and I am attempting to model a swale that drains into a detention pond.  This swale has drainage flowing into it along its length, and drainage from a catch basin system discharging to the head of the swale.  Currently I have the entire drainage area modeled as one subcatchment, with the swale included as part of the T[c] calculation. A pond node models the last catch basin with a culvert outlet discharging to the swale.

What would the best way be to model this?  My thought is to break the subcatchment into multiple nodes, representing the drainage area.  The swale would then be modeled as 4 or 5 reaches, with point discharges along the length of the entire swale.  Is there a better way to make the model than this?  I look forward to your insights.

Shane Mullen

RE: modeling swales

Your approach sounds OK, although the big question is "what effect does the reach have on my model?"  If there are significant routing effects, than breaking it into smaller segments is justified.  But if the reach has little effect to begin with, there is nothing gained by modeling it in greater detail.  You can check the reach summary to see how attenuation and time lag are being introduced.  If the outflow is essentially equal to the inflow, the reach isn't having a significant effect on your results.

RE: modeling swales

From my experience, I am almost certain the Reach is producing no attenuation on the Tt or Tc.  To save time and paper, just leave that subarea as one and add the swale to a common Tc...I think is what you have said youv'e done.

If you need subarea attenuation, you will have to truly break the swale into storage volumes (with embankments and outlets devices) and model them as series ponds.  This will require much more grading and put much more of each lot's yard in the drainage easement...I doubt the reduction in final pond volume will be worth it.  I usually opt to just waste all the space on one lot and leave the others big.

RE: modeling swales


My big question is:

  What are your modeling goals?

I may set up the same site many different ways depending on what I am trying to depict.  If I'm looking at overall peak for a site, I may incorporate all reaches in the Tc.  If I need to know flows at specific points, I may break it down into reaches.  If I need to determine capacity and/or velocity in a channel during an event, I may break it into the reach.  If the reach is long or large enough to have storage or timing effects....

How bout more info?

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