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A Peculiar HGL, any ideas?

Posted by Ross Volkwein on May 7, 2013 at 2:12pm

I'm getting a strange HGL for a fairly simple run. Just running the flow routing module with an external inflow. Using Dynamic Wave with a 1 second time step, 15sec conduit lengthening time step, both normal flow limits. Why would manhole PL120 not be surcharging here? At one point I had several of these patterns at every other manhole. Any ideas of what I could try to determine the issue?

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Replies

Rodrigo Concha May 8, 2013 at 8:42am

Hi,

I saw your model and think that there is an error in your Manning roughness coefficient for all conduits: 0.0013; it is a very low value. I changed it for 0.013 and the model seems to be an expected behaviour
- Robert Dickinson > Rodrigo Concha May 8, 2013 at 9:12am
  
  Thanks, Rodrigo. That was a good observation by you. It also really does help his simulation results.
  - Ross Volkwein > Robert Dickinson May 8, 2013 at 1:01pm
    
    Yes, the switch between normal and St. Venant causing that behavior makes sense.
    
    An extra zero was the problem though, what a simple mistake that really skewed the results! Thank you Rodrigo!
Robert Dickinson May 7, 2013 at 5:15pm

I lowered the flow to 2 better show what is happening in your links. You have a mixture of normal flow for steep links, flat bes slopes and intermediate bed slopes, the hgl has to change to have the same flow in all links.
Robert Dickinson May 7, 2013 at 2:56pm

Hi, Please attach your model and I can comment. The HGL does look odd to me as well.
- Ross Volkwein > Robert Dickinson May 7, 2013 at 3:12pm
  
  Here is the model. The only inputs should be external inflow at the upper most junction and a DWF pattern downstream of this issue.
  - Robert Dickinson > Ross Volkwein May 7, 2013 at 3:32pm
    
    Hi, All that is happening is that the head has to build up those nodes with the spikes in the HGL to push the water downstream.
    - Ross Volkwein > Robert Dickinson May 7, 2013 at 3:54pm
      
      Hmm, I still don't understand how there can be a positively sloped HGL in the direction of flow. Between nodes PL119 and PL120 for example.
      - Robert Dickinson > Ross Volkwein May 7, 2013 at 4:06pm
        
        The flow is the same in each of your links but the depths are different in each link, to get the same flow through each link you need to increase the HGL. Some of the links are steep and use the normal flow equation and not the full St Venant which accounts for the odd slope to the HGL
Robert Dickinson May 7, 2013 at 2:28pm

Are you using kinematic wave or dynamic wave for the solution?