SWMM tailwater conditions

[chrisncivil]

I am trying to examine the effect that different outfall fixed stages have on the volume of flow through the conduit leading into the outfall. I am looking at a submerged outfall.

I am new to SWMM and am trying a very simple model and cannot seem to get it to work. No matter what I change the Fixed Stage to, there is no difference to the flow through the conduit. I am sure that I am missing a basic but vital step. Any help will be much appreciated.

 

[gbam]

chris...Unless you have a coded diversion (can't remember if SWMM does that) the flow should be constant. What happens to your system upstream of the outfall? Does it unseal (open channel flow)? If the system is under pressure there should be an increase in HGL/EGL in your system with an increase of tailwater (submerged). If the system unseals then it shouild reach normal depth at some point upstream and it would not have a significant impact to the system.

 

[beej67]

Your post confused me, Chris. Are you looking at volume, or flow rate? Is it a steady flow analysis, or are you monkeying around with hydrographs?

The continuity equation (conservation of mass) says what goes in must come out, less what stays. If you're doing any sort of steady flow analysis the flow rates will never change no matter what you do with your outlet works, the grade lines will just change to permit the constant flow rate.

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

http://www.campbellcivil.com

 

[Ryb01]

chrisncivil,

Try running your model in "dynamic wave" flow routing.

Here's the methodology from the SWMM manual...

"Dynamic wave routing solves the complete one-dimensional Saint Venant flow equations and therefore produces the most theoretically accurate results. These equations consist of the continuity and momentum equations for conduits and a volume continuity equation at nodes.
With this form of routing it is possible to represent pressurized flow when a closed conduit becomes full, such that flows can exceed the full-flow Manning equation value. Flooding occurs when the water depth at a node exceeds the maximum available depth, and the excess flow is either lost from the system or can pond atop the node and re-enter the drainage system.
Dynamic wave routing can account for channel storage, backwater, entrance/exit losses, flow reversal, and pressurized flow. Because it couples together the solution for both water levels at nodes and flow in conduits it can be applied to any general network layout, even those containing multiple downstream diversions and loops. It is the method of choice for systems subjected to significant backwater effects due to downstream flow restrictions and with flow regulation via weirs and orifices. This generality comes at a price of having to use much smaller time steps, on the order of a minute or less (SWMM will automatically reduce the user-defined maximum time step as needed to maintain numerical stability)."

Hope this helps.