Wednesday, July 5, 2017

How does the CFL time Step Change in one SWMM 5 Link? #Inside_SWMM5

Introduction – the reason for these series of blogs are as an expanded view of the input, engine and output of #SWMM5 It is a companion to the EPA Documentation which I describe here:
I have noticed based on email questions and postings to the SWMM List Sever (a great resource hosted by CHI, Inc.) that many SWMM 5 users do not know about the really outstanding documentation on SWMM 5 posted on the EPA Website It consists of two now and in the near future three volumes on Hydrology, Water Quality, LID's and SuDS and Hydraulics. The documentation is fantastically complete with detailed background on the theory, process parameters and completely worked out examples for all of the processes in SWMM5. It is truly an outstanding aid to modelers and modellers worldwide. It would benefit you to read them (if you have not already downloaded the PDF files)

If you use variable time steps in SWMM5 then based on the CFL time step condition the program will adjust the time step.  The smallest link time step governs the program time step.  You can see the results for one link in Figure 1.  The time step increases as the velocity and depth increase during the simulation.
Variable Time Step (SWMM5 Help File)
Check the box if an internally computed variable time step should be used at each routing time period and select an adjustment (or safety) factor to apply to this time step. The variable time step is computed so as to satisfy the Courant condition within each conduit. A typical adjustment factor would be 75% to provide some margin of conservatism. The computed variable time step will not be less than the minimum variable step discussed below nor be greater than the fixed time step specified on the Time Steps page of the dialog.
        Figure 1.  The time step increases as the velocity and depth increase during the simulation.

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