The Link Time Step in SWMM 5, InfoSWMM and H2OMap SWMM

Introduction:  This set of blogs uses the 1000 year rainfall/runoff/hydraulics model that you can download at http://swmm2000.com/forum/topics/1000-year-simulation-with-rainfall-in-swmm-5 to show the inner workings of SWMM 5 and by extension InfoSWMM and H2oMap SWMM using a QA/QC version of SWMM 5 with extended graphics.   I always hope that seeing the inner workings of a SWMM 5 feature helps to understand the code, sensitivity and importance of a parameter.   It also helps show sometimes when a parameter is not important.  

Discussion:  Here we look at the link time step for a 100 year simulation.   If you use the Variable Time Step in SWMM5 with the CFL Adjustment factor the program will compute the needed link time step at each simulation time step based on the last time steps depth, velocity and width.  The link time step is

Link Time Step = Adjustment Factor * CFL Explicit Time Step for the Controlling Link

The time step is larger for low flows and decreases as the flow in the link increases (Figure 1).     The time step ranges between the maximum step allowed by the user during the simulation and the time step lengthening value in the Dynamic Wave Tab of the Simulation options.  The program will use the minimum of the time steps for ALL links.  The minimum time step at each simulation step is multiplied by the Adjustment Factor.  The time steps used during the simulation are listed in the Routing Time Step Summary table where you can find the average, minimum and maximum time steps.  The smaller the Adjustment Factor the smaller the link time steps during higher flow.

Figure 1.  The Link Time Step over a 100 Year Period for Link Venant