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