SWMM5 uses
the Courant–Friedrichs–Lewy (CFL) condition to compute the variable time step used during each time step of a simulation. In general, the shortest link length with the highest velocity and depth will dominate the time step computations. The CFL time step is the link length over the current velocity plus the current wave celerity. If you plot the velocity and computed time step over time the time step will decrease as the velocity increases (Figure 1). Figure 1 also shows why a hot start file is important in SWMM5. If you start out with a dry network the SWMM5 engine will not be able to have a good estimate of the needed CFL time step. The depth and velocity will be zero.
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| Figure 1 - The relationship between the current link mid point velocity and the time step in a link of SWMM5. |
