**Subject:** SWMM 5 and InfoSWMM Time Step Guide

If you use a variable time step in SWMM 5 or InfoSWMM/H2OMAP SWMM it is hard to gauge the proper value of the conduit lengthening. You want to use a value that does not increase the volume of the network yet does increase the length of the shortest links so you can use a longer time step. A good approximation to the time step that you want to use is shown in the image.

The **Time Step Guide** in seconds is **Link Length / [Velocity + sqrt(g*Maximum Depth)]** with the assumption that the velocity at maximum depth is about the value of the wave celerity for closed links or **sqrt****(g*Maximum Depth). **Normally (unless pumps are involved) the average time step used during the simulation is a good gauge of the time to use for the simulation. For example, in this model run the time step used is 13 seconds which is about the conduit lengthening time step of 20 seconds * adjustment factor of 0.75