1st The time step you use in SWMM 5 is controlled from the top by the rainfall interval (Figure 1):
1. All of your time steps should be less than the rainfall interval,
2. The hydrology time step should be less than or equal to the smallest raingage rainfall interval in your network,
3. The hydraulic time step should be less than or equal to the hydrology time step and should be based on the hydraulic needs of the your network. Short length links, pump and weirs may require a smaller maximum hydraulic time step.
2nd The report time step controls what you see in the graphics output of SWMM 5. If you see a large difference between that you see in the graphics output and the report text file it is because you have a large difference between the report time step and the average time step used during the simulation.
Solution: If there is a large discrepancy in the graphics and report text file then the best solution is to reduce the maximum time hydraulic time step so it is closer to the average time step and also to make the report time step closer to the Maximum time step (Figure 2).
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| Figure 1: Relationship between the rainfall, hydrology and hydraulic time steps. |
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Figure 2: Relationship between the minimum, average and maximum simulation time steps and the report time step. |
Comments
My apologies, i uploaded my images to Posterous and lost them when Twitter bought Posterous and closed down the blogging platform. i have moved almost all of the images to swmm5.net and I will eventually fix them here.
Subject: SWMM 5, H2OMap SWMM 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