Subject:  How to Determine if your model is Unstable in SWMM 5 or InfoSWMM

SWMM 5 and InfoSWMM has a good output feature in the RPT file that tells you the list of links with the highest flow instability during the simulation.  If you look at the link flow with the highest instability value and it looks okay to you then it usually means the rest of your model output is stable.  The index is the number of flow turns for the link during the simulation.  A flow turn occurs when

We call DQ the difference between the New and Old flow,

The value of DQ is greater than 0.001 cfs (we do not want to count small perturbations),

The sign difference between the new DQ and the Old DQ is negative.   In other words we want to count those oscillations in which the DQ value was negative and is now positive or was positive and is now negative. We don't count then when the flow is monotonically increasing or decreasing in the link.

For example, the Link U-104 below has a large number of Flow Turns but a plot of the link flow shows the Flow Turns to mainly unimportant.