Return Periods Using The SWMM 5 Statistics Tool

Return Periods Using The  SWMM 5 Statistics Tool

A great feature in SWMM 5 is the Statistics tool which allows you compute the long term statistics for 10, 25, 50, 100, 500 or 1000 years (and probably longer).  You run the simulation, pick a parameter to analyze then compute the:
A. Return Period of all of the Events,
B. Based on the Overall Time Series of Results, and
C. The Frequency Graph of the Found Events.

The Return Period is normally the Simulation Length + 1, For example, a 100 year simulation has a return period of 101 years.  Or in terms of the SWMM 5 Delphi Code

K := E^.Rank;
R := (Results.Duration + 1 – 2*Stats.PlotParameter)/(K – Stats.PlotParameter);

where the PlotParameter is normally zero so the value for R ends up being

R := (Results.Duration + 1 )/(K);

Return Periods in SWMM 5 Statistics

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