Rain Events – Chicago Design Storm

A design rain event is a synthetic rain event created in order to simulate a critical condition for the design of a given structure or measure (e.g. a stormwater drainage system or a culvert). The Chicago Design Storm (CDS) is a widely used method for generating design storms for urban applications. It relies on an analytical curve that is normally fitted to an observed IDF curve, i.e. a curve describing the relationship between Intensity, Duration and Frequency observed in the rainfall record of a local rain gauge with high temporal resolution. For the analytical curve we use the Sherman equation (Sherman, 1931):

Where i_t is rainfall intensity, t is storm duration, and a, b, and n are coefficients that represent local conditions and the return period. We find these coefficients by fitting the equation to observed IDF-curves using non-linear least squares. 

Using these coefficients, we generate CDS events using the following equations for intensities before (2) and after the peak intensity (3):

Where t₁ and t₂ represent the time from peak intensity (considered here as time zero) and γ is the peak-time ratio (which controls when the peak occurs during the rain event).

Our CDS-events are constructed using a peak-time ratio of 0.5 (i.e. the peak intensity occurs at the middle of the event), the time steps are 5-10 minutes long, and the standard duration is 4 hours.