Very frequent design rainfalls - An enhancement to the new IFDs

The Bureau of Meteorology (BoM) has released new Intensity-Frequency-Duration (IFD) design rainfalls for Australia for probabilities from 1 Exceedance per Year (EY) to 1% Annual Exceedance Probability (AEP). This is suitable for most hydraulic infrastructure design; however Water Sensitive Urban Design (WSUD) requires IFD's for events more frequent than 1 EY. Many stormwater quality or WSUD guidelines recommend a flow threshold of Q3month for the design of stormwater quality treatment devices. Design rainfalls for the 3 month average recurrence interval (4 EY) have not been previously available, with agencies giving their own advice on the approach for estimating very frequent design rainfalls. To address this need, estimates for probabilities more frequent than 1 EY will be provided as part of Phase 2 of the IFD Revision Project. A ratio approach has been adopted, using the at-site Partial Duration Series (PDS) to determine the ratio of the various very frequent design rainfall values with the 50% AEP IFDs. L-moments were used to characterise the distributional properties of the PDS data with the Generalised Pareto Distribution (GPA) found to best represent the at-site PDS. Quantile depths calculated at each site are used to derive the ratios relative to the 50% AEP value. They are then gridded in the smoothing spline software ANUSPLIN. The splines are fitted using three independent variables; latitude, longitude and elevation, with the appropriate degree of smoothing for the fitted functions determined through generalised cross validation. Depths have been calculated from the gridded ratios and the 50% AEP IFD depths for durations from one minute to seven days and probabilities of 2, 3, 4, 6 and 12 EY. The very frequent design rainfall estimates will improve consistency of design flow estimation for WSUD and urban development by providing nationally consistent values for use with the design guidelines.