Different design and analysis methods are recommended by the 1987 and 2019 editions of Australian Rainfall and Runoff.

The 1987 Procedure

The 1987 procedure simulates the conversion of (a) rainfall intensities to peak flowrates, using the rational method, or (b) rainfall hyetograph patterns to flow hydrographs, using rainfall-runoff models such as Horton (ILSAX) and the extended rational method (ERM). The required intensity-frequency-duration data and temporal (time-based) rainfall patterns were usually taken from Australian Rainfall and Runoff 1987, which provides patterns for various average recurrence intervals (ARIs) and storm durations.

DRAINS runs through these 'stacked' storms, defining a 'worst case' result (such as the greatest flowrate in a pipe, or the highest water level in a pit). A design procedure is used to define pit and pipe sizes, and pipe invert levels.

The designed system can then be tested using detailed simulations for minor and major storms.  In minor storms (typically 5 or 10 year ARI events) the designed system should keep overflows from pits within acceptable limits (such as allowable widths of flows in street gutters, or safety limits based on flow depths and velocities). Major storms (typically 100 year ARI events) are used to check the system against additional criteria in extraordinary events.  The designed  system must work satisfactorily for storms of all the durations considered. Usually, this will require further trial and error calculations, varying system components until an effective and economic system is obtained.

The 2019 Procedure

This method is presented in Chapter 5 of Book 2 of ARR 2019, which describes the 1987 temporal patterns and others in Section 5.2.3. It then describes a process leading to the availability of sets or ensembles of 10 rainfall patterns on the ARR Data Hub. These ensembles are sampled from storm bursts taken from large sets of recorded rainfalls, and are available for 12 zones covering all parts of Australia.

Since the patterns in an ensemble are equally likely for each storm duration considered, it is appropriate to design for the median of the 10 patterns, rather than for the single representative pattern assumed in the 1987 data. We need to design and analyse for a 'median result' rather than a 'worst case' one.  This increases the amount of analysis tenfold.

DRAINS has been set up to do this process, incorporating changes to handle the large amounts of results generated.  I-F-D data obtained from the BOM website and temporal patterns from the ARR Data Hub are inputted into DRAINS. Runs are performed in the usual way, and results are presented as a series of bar chart, as shown below, for a single pipe:

Each group of 10 bars is for a given duration. For the 10 minute duration above, the seventh pink bar is the closest to the median, or called the 'upper median', and represents the answer for the 10 minute duration. For the 1 hour duration group, the third (red) bar is the closest to the median for that group.  It is also the highest of the (pink) upper median storms for the various durations, and becomes the basis for the design of the component.  Similar results are obtained for all the components in the system, such as pits, pipes and detention basins. With this information known, the system can be assessed using the same design criteria as the 1987 procedure.

According to Chapter 3 of Book 5 of ARR 2019, the new procedure should be used with an initial loss-continuing loss model. This has been implemented in DRAINS and can be applied with the ensemble median procedure, but the data required is incomplete at present. Further explanation is provided here.

However, the method can be applied with the Horton (ILSAX) and extended rational method hydrological models, as well as storage routing models (for analysis purposes).