This name refers to design and checking procedures in urban stormwater drainage practice that use two levels of storm event. The minor event is used to determine the layout and size of pipes and other elements for convenience and safety. For street drainage systems this is usually a storm in the range of 1 to 10 year ARI or 1EY to 10% AEP. The major event is used to check that the system fails safe. It involves a storm of around 100 year ARI (1% AEP) that will overtax a normal stormwater drainage system, so that considerable overflows will occur along overland flow paths.
These procedures originated in the U.S. and Canada and were adopted in Australian manuals such as Australian Rainfall and Runoff (1987) and QUDM.
In DRAINS design procedures, major and minor storms are selected from the Select Storms option in the Project menu. The design method establishes pipe diameters and invert levels, considering both major and minor flows. Analyses for minor and major conditions must then be run separately, to check the operation of the designed system.