Current practice in Australia for the design of piped urban drainage systems is to consider flow in the pipe as being steady with the pipe flowing full and under pressure; an approach recommended by both Argue (1986) and O¿Loughlin and Robinson (1987). The suggested advantages for the currently recommended approach include the inclusion of:

• energy losses associated with structural elements of the drainage system, and inclusion of
• downstream influences on the flow conditions such as those typically due to backwater effects.

Inherent in this approach, as discussed by Ball (2001), is the assumption that the drainage flows can enter freely the subsurface drainage network. Factors that may negate this assumption include:

• the characteristics of the inlet pits and other structures in the drainage network,
• partial or complete blockage of inlets, and
• the influence of the system hydraulics.

Since the last revision of Australian Rainfall and Runoff there have been a number of studies examined these issues with a resultant need to incorporate the results of these studies into Australian Rainfall and Runoff.

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