Stormwater Drainage Design
The Australian Standard related to stormwater drainage is AS 3500.3.2:2003 “Stormwater Drainage Acceptable Solutions”.
Most councils have guidelines on drainage design. Bega Valley Shire Council has specification D5.
Useful design sources on the internet
http://www.roof-gutter-design.com.au/ This site has calculators for design to AS3500 including eaves gutters, downpipes, pipes and box gutters.
www.bom.gov.au/ The Bureau of Meteorolgy site has an IFD calculator which can produce IFD charts for specific locations within Australia.
ARI – Annual Recurrance Interval
The annual recurrance interval is known as the return period, this is the mean time between occurances of some event, such as a flood or rainfall event.It is used as the foundation of many design procedures based on acceptable frequencies of occurence. For urban stormwater drainage design the ‘event’ will be the exceedance of some rainfall intensity and quantity over time and the resulting stormwater flows.
Time of Concentration
The time of concentration is the time it takes water to travel from the furthest point in the catchment to the point under investigation. To generate peak flow from a catchment, a storm must last at least this long. The longer a storm lasts, the lower the average intensity.
Design IFD rainfall
Design Intensity Frequency and Duration rainfall relationships are generally derived in accordance with Australian Rainfall and Runoff (1987 Engineers Australia). The Bureau of Meteorolgy www.bom.gov.au/ has an IFD calculator which can produce IFD charts for specific locations within Australia.
AS3500.3 contains a procedure for the design of Box Gutters. Minimum width for commercial applications is 300mm and 200min for domestic applications ( ref Pg 32 AS3500).