Annual Recurrence Interval (ARI) for Roof and Surface Drains

[leecivil]

Hi,

Can anyone give me a proper explanation why roof drainage systems (downpipes, gutters, etc.) are typically sized different in ARI than the surface drains it outfalls to?

Example, roof downpipes are designed to 20-Yr ARI along with overflow measures e.g. rainheads
However, it discharges to the pit on the ground with outlet pipes designed to 10-Yr ARI as required by councils.

I have always believed that roof downpipes must allow the greater flows to discharge from the roof to reduce chances of ponding/surcharging and causing damage. The flows through the buried pipes are then acting under head during the designed roof ARI.

Thanks very much.

 

[beej67]

I actually do work in one municipality that requires all roof drains, gutters, and associated MEP to be sized for the 100 year.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

just basic risk assessment - there is higher risk from flooding on the roof than on the ground

 

[leecivil]

Thanks guys. I also agree it's the risk of flooding and damage on the roofs as compared to surface drains.

 

[beej67]

The real answer to this question:

Can anyone give me a proper explanation why roof drainage systems (downpipes, gutters, etc.) are typically sized different in ARI than the surface drains it outfalls to?

...is that the requirements were typically brewed up by different government agencies with different engineering backgrounds (if any background at all) so their expectations of what to design for are basically unrelated.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com