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Multiple return periods in a single drainage system

Multiple return periods in a single drainage system

(OP)
Hello all,

As far as I know, in the design of an urban storm drainage system where drainage pipes pass through residential, local and collector roads, the whole system should be designed for a single return period (say 10-Years) and checked against a higher return period (say 50-Years) for possible flooding.

But what I'm being asked to do is to design the tertiary (residential) pipes for a 10-Year return period, the secondary (local) pipes for 25-Years and the Primary (collector) pipes for 50-Years. The logic being that flooding of residential roads can be tolerated while flooding of collector roads can not. See the attached drawing for an example.


So what is the correct approach for the design. Should the pipes be designed with different return periods based on classification? Or should I design the pipes for the larger return period, but would give me oversized pipes upstream?



Thank You.

RE: Multiple return periods in a single drainage system

It's not uncommon to see tertiary / secondary lines designed for lower return periods. Just do what they ask. If you want to be very clear about what's going on with your system, show the HGLs for all three events in your profiles.

I've seen some regulations for new residential subdivisions, for instance, that require the 25 year storm for all new lines that only carry internal runoff, but 100 year for lines that carry offsite water as well. So you'd have multiple requirements even within the same storm drain network. We usually just show both HGLs on the profile sheet, and provide two pipe charts.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Multiple return periods in a single drainage system

Do as you are asked, and show that when the HGLs pop, the overland flow can be safely conveyed to the higher systems overland within the road network, and not flood houses.

RE: Multiple return periods in a single drainage system

(OP)
Thank you beej67 and Twinkie.

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