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Design Storms

Design Storms

Design Storms

The following is a policy stated in a County regulation--

Detention facility storage volume shall be adequate to attenuate the post-development peak discharge rates to pre development discharge rates for the 2,10 and 100 year storms.
Detention facility outlet structure release rates shall be less than the pre-development peak runoff rates for the 2,10 and 100 year storm events.
The total combined post-development peak flow rate from a development cannot exceed the total pre-development peak flow rate for the 2,10 and 100-year storms.

Why are the 2 and 10 year storms considered if the 100-year storm generates more peak flow (More precipitation)?


RE: Design Storms

A single outlet device designed for the 100-year event will generally be too large for the 2-year event, allowing the peak flow to pass through with minimal attenuation.

This is the reason for the multi-stage outlet controls used on many ponds.  The usual procedure is to start with the smallest event, designing the appropriate storage and outlet controls, and then work up through larger events, adding storage and outlet capability as required to handle the peak flow.

RE: Design Storms

What is the appropriate distribution to use for the 2,10 and 100 year design storms?
24 hr precip with a Type II distribution?
6 hr precip  with a Type II distribution?
24 hr precip with a 6 hour specified distribution?
6 hr precip with a  6 hr specified distribtuion?
24 hr precip with a 2 hour specified distribution?
6 hr precip with a  2 hr specified distribtuion?

All of the above, selecting the storm with the greatest

RE: Design Storms


Hopefully the regulating body has guidlines on the parameters to be used of the design storms.  

Here, in Delaware, we use the NRCS Type II 24 hour storm with the Delmarva Unit Hydrograph.  The 2 & 10 year events are regulated to pre-development rates and the 100 year rate is regulated in a certain geographic area (not in the coastal plain).

RE: Design Storms

I am totally new to HydroCAD.  Would you direct me if I have to calc the flow of a roof drain going into a manhole then out to street.  I know how to calc. it by Q=CIA but don't know how to do it by HydroCAD.

Thank you in advance.

RE: Design Storms


Please start a new thread rather than posting under an unrelated topic.

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