Modified Rational Method-routing

[kingpawn]

What is the largest watershed that the modified rational method can handle for detention pond design? Where can I find documentation on this watershed size?

I have noticed alot of people using a rectangle hydrograph storage method for their pond routing. This would mean that the release rate from the pond is at the peak runoff throughout the storm event. Using this method gives a very small storage requirement. I feel that this method is wrong. PLease advise.

Thanks for your time.

 

[RWF7437]

Opinions vary on the maximum watershed size for use of the Rational Method. FEMA allows its use on watersheds as large as 1 square mile (640 ac) but most others limit its use to far smaller areas.

The Modified Rational Method for sizing detention storage is widely used but you will find it nearly impossible to find documentation for it or a convincing explanation of its validity. Many agencies use it but HARDLY ANY BELIEVE IT. Personally, I consider it BOGUS.

good luck

 

[cvg]

The following thread discusses the application / mis-application of rational method to detention pond design...

thread194-120607