Using rational method for detention volumes 1

[kgf36]

Hi,

I had tried to create a different thread on a similar subject, but it appears that it has not gone through, so I'm trying again. (sorry if I post twice)

So for a small site, the rational method can be used to find storage volumes for detention basins, is that correct? And if so, how can one find the storage volume for the 100 year 24 hour storm if the IDF curve only goes up to 120 minutes?

 

[cvg]

the rational method is not recommended for analyzing a detention basin. the modified rational may be used possibly for conceptual level analysis, however still not recommended. Stick with a rainfall - runoff model which generates a hydrograph and can route the hydrograph through your basin.

 

[tsgrue]

.

I suggest reading the thread “Volume of Runoff Rational Method Formula” in the Storm/Flood Engineering Forum.

Volume of Runoff Rational Method Formula
thread162-198581

.



tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[kgf36]

Thank you for your thoughts.

To clarify my initial post, I have a set of calcs done for a basin using the rational method here that I'm trying to understand. What I am stuck on is that to determine the volume of storage needed, the IDF curve was used for the 100 year storm and for each time increment going from 5 minutes to 120 minutes. And somehow, supposedly,by doing it that way the 100 year 24 hour storm is accounted for. But I'm having a hard time understanding how or why it would be so.

And trust me, if I had done the calcs myself, I would not have used the rational method.

 

[cvg]

you won't understand it because it is not so.

you will need to do as previously suggested, find the 100-year storm rainfall depth and use that to estimate the storm hydrograph. you can then check the basin routing. if this is a "retention" only basin, than design it to hold the entire storm runoff (minus losses). Otherwise, you will need to hold the entire storm hydrograph, up to the peak and then some in order to attenuate the peak runoff released downstream.