Rational method application

[efejr28]

Hello everyone. I am wondering about the rational method applicability. For less than 160ac & retention facilities...
But for more than 160ac. and detention facilities use HEC-1 modeling... ok.
I have seen the rational method where instead of intensity (in/hr), only inches are used. What is the difference of this variation?
What is SCS?

Thank you,
EF

 

[LHA]

I would not use the Rational Method on anything over 10 acres, or over 1 acre unless it is an homogenous watershed (parking lot, ball field, woods).

Inches of runoff can't be used in the Rational Method, so that instance you saw was wrong. Rational Method uses intensity to get a peak flow, only.

SCS = Soil Conservation Service, now called the National Resource Conservation Service. They created TR20 and TR55, which is what you should, by and large be using for anything between 10 acres and 1.5 sq mile. TR55 software is free at NRCS website, it is easy to use, if you've had hydrology coursework.

Over 1.5 sq mi, use a statistical flood model.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[mrwhiteOH]

The issue I see with using rational method for a detention facility is not related to drainage area (Different sources will give differing limiting values, anywhere from 50-250 acres, personally I think about 200 is max.) but rather the fact that a hydrograph cannot be constructed from rational method data, and therefore stage-storage-discharge relationships, as a function of time through a pond, cannot be determined.

There are some quick methods using rational such as assuming a triangular hydrograph using rational peak as max. peak at time equal to time of concentration. This method falls apart for any drainage basin greater than just a few acres.

I recommend using TR-55 or similar method of generating a hydrogragh for your particular design.

regards

KEW

 

[SimonDone]

Reading various sources about the Rational Method, I see recommendations for the maximum catchment area ranging from less than 1 to over 500 hectares. Is there consensus for the maximum size? Why this variation?

Secondly, what should be used for larger catchments? Am I right in thinking that there are no generic methods and that the specific water course flows should be investigated?

With thanks to all,

Simon

 

[LHA]

I am not aware of any consensus regarding watershed size.

For larger areas, use a statistical model. There is one for PA, USA called PSU-4. It is available here:

http://www.dot.state.pa.us/hh/

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve