Merits and Demerits of using Rational Formula 1

[palta79]

Dear All,

I have designed a sewerage network of a small catchment(less than 200 hectare) using maximum value of the run off based on Rational Formula. The pipes were sized using Manning Equation.

Now I am trying to understand the merits and demerits of using Rational Formula and Manning Equation, any other formula which could have been used for run off estimation.

Please can anyone suggests some points in this regard.

Regards

 

[cvg]

this article might shed some light on the pros and cons:

http://www.stormh2o.com/SW/Articles/Voodoo_Hydrology_37.aspx

 

[CPENG78]

Palta79,
There have been extensive discussions on this site as to the pros and cos for the rational method including its limitations. The term "small catchment" is a relative term depending on the actual size of your catchment. For the rational method, I have seen in literature the cut off to be anywhere between 160 to 200 acres. That's only 65 - 80 hectares more or less.

Look up the following threads for initial search: thread194-174764 and/or thread162-198581

It would appear to me that based on your "size" description, that you may need to look at a different method.

 

[cvg]

I have worked for agencies where the cutoff was 640 acres, this is set by local ordinance and based on that size, 200 hectares is well within possible limitations of the method. However there may be better, simple methods than rational for this project.

 

[CPENG78]

CVG,
Very true, it all depends on the local agencies and I have seen the area cut off with different area cut offs. As an example the 160 Acre came from a state agency. I do have to mention again and I agree with you that a different method may be in line for this one.

 

[fel3]

In 1989 I prepared a storm drainage master plan for a small rural community that is adjacent to and down slope of an interstate freeway. As part of my research I obtained the 1960s era storm drainage study prepared by the California Department of Transportation for the freeway. The Caltrans study was primarily devoted to sizing culverts under the freeway. Imagine my surprise when I discovered that they had applied the Rational Method to a watershed measuring just over 5 square miles immediately upstream of my study area and to even larger watersheds that were outside my study area. I "reverse engineered" their time of concentration results and determined that they had used the Kirpich Equation.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[palta79]

Thank you all for your response.