Modified Rational VS. SCS 1

[proud2banerd]

I'm designing infiltration beds for a new parking lot and two new buildings - the site is barely 2.5 acres. The Township has asked that we try to fully infiltrate a 10YR, 24 hr event.

The time of concentration is miniscule--depending on which calc I use, it ranges from <1 minute to 3 minutes.

I tried computing the storage volume, using the Modified Rational Method, but the volumes seem really small--they're about half of what I get when I use Hydraflow to compute the runoff with the SCS method (with a Type III storm distribution).

Can anyone offer advice on what I may be doing wrong?
Am I making the situation far too complicated?
Would it be terrible overconservative if I simply took the 10 yr, 24 hr rainfall (in) from the contour maps in the back of the TR-55 and calculate the total volume for that depth over my drainage area? I know that would neglect the losses due to abstraction, etc, but would that matter since my area is so small and impervious?

By the way, I realize that there are lots of threads covering the whole modified rational issue, but none of them seemed to directly address my problem--so I'm starting a new one!

Thanks!

 

[RWF7437]

First, you're probably not doing anything "wrong" except trying to use the modified Rational Method. Some consider this method bogus while others have derived variations of it ( Los Angeles County for example ) which are less bogus but yield results comparble to the NRCS 484 method or the Santa Barbara Urban hydrograph method.

If you don't live in southern California, your NRCS ( formerly SCS ) method should be acceptable to the reviewing agency. Using it with reasonable soils and antecedent moisture condition assumptions should give you decent results and will take in to account the other losses.

Your next problem will be what to use for estimating infiltration. This is difficult without onsite testing and even then is simply a best guess.

For this small site such rigamarole seems hardly justified but then there are those who think local detention is cost effective.

good luck

 

[proud2banerd]

Thanks! Glad to hear that it may not be due to a huge mistake in my calcss, yet to be discovered. I'll sleep better tonight.

 

[RWF7437]

http://www.weather.gov/oh/hdsc/

 

[cvg]

are you designing a "retention basin"? If so, why are you calculating peak discharges? you need to calculate volume for a 24-hour storm rainfall event. V = C*A*P/12 (P=precip in inches)

 

[RWF7437]

cvg,

I would suspect that proud2banerd is also designing the conveyance system. Even for this tiny site there is probably some piping, catch basins, etc. Also, the basin must have an overflow of some type in case the infiltration doesn't work. For those reasons, he/she need to estimate probable peak flows.

If nothing else, this exercise helps to illustrate the near uselessness of the Rational Method. Modifying it to produce an unrealistic hydrograph doesn't seem to help it very much. It is also likely that peak flows calculated using regional regression equations would result in higher values. So, the Rational Method appears not to be as conservative as conventional wisdom would have us believe.

It seems also, very unlikely that small, local detention or retention basins are effective at all. They are therefor unlikely to be cost effective.

The NRCS methods aren't very much more "scientific" but they do have the advantage of being capable of being calibrated based on rainfall and stream flow gages. This has been done in some areas of the US but, not as many as we might like.

These are only my views, of course, so please feel free to tell us yours and those of your colleagues.

good luck

 

[geosavvy]

I have been trying to pursuade local engineers from using the modified rational, though its been allowed here in the past.

On a few small sites recently both methods were used for comparison and the required storage was around double (sometimes more) when using the NRCS analysis.

Based on what I've seen and read, I too think the Modified Rational is bogus. You are using only a couple of variables to extrapolate a lot of important information.

Using the modified rational method to design a detention pond would be analagous to a geotechnical engineer assigning an allowable bearing pressure based on the color of the soil. In a few situations/geologies it may give you a ballpark idea... but overall, its basically dumbing down an engineering analysis to a series of steps that a 9th grader could perform.