SCS 24 hour storm with Rational Method? (tough one!) 1

[LHA]

Fellow geniuses:

A competitor's software has an option which allows hydrographs to be generated using the Rational Method, but with the intensity from SCS 24 hour storm distribution. The Return Rate, Precipitation (in inches) and SCS Rainfall Type are specified...from there, an IDF curve is (apparently) generated. Where does this intensity come from? Does HydroCAD offer such an IDF.txt file? I would like to check the calcs using my HydroCAD.

I have received calculations to review. They seem to meet the Ordinance, and I don't want to disallow the method out-of-hand. If its validity can be verified, I will accept it.

Can anyone point me to a source which will allow me to verify the logic behind, and to spot check these.

Thank you.

--Steve

 

[pepperlick]

In my experience, using the TR-20 rainfall as the Intensity value in Rational flows will result in a lower runoff. At least in my area (Massachusetts), the IDF curve will yield 'I' values approx. an inch higher (for short Tc's). Remember, 'I' values vary with Tc. Why not accept the TR-20 flow on its merit?

 

[psmart]

Although it is possible to convert a rainfall distribution (such as an SCS 24-hour storm) to an IDF curve, the general practice is to use the published IDF data for the location.

With HydroCAD-7 you can define custom IDF curves in several formats. For details see Start | Programs | HydroCAD | IDF info.

 

[bstone]

On page 43 of the HydroCAD manual (version 7), there is an interesting discussion of the origin of the SCS rainfall distributions. According to that discussion, the SCS curves incorporate the rainfall intensities on 30-minute intervals from IDF curves for a given location and return period. In effect, the SCS curve for a 25-year, 24-hour storm should include intensities for 25-year storms lasting 30 minutes, 60 minutes, 90 minutes, etc. on up to 24 hours.

In my experience, I've found that using IDF curves leads to lower peak discharges than TR-20 or TR-55 predict using 24-hour duration storms. However, the big difference is that the "C" in the rational method is not the same as the "CN" in the SCS method and I haven't found a lot of good references for selecting a "C" so maybe my "C" values were off.

 

[psmart]

Even converting C to CN values is problematic. The rational equation is linear, and produces some runoff for even the smallest amount of rainfall. But the SCS runoff equation is non-linear, and produces no runoff at all until the initial abstraction is satisfied. These and other differences make it extremely difficult to do direct comparisons of SCS and Rational results. For further background please see

http://www.hydrocad.net/6166b.htm

 

[queque]

WHAT IS THE NAME OF THE SOFTWARE?
It semms similar to a synthetic hydrograph described by the Pima County Stormwater Detention/Retention manual in Arizona.

 

[LHA]

It is called Virginia Tech/Penn State Urban Hydrology (VT/PSUHM).

Does Pima manual have anything of substance regarding the algorhythm used to synthesize the intensities?

It gives a hydrograph, so if I can document how it is produced and spot check the math, I will accept it.

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

 

[queque]

Does anyone have a Rational Equation use criteria? Is there a limit on the size of the drainage area that limits it's use?
I have been told that the Modified Rational equation in use in southern Arizon is suitable up to ten square miles.
As a surface water and groundwater modeler, I find that hard to belive. If you cannot calibrate a HEC 1 model in an area with no data, its results and use is questionable. How does the use of a Rational equation for a drainage area beyond 5 acres become justified?

 

[LHA]

More than size of watershed, I feel its validity has more to do with the homogenity of the watershed.

Here in PA, Rational is typically allowed up to 10 acres, but I use SCS for anything over 1 acre. Ten Sq mi is out ot the question, but we have huge changes in topography, cover and soil type over any given 10 sq mi area.

If southern AZ has relatively little change in topography, cover and soil type over any given 10 sq mi area (if the whole 10 sq mi is flat, sand and desert), then I could see a common C value applied.

However, it would also depend on the typical storm type...Rational assumes constant intensity.

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