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





RE: SCS 24 hour storm with Rational Method? (tough one!)
RE: SCS 24 hour storm with Rational Method? (tough one!)
With HydroCAD-7 you can define custom IDF curves in several formats. For details see Start | Programs | HydroCAD | IDF info.
RE: SCS 24 hour storm with Rational Method? (tough one!)
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.
RE: SCS 24 hour storm with Rational Method? (tough one!)
RE: SCS 24 hour storm with Rational Method? (tough one!)
It semms similar to a synthetic hydrograph described by the Pima County Stormwater Detention/Retention manual in Arizona.
RE: SCS 24 hour storm with Rational Method? (tough one!)
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
RE: SCS 24 hour storm with Rational Method? (tough one!)
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?
RE: SCS 24 hour storm with Rational Method? (tough one!)
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