Design Storm Duration

[PJC69]

I have modelled a residential development for which we are required to attenuate the post development peak discharge to the level of the pre-development peak discharge for all events up to 1 in 25 year storm. The catchment is small (2 Ha) and the time of concentration 10 minutes. I have received back comment from the local authority who do not appear to be familiar with the SCS Methodology of using synthetic 24 hour storms with nested rainfall intensities. They comment ... it appears that the option selected in your programme only shows storage required after 11 hours of precipitation, and it is suggested that this antecedent condition is clearly intended for much larger catchments than your development. It is our experience that the critical storm is likely to be between 20 and 40 minutes duration, and storage volumes in the attenuation pond will be significantly more that Hydrocad calculates using the 24 hour storm.....
Anyone been faced with a similar response from the regulating authority and how did you convince them that the method used is correct ?

 

[RWF7437]

It is probable that the reviewing agency is using the so called "Modified Rational Method". If so, they should tell you that they require that method be used. For small drainage areas, such a method will typically yield required storage volumes larger than the TR-55 method.

Whether either method is "correct" is very doubtful. No one knows the answer to that question with any certainty.

good luck

 

[psmart]

Do your regulations require the Rational method or the SCS/NRCS procedure? They should state specifically which is acceptable. If the SCS method is accepted, a 24-hour storm is used in virtually all cases. If they need some of the reasoning behind the 24-hour storm please see

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

 

[PJC69]

The regulations do not prescribe the method to be used. The local authority did note that Hydrocad offers the option of checking using the Rational Method, and they recommend that I establish the critical storm for this particular catchment and pond configuration to confirm the results presented in my report (which made use of the SCS procedure). My concerns are that comparing peak outflows using different methods is not the point. I'm comparing before and after (development) outflows in order to assess the required storage volume and therefore its paramount to use one method. Rational method predicts peak runoff but not runoff volume and therefore is not suitable for routing calculations to determine storage volume.

 

[RWF7437]

The documentation for WIN TR55 says the method may be used for drainage areas as small as 1 acre to as large as 25 square miles. The Modified Rational Method is probably very conservative as compared to the TR-55 method and has little scientific basis, if any. You are correct that comparing the two methods doesn't make much sense. Why not calculate several storms of different durations, using the TR-55 method, to see if storm duration affects required storage volume appreciably. You may not be able to do this in HydroCad but other programs are available. SMADA is one such program.

You may also want to use the free program NFF (National Flood Frequency) which uses regional regression equations to calculate flows and gives you a "generic hydrograph" for both urban and rural basins.

Whatever method you use, route several storms through your basin to see if the volume seems reasonable.

good luck

 

[psmart]

PJC69, I agree that comparing Rational to SCS results is a pointless exercise. Pre vs Post is the real issue, and this must be done with the same methodology.

The request to determine the "critical storm" is probably based on experience with the Rational method. Proper use of the Rational method does require the determination of the critical duration (and the associated intensity) for any given point in the watershed.

However, the SCS method uses a "nested" synthetic rainfall distribution that effectively tests all durations in a single calculation, and does not require a specific determination of the critical duration.

The only advice I can offer is for the agency to consult some of the reference materials on the SCS method. (See my previous link.)