Graphical Peak Discharge Method for 25-year, 6-hour storm

[ewbeng]

I am trying to determine the pre- and post-development runoff volume for the 25-year, 6-hour storm (per City requirements). I've brushed up on the various methods and would like to use the Graphical Peak Discharge Method. Where I get confused (and maybe I've selected the wrong method) is the TR-55 mentions the 24-hour storm. Is this method valid only using the 24-hour storm or could the 6-hour storm rainfall be entered in as P in the equation Q=(P-0.2S)^2/(P+0.8S)?
Would that be valid as long as the 6-hour duration was used for both pre- and post-development calculations?

Similarly, moving forward to the steps to determine the peak runoff rate - am I reading it correctly that in the sheet flow calculation, the 2-year, 24-hour rainfall is always used?

And in computing Ia/P, the 25-year, 24-hour rainfall amount would always be used?

Thanks for any insight.

 

[psmart]

The SCS runoff equation you cited can be used to calculate the precipitation excess (runoff) for any rainfall depth. However, the graphical peak discharge method involves the use of peak discharge graphs (TR-55 Exhibit 4) which were developed specifically for the SCS 24-hour storm distributions, so I don't think you can use the procedure for other durations.

This begs the question, why not use the full SCS hydrograph generation procedure, which can be used with any storm distribution or duration? This is SOP with HydroCAD. If you're not using HydroCAD, you might want to post in the general Storm/Flood forum for a wider audience.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[ewbeng]

psmart,
Thanks for the quick reply and information. As I mentioned, I had to brush up to get to where I'm currently at and figured I must be going down the wrong path (Graphical Peak Discharge Method) ... so the full SCS hydrograph generation procedure? ... is there a manual procedure for that or is it using the WinTR-20?
Also, I now see I posted in the wrong area - I had been reading a thread in this area and simply added my question. I'm 0 for 2 today!!

 

[psmart]

You'll need software to do the full SCS hydrograph procedure. You can use WinTR-55 or WinTR-20, but the free HydroCAD Sampler is much easier to use and should do what you need.

http://www.hydrocad.net/sampler.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[ewbeng]

I'm having a hard time finding guidance on using the full SCS hydrograph generation procedure. (Again, I'm having to brush up ...) I've gone back through as much information as I can find ... and it's all a vague overview, or at least doesn't seem to give me enough information to actually put into practice.
What I'm finding usually mentions the SCS hydrograph then does an example using another method - which is how I started down the Graphical Peak Discharge Method. Other information I've found provides an example, with a handful of "givens" which I do not happen to have for my situation.
Any suggestions on where to get started? I'm not opposed to putting in the time to "relearn" the procedure, but am having a hard time getting started.
Note, our County flood control does provide the RAINFALL DISTRIBUTION COEFFICIENTS FOR VARIOUS DESIGN STORMS as well as an S-CURVE for HMS.
Thanks for any direction on getting started.

 

[psmart]

Your local stormwater regulations should provide details on the required/acceptable methodology. I'm guessing that the s-curve is a mass curve. Not sure about the coefficients. You'll need to read the regs carefully. Depending on how much time you have available to perform the study, you may want to consult a local engineer with prior experience.

Peter Smart
HydroCAD Software

http://www.hydrocad.net