Pond Pack modeling a 6 hour storm duration 1

[jleiben]

Does anyone have some guidance for using Pond Pack to model a 100 year 6 hour storm event? We're being forced to compare a 100 year 24hr and a 100 year 6 hour storm event to determine which one produces a higher peak value. My intuition would have me believe its the 6 hour duration, but I'm having a difficult time setting this up in Pond Pack to validate my thinking. This is for designing drainage ditches along the perimeter of a Valley Fill in Western VA. Any thoughts? Thanks in advance.

 

[Rye1]

I'm not sure about pond pack but it would be easy in hydrocad. This issue comes up a lot in Charlotte NC. The City storm water designs require street drainage to be meet the 6 hour duration storm but the FEMA floodplains are mapped using the 24 hour duration storm.

Robert Billings

http://www.newrivereng.com

 

[psmart]

The 6-hour storm is typically taken from the middle 6 hours of the corresponding 24-hour storm. As such, the peak rainfall intensity is exactly the same, assuming that the 6 and 24 hour rainfall depths are specified correctly.

Although the peak rainfall intensity will be the same, the 24-hour peak runoff will be greater because of the larger overall rainfall depth and the non-linear nature of the SCS runoff equation. This causes the initial abstraction to be satisfied earlier in the 24-hour storm, so the peak rainfall intensity will produce a larger peak runoff.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[jleiben]

Thank you both for your responses, but I still need some guidance with Pond Pack. We do not have HydroCAD, though I have a trial version, but ideally I would like to know how to do this in Pond Pack. I realize what the 6 hour storm would be with relation to the 24 hour storm, but I still haven't been able to determine how to mimic it in Pond Pack. Any thoughts or anyone with experience doing as such?

 

[jleiben]

Could you also explain how to set this up in HydroCAD then? As i said I have a trial version and have it working, but would like to make sure I'm using it properly to validate the numbers i'm getting. Thanks.

 

[psmart]

HydroCAD includes 6, 12, and 24 hour versions of the Type II and III rainfalls. You will find them together on the rainfall list. Just pick the storm distribution you want and set the appropriate depth for your location.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[jleiben]

For some reason I've been assuming that the 6-hour storm would produce a larger peak than the 24-hour storm, however, its actually producing a lower peak value. Does this make sense or am i missing something in how I'm running these two different storms? I'm basically using the NOAA depth values for the two different storm durations (4.05" vs. 5.53").

 

[psmart]

Please see my previous reply:

"Although the peak rainfall intensity will be the same, the 24-hour peak runoff will be greater because of the larger overall rainfall depth and the non-linear nature of the SCS runoff equation. This causes the initial abstraction to be satisfied earlier in the 24-hour storm, so the peak rainfall intensity will produce a larger peak runoff."

Remember, these are "nested storms". The 24-hour storm contains the entire 6-hour storm nested at it's center, so the peak rainfall intensity will be the same. But the additional rainfall during the first 9 hours of the 24-hour storm causes an increase in the peak runoff.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[gbam]

I just opened up Pond Pack and there are a number of ways to model the precipitation. Can you create a second run using the 6-hour storm? How did you run the 24-hour?
The last time I ran Pond Pack I imported a runoff hydrograph created from HEC-1. I did this for existing and proposed condition since it was an existing basin. Then I performed a comparison using excel.

 

[Twinkie]

People still use Pond Pack?

 

[cvg]

note that not all rainfall distributions are similar to SCS and may not be nested so that peak runoff may be higher with 6-hour storm. also, you may not be using initial abstraction / constant loss rate in your analysis. you may need to factor in aerial reduction and basin storage and routing which might also affect your runoff depending on the watershed size. a blanket statement as made below is not always true. It is not uncommon for a shorter duration, intense storm to produce higher peak runoff for certain watersheds.

the 24-hour peak runoff will be greater because of the larger overall rainfall depth and the non-linear nature of the SCS runoff equation. This causes the initial abstraction to be satisfied earlier in the 24-hour storm, so the peak rainfall intensity will produce a larger peak runoff

 

[psmart]

I believe I set my comments in the proper context. My comment is true for standard SCS runoff calculations using a nested storm. Other runoff procedures, non-nested storms, and hydrograph routing calculations may produce different results.

jleiben, what rainfall distribution are you using? Is it one of the SCS/NRCS distributions, or something else?

Peter Smart
HydroCAD Software

http://www.hydrocad.net