Pond Surface SCS CN

[Woody2185]

I'm currently working on a project with an area of 38 acres. The requirements are to reduce the discharge resulting from the 25, 50, and 100 year 24 hour storms by 25% after development. That's not my problem, just a little background.

The site is situated in a wetlands area. The local agencies recommend using the heavily wooded condition for predevelopment rates, which is fine. The developer would like the detention pond to be an 8 acre wet pond. What CN value should I use to model this pond post-development? Should I assume that all rainfall that falls over this 8 acres will runoff? The DOTD manual I am referencing does not provide any values for this condition.

 

[jgailla]

The curve number for a pond is 100. There is no storage available which would reduce the number.

 

[psmart]

The rainfall over the 8-acre wet pond is going to "runoff" completely, so a CN value of 98 is commonly used. Of course, the "runoff" would be included as part of the inflow for the wet pond.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[jgailla]

psmart,
I would have to disagree with using a CN of 98, even though it wouldn't have any practical effect because the models aren't accurate to that extent.
Using a CN of 98 implies that some of the stormwater is being stored in the system. Pavement has cracks, grass can store quite a bit of water, but how can a water surface do anything but rise? The water falls directly on the surface, so there is no time of concentration and no losses.

 

[RWF7437]

CN = 98 is used for lake surfaces to account for water "lost" by evaporation, seepage into the lake bed, as well as storage on overhanging vegetation or pond weeds. But, as jgailla says, won't make any practical difference.

 

[Woody2185]

Thanks, I know that the difference between 98 and 100 would be minimal, just wanted to be sure that I was ok with my assumption.