Runoff Curve Number for Artificial Turf?

[Yakman256]

Hey Guys,

I have an interesting problem in that a High School wants to replace the existing turf field with artificial turf and the question is ... Will the existing stormwater basin be able to handle the runoff without modifications? (I Know...Cost is no object when its tax payer money) The existing basin is located close to a stream within a high quality watershed (150 ft buffers) in PA and I'd rather not approach PaDEP with any earth disturbance within 150 feet of a stream even if I'm expanding an existing facility. (I'd rather bang my head against a brick wall)

Anyway, the cross section is the turf membrane, then 4-6" clean stone over some perforated pipe that eventually will discharge to the basin. It stands to reason that the runoff from the field will be exposed to the earth and there will be an infiltration component. In the Past I've modeled the turf fields as gravel (CN=85) but I think that's probably conservative.

Does anyone have any idea what the actual runoff coefficient is for Artificial Turf?

Thanks in advance

 

[fel3]

I have never had to deal with artificial turf, so I did a Google search for ["runoff coefficient" for "artificial turf"] and came up with lots of hits. You could also try "synthetic turf" in this search instead of "artificial turf".
[ul]
[li]ADS used C=1.00 in this example calc:

http://www.ads-pipe.com/pdf/en/a5.19a-advanedge_drainage_layout_for_synthetic_turf_fields,_07-12.pdf

[/li]
[li]This study has an in-depth discussion in the middle of the report:

http://www.kirklandwa.gov/Assets/Planning/Planning+PDFs/FieldTurfstudy.pdf

[/li]
[li]HydroCad has some advice regarding artificial turf:

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

[/li]
[/ul]

In reviewing these and others, it looks to me like values between 0.75 and 1.00 are typically being assigned to artificial turf.



==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[psmart]

Artificial turf, combined with a stone base, is generally designed to intercept all the rainfall, so there is no "runoff" in the usual sense. Instead you get nearly 100% capture of the rainfall. So you would generally use a very high "runoff" coefficient (C=1.0) or CN value (CN=98+).

How you model the intercepted rainfall depends on the design of the stone bed: Is is sloped to drain rapidly, or does it retain or detain the rainfall? The later cases are often modeled as detention (pond) storage.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Twinkie]

I had a very similar project, in PA as well, a few years back. In the end I ended up using a CN of 98, the underlying soils were hydric D, and I had room in the existing basin to account for the high CN.

If I were you, I would do a quick model, see if your existing basin will still work with the CN=98. If it does, then you are all set, you may be able to modify the outlet structure as well, which shouldn't push you back to the DEP if you arent modifying the basin shape.

 

[Twinkie]

I had a very similar project, in PA as well, a few years back. In the end I ended up using a CN of 98, the underlying soils were hydric D, and I had room in the existing basin to account for the high CN.

If I were you, I would do a quick model, see if your existing basin will still work with the CN=98. If it does, then you are all set, you may be able to modify the outlet structure as well, which shouldn't push you back to the DEP if you arent modifying the basin shape.

 

[Yakman256]

Thanks guys

Like I said... In the past I always modeled the Artifical Turf Areas as Gravel CN=85 and used the minimum 5 min time of Concentration. I did a turf field for a nearby college and one of the maintenance guys told me that he has never seen the discharge pipe running with more than a trickle. Who knows, maybe he just wasn't there to witness the peak. I Still think there will be some amounts of infiltration and I also think that there must be some attenuation in the system but without some hard data to back up my assumptions I'll stick with the numbers I usually use.

 

[beej67]

I would think the thing to do is either model it as a storage basin, or back your way in to an "effective CN" based on a sensitivity analysis that matches the results of a storage-and-release case. I'm surprised turf suppliers haven't done such an analysis in order to provide an "effective CN" they can recommend to engineers.





Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[chicopee]

With artificial turf, wouldn't you have some sort of closed cell foam backing not allowing infiltration of storm water?