CN Adjustment for Infiltration 1

[DASkurky]

I am looking to adjust a CN to a detention pond based on an offline uphill infiltration BMP.

In Georgia, if there is an offline BMP (enhanced swale, bioretention garden, etc.) which infiltrates the "first flush" (1.2" storm) volume, then there is an adjustment factor which can be applied to the detention routing to account for the loss in volume. The table relates percent impervious and HSG to a percent reduction in CN (up to 90%). There is no reference to this table to trace the math supporting it.

I am infiltrating more than the 1.2" storm in an upstream BMP and would like to adjust the CN for this watershed for when it reaches the detention pond downstream.

I know Hydraflow Hydrographs has an exfiltration function but I am reluctant to use it. I would rather keep the BMP and the pond as two separate systems and use the reduced CN for the detention routing.

Does anyone have any experience with this??

Thanks,
Dave S

 

[psmart]

Another option would be to model the upstream BMP as a separate pond, using the appropriate parameters to simulate the retention and infiltration. This might be easier to justify than a non-standard adjustement of the CN values.

Otherwise, you might be able to use the equation for potential maximum retention to calculate a CN that includes the effect of the BMP:

First calculate S from the standard CN using:
S = 1000/CN - 10

Then add the BMP exfiltration depth to the result and calculate an equivalent CN using:
CN = 1000 / (S+10)


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[DMcGrath]

Virginia is starting to implement a "Runoff Reduction Method" that accounts for temporary storage on site to reduce the overall amount leaving the site. Effectively you calculate the volume of storage as an initial abstraction (S), then back into a revised CN. psmart notes this method above. All the documentation, including active spreadsheets can be found at

http://www.dcr.virginia.gov/lr2f.shtml

 

[DASkurky]

PSmart and DMcGrath, thanks for the input.

I am not sure how to "add the BMP exfiltration depth" in the CN revised equation.

Modeling two ponds in series is probably the simplest solution. And may produce the most accurate results.
Thanks!

 

[psmart]

If the BMP will infiltrate 1.2", just add 1.2 to the S value before solving the second equation.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Drew08]

S is a potential retention, not actual retention. Adding some depth to S will not remove that depth from the total runoff.

While the BMP infiltration is an infiltration, it is first diverted flow (you mentioned it is off line). Mathematically it can be modeled as any loss, i.e. pump away, evaporate, fall off a cliff, etc). Model the diversion structure, not the BMP. However, if you want an equivilant CN, you may want to treat the BMP infiltration simple as a reduction in the depth of runoff (Q). Try this:

Assuming:
Post construction runoff:
Q(2) = Q – F ; and,
Q(2) = (P - 0.2S(2))^2 / (P + 0.8S(2))

Pre construction runoff:
Q = (P - 0.2S)^2 / (P + 0.8S)

Combine to:
(P - 0.2S(2))^2 / (P + 0.8S(2)) = (P - 0.2S)^2 / (P + 0.8S) - F

Iteratively solve for S(2); and solve for:
CN(2) = 1000 / (S(2) + 10)

Where:
S, CN, Q = original storage, CN and runoff prior to constructing the BMP.

S(2), CN(2), Q(2) = new storage, new CN and new runoff after construction BMP.

F = depth of BMP infiltration related to the area of the watershed.

Using this method the actual depth of runoff is reduced by exactly the depth of BMP infiltration.

Note that this will only modify the CN of the subwatershed contributing to the BMP, not the entire watershed. Use a weighted CN method to determine the modified CN for the entire watershed.

 

[psmart]

Drew08 - That's true. To use my simplified approach you would need to adjust by the estimated potential maximum retention for the BMP, rather than just the actual first-flush volume. Of course, that value may be hard to determine, so I would tend to go with my initial suggestion of a separate modeling step to simulate the flow reduction created by the BMP. This could be a pond, diversion structure, or other mechanism that will reduce the flow.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[DPAJR]

If you know S for bare soil, say from an NRCS Map Unit Description for the soil type, and solve for CN how would you adjust your new CN for conditions like Poor Fair or Good with respect to usage or vegetation?

Would looking in the Hydrocad CN table for an equivalent CN value and comparing the differences be a good idea? Or would you possibly just add or subtract 10 for each adjustment?