I continue to have concerns about the application of channel protection criteria per 9VAC25-870-66.B.3 in conjunction with the karst loss modification values, specifically if discharging to a karst feature.

There are significant problems with applying channel and flood protection independently to each karst feature per 3.306.A.

General best practice dictates that the existing hydrologic flow regime to a karst feature should maintained to the extent possible.   The application of energy balance does not result in an allowable post-development discharge regime that is similar to the pre-developed flow. 

Even in a case where no impervious area is added to a drainage area to a sinkhole, the application of the karst loss adjustment factor, and then energy balance results in a lower peak discharge to the karst feature due to the improvement factor alone.

For example, if:

Pre -dev peak discharge = 0.25 cfs

Pre-dev runoff volume = 500 cf

In this case, we will say the area is not developed, and so pre- and post- numbers are the same.

Apply KAF – presumably the whole drainage area to the karst feature is “in karst”, so the KAF is 0.33. 

0.25 cfs x 0.33 = 0.0825 cfs

New Pre-dev peak discharge is 0.0825 cfs.

The table indicates that the KAF is applied to peak flow rate (does not mention volume, which seems like another issue altogether when applying KAF to energy balance methodology), so presumably the energy balance equation now looks like this:

Q-1 allowable = 0.8 (0.0825 x 500) / 500

Q-1 allowable = 0.066

This allowable is 80% of the original peak flow.    Or only 26.4% if you look at the unadjusted pre-development peak, which I believe there is a good case for (see further explanation on this point below).

For a scenario where some of the area to the karst feature is developed:

Post- peak discharge = 2.5 cfs

Post- volume runoff volume = 2000 cf

Q-1 allowable = 0.8 (0.0825 x 500)/ 2000

Q-1 allowable = 0.0165

Or only about 20% of the original karst-adjusted peak flow to the feature.  This does not result in maintaining the existing hydrologic regime. 

Furthermore, if the drainage area discharges to a karst feature, KAF may not even make sense, as the entire amount (100%) of predeveloped surface runoff would make its way into the feature.  I believe the KAF is meant to be applied more at a landscape scale, where your surface point of discharge may need to be adjusted due to karst losses across the landscape.  In that scenario, the adjusted peak represents the water that does not enter the karst system.

 This is different than analyzing a point of discharge directly into a karst feature, where all of the runoff does enter the karst system, and therefore should not require an adjustment. If you look at the allowable from the second scenario compared to the unadjusted pre-development peak, the allowable discharge is only 6.6% of the pre-development peak, and nowhere close to the existing hydrologic regime.

Remaining comments from the earlier version.  Comments that have been addressed by revisions have been removed.

1. Pg 20 – 3.302.4.A - Is there a preferred or recommended methodology for mapping the water surface elevation of a natural system?
2. Pg 43 – 5.200.B – “…should deviate from the pre-development condition by no more than plus or minus 10%” May consider adding “in total area” (vs. peak flow pre-to-post)
3. 5.302.D – Please confirm/clarify that the CN adjustment for HSG correction is meant to be used for quantity compliance modeling only, not for the VRRM spreadsheet. I don’t believe the VRRM spreadsheet will allow a different soil group pre to post (if you use the Redevelopment Spreadsheet).  How does this impact the CN adjustment if you are using the CN adjustment provided by the VRRM spreadsheet?
4. Pg 40 – CN Adjustment table – there are several other land cover categories that I use on a regular basis.  This appears to lump many diverse land cover categories into “Open Space”.

Thank you for the opportunity to comment.