What's Happening with the Virginia Stormwater Management Program?

If you are trying to keep up with the latest changes associated with the draft Virginia Stormwater Management Program (VSMP) Permit Regulations Parts I, II and III (4 VAC 50-60) please read on for a concise summary of where the regulations currently stand.
◊  Proposed Implementation Schedule

  • USEPA issued final Chesapeake TMDL on December 29, 2010.
  • General Assembly requires new regulations 280 days from final Chesapeake Bay TMDL – October 7, 2011.
  • Localities have no sooner than 15 months and not more than 21 months after regulations are effective to adopt new regulations  or provide 6 months’ notice for the request to turn review authority over to DCR.

The following are the highlights of the latest DRAFT of the regulation regarding water quality and quantity control,   The draft regulation can be found at http://www.dcr.virginia.gov/lr2d.shtml.   Note:  These are subject to revision by the Regulatory Advisory Panel (RAP), DCR, and the General Assembly.   The next RAP meeting is scheduled for February 28, 2011.
◊  Quality Control

  • New Development:
    • Removal of  at least 0.36 lb/acre/year of  phosphorus; the current regulations requires approximately 0.45 lb/acre/year of phosphorous.
    • Re-development:
      • If there is no net increase of impervious cover and ≥ 1 acre disturbed, then the requirement is to reduce the phosphorus load  at least 20% below pre-development levels,
      • If there is no net increase of impervious cover and < 1 acre disturbed, then the requirement is to reduce phosphorus load at least 10% below pre-development levels, or
      • If there is a net increase in impervious area, then follow the new development criteria.
      • Use the Runoff Reduction Methodology to determine the volume of water quality control based on the 90% storm event, which is approximately 1.0 inch.
      • Best management practices (BMPs) shall be as outlined on the Virginia Stormwater Management Clearinghouse website – http://www.vwrrc.vt.edu/swc/.

◊  Quantity Control – Channel Protection

  • For Manmade Conveyance,
    • Show the post-development 2-yr, 24-hr peak release rate does not cause downstream erosion or
    • Control the peak release rate not to exceed the 1-yr, 24-hr rate based on Energy Balance Equation.

                       Energy Balance Equation:  QPost ≤ I.F. (QPre * RVPre ) / RVPost

QPost = Max. allowable peak flow rate from a developed site
QPre = Peak flow rate from pre-developed condition
RVPost = Volume from a developed site
RVPre = Volume from pre-developed condition
I.F. = Improvement Factor (0.8, > 1 ac; 0.9 ≤ 1ac)

  • For Restored Conveyance,
    • Provide a release rate that is consistent with ‘design parameters’ of the restored system or
    • Control the peak release rate not to exceed the 1-yr, 24-hr rate based on Energy Balance Equation.
    • Natural Conveyance
      • Control the peak release rate not to exceed the 1-yr, 24-hr rate based on Energy Balance Equation.

 ◊  Quantity Control – Flood Protection

  • Show there is no current flooding of the downstream system for 10-yr, 24-hr storm event.
  • If there is current flooding of the downstream system  during the 10-yr, 24-hr storm, then
    • Control the maximum release rate to avoid localized flooding or
    • Control the post-development maximum release rate to less than or equal to the pre-development release rate for a 10-yr, 24-hr storm event.
    • Limits of downstream system analysis
      • To the point of analysis where the project area is less than 1% of the total drainage area,
      • To the point of analysis where the  project maximum release rate is less than 1% of the existing flow rate of the drainage area, or
      • Where the outfall meets a floodplain.

This, of course, is a technical summary – please contact me if you would like to understand how this may apply to you.