CWP Runoff Rundown Fall 2009

Is Runoff Volume the "Real" Pollutant? If So, Can I Still Get My Plan Approved?

Volume as a pollutant? We have been focusing on total suspended solids, total phosphorus, total nitrogen, bacteria, and other common urban pollutants for a long time now - have we been missing the root cause of stream impairments and stormwater pollution - too much runoff volume? Isn't reducing the overall volume of runoff the best way to reduce commensurate pollutant loads (not to mention downstream channel erosion problems)?

If you look around in the stormwater world these days, volume seems to be the buzz. The September, 2009 issue of Stormwater Magazine has an excellent article by Andrew Reese entitled "Volume-Based Hydrology". If you have any questions or doubts about the role of volume in stormwater management, start with this article.

Now for the next challenge: how do we fold this concept into the process of drafting, reviewing, and approving stormwater management plans for development sites? After all, most local codes and design manuals use compliance tools that are still based on peak rate control and pollutant removal through BMPs.

This, frankly, has been one of the hobgoblins vexing the mass application of LID. Many people (including developers, design consultants, and plan reviewers) are fond of the concept, but the mechanics of putting this stuff on a plan and getting it approved by the local authority has not always been smooth. Approval of LID or volume-based stormwater solutions is often a special (read -- prolonged) loop in the plan approval process. The local reviewer gages compliance by using the tools of the trade - code language, checklist, standard hydrology and hydraulics methods, and/or design manual -- and LID practices don't always fit in so neatly, and therefore is not duly credited. In other words, there isn't a compliance metric that allows LID and traditional practices to work in concert to achieve compliance. More often than not, the traditional practices achieve compliance while the LID practices are considered expensive window dressing or "demonstration" projects.

Help is on the way!

Many localities and states are blazing new territory with regard to computation methods and credit or compliance systems in order to level the playing field. Here are a few examples:

Runoff Reduction Method: The Center, along with the Chesapeake Stormwater Network, developed this method for Virginia's proposed stormwater regulations. The method accounts for the runoff reduction capabilities of both LID-type and conventional practices. The method is accompanied by a compliance spreadsheet that allows site designers to experiment with 14 different categories of practices, including rooftop disconnection, sheet flow to conservation areas, rainwater harvesting, and permeable parking, in addition to more conventional practices. The final outcome of the spreadsheet is a post-construction pollutant load for total phosphorus, as well as an adjusted curve number that can be applied to larger storm events.

The technical basis for the Runoff Reduction Method can be found in the Runoff Reduction Technical Memo. The spreadsheet can be found on the Virginia Department of Conservation & Recreation website (scroll down to "Revised Runoff Reduction Spreadsheets and Background Documents;" the latest spreadsheet and documentation is dated September 30, 2009). All of the supporting BMP specifications can be found on the Virginia Stormwater BMP Clearinghouse, maintained by the Virginia Water Resources Research Center (scroll to the bottom of the page to download BMP specifications).

Capitol Region Watershed District (Twin Cities, MN), Volume Reduction Worksheet: The Twin Cities seem to be at the forefront of infiltration and runoff reduction. The worksheet is also in spreadsheet format, and accounts for various "volume reduction practices" as well as "subsurface storage and infiltration volume." The spreadsheet provides handy storage volume credits for seven different types of practices. Volume reduction is compared to a standard of 1-inch x site impervious cover.

Coastal Stormwater Supplement to the Georgia Stormwater Management Manual: The Center developed the Coastal Stormwater Supplement to aid coastal communities in Georgia in addressing the unique challenges and opportunities of coastal stormwater. The Supplement recommends reduction of the first 1.2 inches of rainfall through the use of many of the practices discussed in the Runoff Reduction Method. A spreadsheet has been developed to allow designers to comply with the recommendation. This spreadsheet accounts for only runoff reduction, and not pollutant loads (in contrast to Virginia's version). Also, this version credits the specific designed volume of a practice, rather than relying on percent removal estimates. The spreadsheet and the Stormwater Supplement are available here for download from the Chatham County-Savannah Metropolitan Planning Commission.

We're sure there are many other budding methods and techniques out there to account for runoff volume reduction. If your community has developed or is working on such a method, please let us know. Also, let us know if this type of tool is the "missing link" in your community's efforts to incorporate LID or innovative practices. We'll let you all know about the results in a future installment of Runoff Rambling.

As always, feel free to email your comments and suggestions to Dave Hirschman: djh@cwp.org.

-The Runoff Ramblings Team