by Brian Van Wye | Development is often thought of as an enemy of healthy rivers and streams, especially when undeveloped, natural sites are replaced with rooftops, pavement, and other impervious surfaces from which rainwater washes off with erosive, pollutant-laden volume. However, in a highly urbanized area like the District of Columbia, most of the land was developed long ago without the benefit of adequate controls for reducing stormwater pollution. In these areas, “development” today generally means the re-development of an already heavily impervious area and a chance to update the site to more protective standards. With the District Department of the Environment (DDOE) expected to soon promulgate a new, river-protecting stormwater retention standard, such development should soon capture a substantial amount of the stormwater that would otherwise run off.
The new standard will require regulated development sites (those disturbing 5,000 square feet or more of soil). This standard is federally required under the draft U.S. Environmental Protection Agency Municipal Separate Storm Sewer System (MS4) Permit for the District. It is critical to ongoing efforts to restore the Anacostia and Potomac Rivers and Rock Creek, and DDOE wants to implement the requirement in a way that minimizes any chilling impact on District development. DDOE’s analysis has shown that pairing this new retention standard with a carefully designed, first-of-its-kind stormwater retention trading program would not only provide greater flexibility for development projects, but would leverage their associated water quality benefits to further increase annual stormwater retention in the District.
Under stormwater retention trading, all regulated development sites would have to retain a minimum amount of the required 1.2” volume on site, but they would have the option to purchase Stormwater Retention Credit (SRC) from another site to satisfy any portion of the 1.2” volume that is not retained on site. DDOE would certify SRC, measured in gallons, on public and private properties after inspection of stormwater retention practices that comply with a DDOE-approved stormwater management plan for the site. DDOE would only certify SRC for any volume retained above and beyond existing site conditions or regulatory requirements but would not certify any volume above a retention ceiling corresponding to the 1.7” storm. DDOE would not sign off on the regulated development site’s Certificate of Occupancy until all retention practices, on site and off site, were inspected and operating, and would require maintenance of stormwater retention practices and periodic inspections for sites generating SRC.
This innovative program would allow a regulated development site with high retention costs (including opportunity costs) to purchase SRC from a site with low retention costs, creating a market for stormwater retention retrofits at sites with little or no existing retention practices. To assess the potential for this program to provide greater flexibility for development while also achieving greater annual retention for District waterbodies, DDOE conducted a preliminary analysis (see Table 1) and found that allowing high-cost site 1 to purchase SRC from low-cost site 2 could not only result in a 53% increase in total stormwater volume retained on an annual basis, but could also produce this benefit at a 30 percent capital cost savings.
The increase in overall annual stormwater retention under scenario B results because retention retrofits are installed on additional land area, which otherwise would have little or no retention in Scenario A, and because many of the storms that occur in a year are less than 1.2”. Figure 1 illustrates the size of 2009 rainfall events, with the blue line corresponding to retention practices capable of holding the 1.2” storm volume, the red line corresponding to 0.75” of retention practices, and the yellow line corresponding to 0.45” of retention practices. As shown in Figure 1, a 1.2” retention practice does not utilize its full capacity during most storm events, and SRC for any volume above a 1.7” retention ceiling provides diminished retention benefits. By contrast, with SRC trading, lower capacity retention practices on multiple sites use their full capacity more frequently, so during most storm event, the total volume retained is greater.
Though this analysis is somewhat simplified, the potential benefits in terms of stormwater retention and flexibility for development are clear. Also, with more sites installing retention practices, there should be greater retention of the most heavily polluted run-off associated with the “first-flush” volume toward the beginning of a rainstorm. Additionally, DDOE’s analysis suggests that the trading program would likely shift retention from the densely developed downtown to less densely developed parts of the District, increasing retention in areas draining to the relatively vulnerable tributaries. Because many of these less densely developed areas are also less affluent than the downtown business core, this shift in retention would likely result in more rain gardens and other green infrastructure in these areas, providing a net environmental justice benefit.
DDOE is optimistic that its stormwater retention trading program will be a genuine win-win, easing the transition to a progressive new stormwater retention standard by allowing greater flexibility for regulated development and also achieving greater benefit for District waterbodies. As the first jurisdiction in the country to test such an approach, DDOE expects some challenges in implementing an accountable and transparent program that avoids being so cumbersome that developers don’t participate. By closely evaluating the program’s operation and impacts, DDOE expects to learn important lessons that will allow the District to optimize the program and that may provide guidance for other jurisdictions.
Brian Van Wye is an Environmental Protection Specialist in the Storm Management Division at DDOE.