Stormwater Standards for the future

Tuesday, September 29, 2015

UNHSC, in collaboration with VHB, SRPC and the Town of Durham recently completed a study modeling the pollutant load reduction potential of simple stormwater regulations.  The goal of this study was to evaluate the net effect that enhanced local stormwater regulations could have on minimizing future pollutant load increases from future impervious cover growth in the Oyster River watershed in New Hampshire.  As future development and land use changes are inevitable, early adoption of enhanced local stormwater regulations provides the greatest opportunity to minimize increased pollutant loading from future development.  Model stormwater regulations developed for the Southeast Watershed Alliance (SWA) in December 2012 provide a template for enhanced stormwater regulations that address future development and redevelopment projects (SWA 2012). The regulations would apply to new development and redevelopment projects that are subject to site plan and/or subdivision review by the Planning Board. This would include most, if not all, commercial or mixed use development projects and residential multi-family or subdivision projects. 


Based on the study approach, approximately 500 acres of additional impervious cover (IC) area could be developed in the Oyster River watershed over by the year 2040 due to future residential and commercial development activity.  This represents approximately a 40 percent increase over the 1,290 acres of IC area estimated to exist in the watershed based on 2010 aerial imagery. This added IC area is estimated to increase the average annual pollutant loads for Total Suspended Solids (TSS), Total Phosphorus (TP) and Total Nitrogen (TN) by approximately 217,700 (~109 tons), 1,060 and 9,950 pounds, respectively, if no local stormwater regulations are adopted and no enhanced stormwater treatment was provided.


Results indicate that early and widespread adoption of the proposed local regulations could reduce the future average annual pollutant loads by approximately 40 to 70 percent depending on the pollutant and would potentially prevent the discharge of approximately 147,144 pounds (~74 tons), 450 pounds and 4,900 pounds for TSS, TP and TN, respectively, from entering the Oyster River and Great Bay estuaries.


Potential savings in deferred costs or cost avoidance gained through early adoption of stormwater regulations and enhanced treatment were estimated to be $14 million in 2014 dollars.  It is expect that if the regulations were extended beyond the Oyster River watershed to include the entire Great Bay watershed, the potential future cost savings could be in the hundreds of millions of dollars.

The model SWA stormwater standards can be found here:  Model Standards

A fact sheet detailing project results can be downloaded here: P2 Fact Sheet

The final report for the future modeling results can be downloaded here: Future Modeling Report