Field Test Site

The UNH Stormwater Center’s field site is adjacent to a nine-acre commuter parking lot in Durham, New Hampshire. The contributing drainage area—curbed and almost completely impervious—generates stormwater runoff typical of developed urban and suburban subcatchments. Installed in 1996, the lot is composed of standard, dense-mix asphalt. For nine months every year, it is used near capacity by a combination of passenger vehicles and bus traffic. The pavement is frequently plowed, salted, and sanded during the winter.

Literature review indicates that the lot’s contaminant concentrations are above, or equal to, national norms for parking lot runoff. The runoff time of concentration is 22 minutes, with slopes ranging from 1.5 to 2.5 percent. Local climate is coastal, cool temperate forest. Average annual precipitation is 48 inches, uniformly distributed throughout the year with monthly averages of 4.1 (+/- 0.5) inches. The mean annual temperature is 48°F, with an average low of 15.8°F in January, and an average high of 82°F in July.

The adjacent field site contains three classes of stormwater treatments: conventional Best Management Practices (BMPs) such as swales and retention ponds; Low Impact Development (LID) designs such as treatment wetlands, and filtration and infiltration designs; and manufactured BMPs such as filtration and infiltration units, and hydrodynamic separators.

Since prior research has demonstrated that stormwater treatment performance varies widely in response to site-specific contaminant loading, the site was designed to test treatments under similar conditions. The parallel but separate configuration normalizes the stormwater treatment processes for rain event and watershed-loading variations. Each treatment is uniformly sized to address a Water Quality Volume (WQV) that targets a rainfall-runoff depth equivalent to 90 percent of annual volume of rainfall, or one inch of rainfall.

Rainfall runoff from the lot is channeled into a distribution box with a floor that rests slightly higher than the outlet invert elevations. This insures that runoff will scour the floor, thereby preventing sedimentation. From the distribution box, runoff flows into a network of pipes that distribute an equal quantity into each stormwater treatment. Effluent from the treatments is then piped into a centralized sampling gallery. There, automated samplers are programmed to test water quality and monitor flow volume from each treatment. A detailed quality assurance project protocol governs all analyses.