In the West River sewershed, URI’s GreenSkills team installed eight bioswales in the in the curb strip of West Park Avenue. Two sites, a treatment and control site, were chosen based on conditions related to slope, soils, expected flows, and neighborhood support. The treatment site had eight bioswales and two underground cisterns that collected stormwater and the control site remained unchanged.
Hydrologic instruments attached to v notch weirs (see image below) measure flow at the outlets of these storm sewersheds in order to analyze how much water is coming in and how much is leaving. Instruments were also installed directly in four of the bioswales to see how water moves through the systems. Water quality characteristics like metals, nutrients, and conductivity were measured on samples taken over the course of selected storms. By comparing the treatment and control sites, researchers from Yale F&ES are able to measure the effect of green infrastructure on water quality and quantity.
A v-notch weir for measuring flow (left), Researchers Kelsey Semrod and Gabe Benoit of Yale School of Forestry install monitoring instruments in a cistern & bioswale (center, right).
Bioswales captured over 50% of stormwater from West Park Avenue during this one inch December storm and 0.5 inch October storm, significantly reducing the amount of water that traveled to the treatment sewer. These figures show water discharge during the storms at the three locations: green infrastructure is represented by the green line, the treatment sewer (where bioswales were installed) is represented by purple triangles, and the control sewer (where no changes occurred) is represented by the red circles. The difference in water traveling to both sewers is clear by comparing the red line (control) and the purple (treatment).
While the hydrologic impact of green infrastructure varies by storm, bioswales and cisterns have proven to collect large quantities of stormwater; on average, together they capture 77% of street runoff and bioswales alone capture 56 % of street runoff. Total rainfall, rate of rainfall, and length of storm were not factors in capture of stormflow by the bioswales. Seasonality may affect bioswale functioning when leaves remain along the street, blocking inlets, and are not effectively removed by street sweeping operations. Trash, leaves, dirt, and other debris can block the opening to bioswales, and a layer of fine sediment from stormwater runoff prevents fast infiltration if not consistently removed and maintained. Additionally, the bioswales were not consistently cleaned out throughout the research study, and therefore were not collecting stormwater to their full capacity. New Haven’s sandy soils allow for fast infiltration, and if bioswales are maintained, rain gardens are expected to capture much greater percentages of total stormwater accumulated. In cities like New Haven that are considering sewer separation, bioswales may serve as a cost-effective alternative to this expensive and disruptive procedure, and even be used in tandem with these efforts.
At the second study site, URI’s GreenSkills team installed seven bioswales in the curb strip along two streets in the Newhallville neighborhood of New Haven. This neighborhood was selected because of its location in the uppermost reaches of a separated storm sewer system, which allowed researchers to more accurately measure the flow and pollutant reductions associated with bioswales implementation. The study site consisted of three small sewersheds – two on Daisy St. and one on Watson St. Researchers conducted monitoring of stormwater discharge and water quality in the sewersheds for 5 months prior to bioswale installation and 8 months after to evaluate the performance of bioswales in managing and treating urban stormwater.
Kevin Dahms working on site in Newhallville.
Similar to the first study site, v-notch weirs and water level monitors were used to continuously monitor discharge from each of the sewersheds. The researchers also utilized flow measuring devices at the inlet to three of the seven bioswales – one in each sewershed – to determine flow into each bioswale. Water quality parameters, nutrient concentrations (nitrogen and phosphorous) and total suspended solids (TSS), were measured at the outlet of each sewershed during select storm events before and after bioswale implementation.
Since the Newhallville neighborhood is in a separated sewer system, stormwater runoff from these sewersheds does not contribute to combined sewer overflows as it does in the West Park area. However, flow reduction in the separated storm sewer is still an important metric since it influences pollutant loading and flooding, which are two concerns for the City of New Haven. By developing a rating curve for storm sewer discharge and rainfall amounts before and after bioswale implementation (See Figures below), researchers determined that the bioswales prevented approximately 885,000 gallons of stormwater from entering the sewer system annually. Lower discharges were also associated with reductions in mass fluxes of nitrogen (N-NO3), phosphorous (P-PO4), and TSS. During specific events where water quality was measured, these reductions reached levels of 63%, 67%, and 89%, respectively.
The below rating curves for storm sewer discharge and rainfall amounts before and after bioswale implementation show that the bioswales prevented approximately 885,000 gallons of stormwater from entering the sewer system annually.
This research serves as one of the first studies on bioswales in Connecticut, helping researchers better quantify the effectiveness and feasibility of green infrastructure implementation. Green infrastructure installation in New Haven may serve as an example for other small cities to both construct and monitor bioswales.
The results from the Newhallville study site are encouraging as the City of New Haven plans to install 200 bioswales to reduce flooding in the Downtown area and improve water quality in the harbor. Affirming the findings from the West Park project, these results demonstrate that New Haven’s sandy soils have a high infiltration capacity, which not only reduces runoff but also pollutant loading. The City of New Haven will use the results from this study in their siting, sizing, and design of the larger scale implementation in the future. This research has added significant value in high-visibility education, demonstration, and community engagement efforts, building a case for additional investments in New Haven and other cities.
Bioswales on West Park Avenue may now be seen on Google Earth!