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.
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. 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. Look out for 200 more of these bioswales throughout New Haven in the next few years!
Bioswales on West Park Avenue may now been seen on Google Earth!