375 - Evaluating the Water Quality and Hydrologic Performance of Bio-swales in North Carolina

Bio-swales are an emerging technology used to treat and convey roadway runoff. This stormwater control measure (SCM) is a combination of a conventional, vegetated swale and bioretention cell. The practice includes an engineered media, gravel layer, and perforated underdrain beneath the surface of a grassed channel. North Carolina State University monitored the performance of four bio-swales treating highway runoff. Water quality and hydrologic data were collected at the inflow, overflow or end of the bio-swale, and underdrain for a minimum of 53 storm events. Exfiltration was estimated as the difference between inflow and the sum of overflow and underdrain flow. As a percentage of inflow, overflow, underdrain flow, and exfiltration ranged from 0 to 76 percent, 0 to 87 percent, and 0 to 100 percent per storm event. There were significant (α = 0.05) reductions between inflow and overflow and underdrain peak discharges. The median total nitrogen (TN) concentrations leaving the underdrains were between 0.63 and 1.25 mg/L; the median total phosphorus (TP) concentrations measured at the underdrains ranged from 0.10 to 0.29 mg/L. Median total suspended solids (TSS) concentrations at the underdrains were between 10 and 18 mg/L. This preliminary research indicates bio-swales should include a forebay, have a minimum bottom width of three feet, and have a longitudinal length of 75 feet per one acre of watershed to increase this SCM’s water quality and hydrologic performance.