Maintenance of linear rights-of-way is a major concern of Departments of Transportation (DOTs). Swales are a stormwater control measure (SCM) placed in the roadway right-of-way that remove pollutants and safely convey stormwater. A common SCM, swales often erode from concentrated water flow and sparse vegetation cover (NCDEQ, 2017a). A potential means for limiting swale erosion is to replace the typical turf grass with an alternative lining, such as riprap or native grasses. This research examined the impacts of maintaining eroding swales with riprap and native grasses by comparing pollutant concentrations and loads from turf-lined conventional swales and bioswales that have been replaced with a riprap or a native grass lining.
A total of eight swales (four conventional and four bioswales) located at North Carolina State University’s Sediment and Erosion Control Research and Education Facility (SECREF) were utilized for this research. Swale parameters tested included swales lined with riprap vs deep-rooted, native grasses, a slope of 1% vs 4%, and conventional swale vs bioswale. Each swale was tested with a medium and large storm event (i.e., 0.75 and a 1.5 inch events, respectively) to examine the water quality effects. During the simulated storm events using synthetic runoff, flow volumes and discharges were measured, and water quality samples were collected for total suspended solids (TSS), total Kjeldahl nitrogen (TKN), ammoniacal nitrogen (NH₃), nitrate/nitrite nitrogen (NOₓ), total nitrogen (TN), total phosphorus (TP), ortho-phosphate (OP), and a series of dissolved metals (Cd, Cu, Pb, and Zn). Class B riprap and a 50/50 combination of River Oats (Chasmanthium latifolium) and Big Bluestem (Andropogon gerardii) was chosen as the alternative linings for this study.
All four conventional swales had significantly and substantially higher volume reduction than those in literature and crediting documents (Davis et al., 2012; NCDEQ, 2017). Both native grass and riprap-lined conventional swales on the 1% slope significantly reduced TSS concentrations. Native grass-lined conventional swales tended to reduce nutrient and dissolved metal concentrations more so than those lined with riprap. Both native grass and riprap-lined conventional swales had generally high nutrient and dissolved metal load reduction. Turf-lined swales tended to have better dissolved metal concentration reductions than those lined with native grasses and riprap. Each of the riprap and native grass-lined swales had substantially more volume reduction than the turf-lined swales. Mean Manning’s roughness coefficients for native grass-lined swales for low and high flow conditions were 0.187 and 0.078, respectively. Alternative liners herein are a competitive option over turf because of their simple maintenance procedures.
Results indicate that riprap-lined bioswales significantly reduced volumes more than native grass-lined bioswales. Bioswales herein had volume reductions within range, and slightly lower than those noted by other researchers (Poresky et al., 2011; Osouli et al., 2017).
Peak flow reduction results for all four bioswales are consistent with literature (Ainan et al., 2003; Wu et al., 1998). Native grass-lined bioswales significantly reduced TSS concentrations in the underdrain of the 1% slope as well as the overflow and underdrain of the 4% slope. Native grass-lined bioswales tended to reduce sediment and nutrient concentrations in the overflow and underdrain more so than those lined with riprap.
This research provides evidence for the use of native grasses and riprap as alternative linings, especially using native grasses in conventional swales. However, continued research on conventional and bioswales under further design parameters (such as a wider range of slopes, various swale lengths, “real” rainfall/runoff, and the implementation of an IWS zone in bioswales) is necessary to more fully understand the impacts of alternative liners herein.
