Swales are the most widely-used stormwater control measure (SCM) in the transportation environment, including linear rights-of-way, highway interchanges, operations and maintenance facilities, and secondary roadways. They are predominant because they are of low expense and easily fit into typical DOT rights-of-way. Swales are usually constructed with turf grass and other low-lying grasses, and recent research (much of which was conducted in North Carolina) demonstrates modest to moderate effectiveness regarding pollutant reduction. However, there are many circumstances when grass-lined swales are impractical, especially when located under a tree line so that the vegetation is shaded or in areas with steep slopes. This situation is conducive to rock lining in lieu of vegetation. Moreover, native deep-rooted grasses are often adapted to growth in swales, and these grasses have different – and less costly – maintenance needs than conventional low-lying turf grasses that need to be mowed. There is little to no information available on how rock-lined and native-deep rooted grass-lined swales perform with respect to water quality. Additionally, research is needed to assign a manning’s roughness coefficient by deep-rooted grasses to flow.
Four existing swales and four existing bioswales located at NC State’s Sediment and Erosion Control Research and Education Facility (SECREF) will be used to evaluate water quality performance of alternatively-lined swales. Four of the swales to be tested are currently conventional grass-lined, with the other two being bioswales. Data from previously-conducted experiments will serve as a basis to compare the performance of alternative swale linings. Two different linings will be retrofitted into the existing swales/bioswales. One will be a rock lining (Class B, as specified by NCDOT); the second, a deep rooted native grass mix. Field tests will be conducted in 2021, after a thorough literature review is conducted in autumn 2020.
The research team will reach out to NCDOT Roadside Environmental Unit (REU) staff and also reach out to local contractors to determine the costs and availability of alternative swale linings to help NCDOT determine future swale designs. If alternatively-lined swales prove cheaper to maintain while providing at least comparable water quality treatment, the potential for less expensive, yet still safe, operations is likely.
NCSU faculty, staff and students will conduct the following tasks as part of this proposal:
- Conduct a literature review regarding alternative swale linings, included in this is an assessment of local availability and costs
- Retrofit eight swales (four conventionally-drained and four bioswales) at SECREF. Characteristics of each of the 8 swales as outlined in Table 1. They include two liner types (rock and deep-rooted grass) and slopes (1% and 4%)
- Run water quality tests, collect data at SECREF and then analyze and synthesize data
- For two of the swales planted with deep-rooted grasses (Table 1), a manning’s roughness coefficient will be determined, using known flow rates, swale geometries, and slopes, while calculating flow height.
- Based upon information gained from (3) and (4) above, NCSU will deliver design guidance to NCDOT personnel to help compare performance and maintenance needs of various swale linings under various conditions.