Storm Water Infiltration and Pollinator Habitat Zones Along Highways

Vegetated stormwater control measures (SCMs) are a critical tool in North Carolina Department of Transportation's (NCDOT’s) toolbox of practices for reducing stormwater runoff and its impact. Infiltration is a key feature of these SCMs. Infiltration reduces runoff volume, and sediment and nutrient loss. Viable vegetation with proper rooting is essential to achieving infiltration. Establishment and maintenance of vegetated SCMs is often problematic due to poor soil conditions prior to seeding, associated with compaction, and topsoil removal. The result can be poorly vegetated areas which generate high runoff volumes relative to undisturbed areas. Recent research has demonstrated that applying tillage to ameliorate compaction on construction sites greatly enhances success in vegetation establishment, increases infiltration, and reduces runoff and erosion. When a strong stand of vegetation is established, the tillage effect appears to remain for at least 2-3 years, based on the period of monitoring, but likely much longer if the area is not disturbed. Incorporating compost improved the vegetation in some cases and helped in preventing recompaction by mower traffic. The effects are likely very specific to soil types and conditions, but relatively high infiltration rates (20-30 cm h-1) in tilled soils were found at most sites during the 2-3 year monitoring period. Current results of a highway installation are also promising, with runoff volumes reduced by roughly 50% when compost was tilled into the soil. While all of the project team's research into infiltration improvements through tillage has involved planting various grass mixes, there may be many locations where other plant types may be more beneficial. NCDOT has been planting wildflower areas for 30 years and the expertise developed in this program could be tapped for this project. Deep-rooted perennials may provide additional structural support for the soil to maintain the initial high infiltration rates typical after tillage. Mowing in these areas would be reduced to once per year, or even less. Furthermore, the plants could be selected to be both aesthetically pleasing to the public and to benefit pollinators. There is great concern about declining pollinator populations, and one of the main contributors to the decline has been reductions in suitable habitat. An on-going project has suggested that wildflower planting along highways significantly increases pollinator populations (O’Brien, personal communication). The combination of benefits – high infiltration, low maintenance, aesthetics, and pollinator habitat – suggests that establishing these areas along roads would be a multifunctional Best Management Practice.