Green Infrastructure Retrofits with Impervious Area Reduction by Property Type: Potential Improvements to Urban Stream Quality

Abstract

Building on concepts that relate stream health to extent of watershed impervious area and the capability of green infrastructure (GI) measures that remove runoff volume to reduce effective impervious area, the extent of GI retrofits possible in existing urban watersheds with stormwater-induced impairments was investigated with impervious area reduction as the key metric. Potential reduction in directly connected impervious area (DCIA) via GI retrofits was estimated for two watersheds in Mecklenburg County, North Carolina; one is dominated by commercial and the other by single-family residential development. The extent of possible DCIA reduction and the differences in contribution by property and ownership type were examined relative to potential stream quality improvement goals. The results indicate that GI retrofits are needed on all property types to significantly impact aggregate DCIA reduction. Private commercial property plays a significant role, providing up to 45% and 35% of total DCIA reduction potential in the commercial and single-family watersheds, respectively. Public property also has great potential, contributing nearly 35% of total DCIA reduction in both watersheds, with the majority attributed to roadway retrofits. Single-family property contributes the remaining 2%–3% in the respective watersheds. The percentage of DCIA remaining in each watershed under a range of retrofit scenarios is not promising relative to a target stream health threshold of 1% impervious area; however, potential incremental DCIA reduction within an urban watershed may be a better indicator of GI’s ability to improve stream quality.