Cost-Benefit Analysis of Green Infrastructure at WMATA Stations

Abstract

WMATA operates across the District of Columbia, Maryland, and Virginia, each of which has its own stormwater rules. Compliance with these rules is required whenever WMATA undertakes major construction projects. The question is therefore not whether WMATA must comply, but which compliance option is most practical and cost-effective at a given station. This report compares green infrastructure (GI) with two alternatives: gray infrastructure, such as underground stormwater vaults, and off-site compliance, such as purchasing credits in D.C.'s Stormwater Retention Credit (SRC) market. GI includes bioswales, rain gardens, bioretention, and street planters. Unlike gray infrastructure or credit purchase, GI can also provide co-benefits, including reduced localized ponding, modest local cooling, and improved station appearance and rider experience. These benefits are treated qualitatively rather than monetized. The analysis combines a regulatory review, flood-risk screening from the TPB Climate Change Vulnerability Assessment, EPA National Stormwater Calculator modeling, and benchmark cost data. Five stations were selected for analysis: Benning Road, Archives-Navy Memorial-Penn Quarter, King St-Old Town, Deanwood, and West Hyattsville. They were chosen from a flood-ranked set of WMATA stations using a qualitative filter that also considered reported drainage problems, site suitability for GI, jurisdiction, and equity. They are therefore not simply the five highest-risk stations, but a comparative group suited to testing where GI may be useful. The findings show that GI improves stormwater performance at all five stations. Increasing GI coverage reduces annual runoff, increases wet-day retention, and raises the amount of rainfall a site can hold before runoff begins. The strongest modeled gains appear at Benning Road, Deanwood, and West Hyattsville, where the largest scenarios reduce annual runoff by roughly 80 percent and raise wet-day retention to about 87 to 88 percent. Archives and King Street also improve, but more modestly because of site constraints and smaller modeled interventions. GI's value depends on jurisdiction and site conditions. D.C. provides the clearest market benchmark through SRCs. Maryland makes distributed GI the preferred starting point under ESD-to-MEP, though WMATA's project-level use of nutrient credits should be confirmed case by case. Virginia relies more on numeric phosphorus standards than a credit market. The report recommends prioritizing GI where on-site retention has legal value, the site is physically suitable, and modeling shows strong performance. Benning Road is the highest priority, followed by Deanwood and West Hyattsville. Archives should be pursued through coordination with area partners, while King St-Old Town is best suited for studying and expanding existing street planters. Overall, GI should be used selectively as a site-specific compliance tool, with additional co-benefits that are not provided by other compliance options.