Green Build-out Model

Casey Trees and LimnoTech developed the Green Build-out Model to quantify the stormwater benefits of trees and green roofs for different coverage scenarios in Washington, DC. This research was funded by the US Environmental Protection Agency (EPA) through a Water Quality Cooperative Agreement. The model was applied to an “intensive greening” scenario and a “moderate greening” scenario.

Nearly all of the waters in Washington, DC, including the Anacostia River, Potomac River, and Rock Creek, are seriously polluted by sewage discharges and urban stormwater runoff. The Green Build-out Model demonstrated that trees and green roofs can be used to achieve substantial reductions in stormwater runoff and sewage discharges to the rivers.

Key findings show:

• For an average year, the intensive greening scenario prevents over 1.2 billion gallons of stormwater from entering the sewer systems, resulting in a reduction of over 1 billion gallons in discharges to the District’s rivers, and a 6.7% reduction in cumulative combined sewer overflows frequencies (74 individual CSO discharges).
• Reductions in stormwater runoff volume of up to 10% across the city, with up to 27% reductions in individual sewersheds under the most intensive greening scenario
• Reductions in combined sewer overflow (CSO) discharge volumes are 6% for the moderate greening scenario and 22% for the intensive greening scenario
• WASA could potentially realize between $1.4 and $5.1 million per year in annual operational savings in the area due to reduced pumping and treatment costs.
• General hydrological relationships, including unit area planning factors, and modeling methodologies that are transferable to other municipalities.