Green Stormwater Infrastructure (GSI) is a complimentary and sometimes alternative systems to traditional or "gray infrastructure" to manage runoff from developed lands. Traditionally, stormwater runoff has been collected and conveyed in closed systems to off-site locations where it is then discharged, without treatment on surface waters. The series of pipes, catch basins, and storm drains that result is known as ‘gray infrastructure.’ Because ‘gray infrastructure’ does little to improve water quality and reduce water quantity, stormwater discharges from these systems often contribute to unhealthy stream flows marked by chronic flash flooding, altered stream morphologies, elevated nutrient and contaminant levels, excessive sedimentation, loss of species diversity, and higher water temperatures.
Green Stormwater Infrastructure (GSI) utilizes infiltration, evapotranspiration, storage and reuse - mimicking natural hydrologic processes. GSI is decentralized by design and either prevents runoff from occurring or treats it as close to the source as possible. We define it as "systems and practices that restore and maintain natural hydrologic processes in order to reduce the volume and water quality impacts of the built environment while providing multiple societal benefits."
Infiltration is a natural process by which water moves into and through soil and other porous materials. Movement through the media is dominated by gravitational and capillary forces. Gravitational forces pull water down vertically through the soil. Capillary forces pull water horizontally and laterally within the soil profile.
Evapotranspiration (ET) is a process by which water is transferred from the earth’s surface into the atmosphere. ET is the sum of water surface evaporation, soil moisture evaporation, plant surface evaporation, and plant transpiration, a process in which trees and other plants absorb water through their roots and transfer it up to the atmosphere through leaf pores. Evapotranspiration happens naturally and is an important and critical part of the water cycle because it represents a considerable loss of water from a watershed.
Capture and reuse practices are designed to intercept and store runoff from impervious surfaces, such as rooftops, reducing the volume of stormwater runoff exiting a site. The stormwater is contained in a vault or other storage vessel and reused for irrigation, car washing, or other non-potable uses. A water budget, which calculates expected water captured and used, is typically developed for larger systems. The reuse of stormwater for potable needs and human consumption is not recommended. Storage and reuse has minimal direct effect on water quality, however, indirectly it helps mitigate erosion and nutrient loading by reducing stormwater runoff volumes and the magnitude and timing of peak flows.