Managing Street Runoff with Green Streets

Streets represent a large portion of the impervious area in urban areas, and properly managing runoff from those streets is critical in meeting regulatory requirements in both our sewer and open channel systems. Green Streets have become a huge component of the City of Portland's efforts to manage street runoff. The combination of soil and plants slow down, retain, and clean urban stormwater. There are approximately 500 facilities currently installed, with an expected increase to 1,500 by the year 2013. Green Streets are built by private developers to meet city requirements for new development and redevelopment projects, and by the City to manage sewer capacity and water quality issues. Data from a number of green street facilities has been collected through a combination of continuous flow monitoring and flow simulation tests. The Glencoe Rain Garden has retained 88% of the rainfall runoff from the site over the past four years. Peak flows have remained below the design level for all storm events during that period with no basement sewer backups, with peak flows from the most intense rainfall events reduced by an average of 90%. The SW 12th & Montgomery Street Planters have been flow tested several times using a combined sewer overflow design storm (over 2 inches of simulated rainfall in 6 hours) with volume retention ranging from 50% to 74%. A number of stormwater curb extensions have also been tested and have performed very well. All reduce large peak flows by 70–90%. Volume retention has varied between 61% and 96%. Green Streets in Portland have consistently performed well with only small variations because of location, design configuration, and antecedent conditions. They appear to be a very effective way of reducing peak flows and flow volumes while reducing impervious area, improving aesthetics, and filtering pollutants.