Estimating Stormwater Infiltration and Canopy Interception for Street Tree Pits in Manhattan, New York

Abstract

Estimates of the amount of stormwater captured by urban trees have focused on the rainfall intercepted by leaves and branches, while the amount of stormwater runoff that flows into a tree pit from the surrounding sidewalk has not been well quantified. This study estimated the amount of stormwater that infiltrates into a tree pit by first calculating the tributary drainage area that drains to street tree pits of varying sidewalk widths and slopes. With Manhattan, New York, as a case study, the study used i-Tree software to find that for street trees in Manhattan, stormwater infiltration greatly exceeded canopy interception, by a ratio of 3 to 1: stormwater infiltration averaged 6842 L/yr, while canopy interception averaged 2228 L/yr. The results contradict prior research that asserted that canopy interception is the primary means by which street trees provide stormwater control. The study also provides a method to calculate street tree drainage areas that would improve estimates of the amount of stormwater captured by street trees, by highlighting the dominant role played by sidewalk widths and slopes. Infiltration averaged 4221 L/yr for a sidewalk width of 3 m and 14774 L/yr for a sidewalk width of 10 m. Infiltration also averaged 5607 L/yr for a street slope of 0.5% and 18,383 L/yr for a street slope of 10%.

Keywords

• street tree
• stormwater
• canopy interception
• infiltration
• i-Tree