Stormwater infiltration capacity of street tree pits: Quantifying the influence of different design and management strategies in New York City

Abstract

Street trees are abundant in the urban landscape and provide many ecosystem services including stormwater management. For trees housed within tree pits, the ability to mitigate stormwater runoff can be modulated by the permeability of the soil within the tree pit itself. Thus, developing a better understanding of how tree pit design and management impact soil permeability can be important to quantifying, and potentially improving, the stormwater benefits of street trees. To this end, water infiltration rate was measured at forty tree pits representing the variety of physical conditions commonly seen in New York City, including the presence or absence of a tree pit guard, the size of the tree pit, the size of the tree, the presence or absence of ground cover planting, the presence or absence of mulch, and the elevation of the pit’s soil surface relative to the sidewalk. An initial analysis of results first tested the impact of each physical condition on infiltration rate individually. Genetic programming was then used to investigate interactive effects between the physical conditions, and to develop a statistical model that captured 66% of the variability in the observed infiltration rate using simple physical features of a tree pit. Results showed that the most significant factor influencing the infiltration rate was the presence of a guard around a tree pit, with guarded tree pits having higher infiltration rates. Additionally, higher infiltration rates in guarded pits were associated with larger pit areas, built-up surface elevations (binary) and the combined presence of ground cover planting (binary) and mulch (binary). Tree size, as measured by circumference at breast height, was found to be a less significant indicator of the infiltration rate. The statistical model, together with the study measurements, can be used to estimate the stormwater benefits of different tree pit management strategies, inform designs for improved stormwater management, and help identify useful observations or measurements for a street tree census.