City Trees

Trees: First Line of Defense against Stormwater Runoff Urbanization is a major cause of increased stormwater runoff and deterioration of water quality in natural systems. Low Impact Development (LID) is an approach to apply nature-based solu- tions to reduce the impact of impervious areas on stormwater volume and quality. It includes practices such as bioretention, permeable pavement, rainwater harvesting, and green roofs. One of the practices that does not get mentioned frequently is increasing the tree canopy as an LID practice. In fact, trees are the first line of defense against increased runoff. Trees capture a portion of the rain through interception (that later evapo- rates directly from the canopy) and they slow down the runoff as throughfall trickles from the leaves, increasing infiltration in the soil (Figure 1). Unfortunately, trees are the first casualties of development and thus not only pervious land is paved, but also a natural defense mechanism is eliminated in the process. Trees can be effectively used as a stormwater man- agement practice. Trees are commonly planted in sidewalks in cities. The traditional approach of planting trees along a sidewalk is not conducive to reducing runoff and is often unhealthy for the tree (Figure 2). An improved design, known as tree box filter, can greatly increase stormwater absorption and retention and provide the tree with increased space for root growth. The design con- sists of suspended pavement and structural cells installed underneath the pavement. The cells are then filled with uncompacted soil and planted with trees (Figure 3). Other options include a stormwater tree pit, which consists of an opening in the pavement that receives stormwater from the side- walk or the road and is planted with trees. Bioswales, installed in road medians or parking lots, can also be planted with trees. In addition, trees can be a part of green roofs that double as parks. Klyde Warren Park in downtown Dallas is a great example of a green roof installed on top of an underpass (see photo). As a stormwater management specialist, I encourage all engi- neers and landscape architects to consider the integration of trees in any stormwater management project they work on. While trees are usually included in projects for aesthetic, health, and heat island reasons, with the correct design, they can also be an effective nature-based stormwater management structure. — Fouad Jaber, Ph.D., P.E., Associate Professor and Extension Specialist, Biological and Agricultural Engineering Dept., Texas A&M AgriLife Extension Service Figure 1. Water processes in a tree. (Figures by Tina McKeand from the 2013 EPA publication Stormwater to Street Trees: Engineering Urban Forests for Stormwater Management.) Figure 2. Traditional sidewalk with tree. (EPA, 2013) 16 CityTREES

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