Trees can help cities better prepare for severe weather events

November 21, 2012 under CANR News, Cooperative Extension

In the aftermath of Hurricane Sandy, many cities are taking a look at how they can better prepare for severe weather events. A low-tech – but effective – solution is to plant trees, says Sue Barton, ornamental horticultural specialist for the University of Delaware.

“A single mature tree can intercept several thousands of gallons of stormwater. Plant more trees in the right places and you can mitigate the impact of storm events,” says Barton.

She points to the research of David Nowak, a forester at the U.S. Department of Agriculture’s Northern Research Station in Syracuse, N.Y., who has analyzed the role that “urban forests” play in controlling runoff and flooding, reducing the costs of stormwater management facilities, and decreasing water pollution.

An “urban forest” doesn’t necessarily mean a tree-filled area the size of Central Park. Instead, researchers like Nowak look at the overall tree coverage in a community. The average urban tree canopy in the U.S. is 23 percent. But the tree canopy in the New Castle County metro area is estimated to be just 19 percent, and the city of Wilmington’s tree canopy is 16 percent.

“Philadelphia and Wilmington have experienced water overflow situations after decent-sized rains, not just storm events like Hurricane Sandy,” says Barton, a Cooperative Extension specialist and associate professor in the Department of Plant and Soil Sciences. “The stormwater management systems in these cities were engineered many years ago and they can’t handle the water flow after a big rain – which means raw sewage and other organic material bypasses the treatment plants and go directly into streams.”

Fixing antiquated stormwater systems isn’t cheap. “One of Nowak’s greatest contributions may be his research into the economic benefits of trees,” says Barton. “He came up with a way to put a dollar cost on how much trees can save a community. He looks at the cost of trees and tree maintenance relative to the costs of updating aging stormwater systems.”

In Wilmington, the Delaware Center for Horticulture (DCH) has been a driving force behind stormwater mitigation efforts that include planting trees and shrubs, establishing rain gardens and installing underground holding tanks. All three of these elements were included in a stormwater project at the Trolley Square Acme that was completed in June 2011.

The 9,000-square-foot project filters, slows and absorbs rain that falls on the roof of the Acme and its 1.42 acre parking lot. Comprised of 19 shade trees, more than 2,800 shrubs and smaller perennial plants, a rain garden, and underground holding tanks, the project captures an estimated 70 percent of the site’s annual rainfall, providing relief to the city’s combined stormwater and sewer system.

Gary Schwetz is a senior project analyst at DCH and was instrumental in the development and execution of the Acme project. His advice to those who want to use trees to intercept stormwater: “Think big.”

Schwetz doesn’t mean you need to plan a big project – like the 2,819 or so living things planted at the Acme — but that you need to include big trees.

“Large trees are better at absorbing rainwater and mitigating air pollution,” says Schwetz.  “A 20-foot tree will have eight times the environmental benefits of a 10-foot tree.”

Of course, it can be tough to grow a big tree in the narrow space between a city sidewalk and the street, or in a city backyard. It can even be tough for big trees to do well in public spaces like Rodney Square, which little by little has seen its grassy area reduced and covered by pavers and other impervious surfaces.

Schwetz and fellow DCH staffers worked on an innovative landscape project that will help big trees flourish at Rodney Square. Other partners were the city of Wilmington and the Delaware Department of Transportation.

What makes the project different, says Schwetz, is the use of a new structural cell technology as the planting medium. These milk-crate-like structural cells can support sidewalks and hold a high volume of good quality soil, creating conditions in which large trees should be able to thrive.

Rodney Square isn’t the only place the city of Wilmington has been planting trees lately. Some 250 trees were planted by the city in the last year and a half. And, one year ago, the city hired Mandy Tolino has its first-ever urban forest administrator.

“Trees and the green infrastructure improve water quality by helping slow water down during a storm, as well as by reducing erosion,” notes Tolino.

Recently, she has been involved in a pilot tree trench installation at Brown Burton Winchester Park, at 23rd and Locust streets. On the surface, this tree trench looks like an ordinary row of trees. But underground, the trench is lined with a permeable fabric and filled with gravel. During a rainstorm, water flows through a storm drain to the trench, where it’s stored in the empty spaces between the stones before slowly infiltrating into the soil below.

There will be a public dedication of the Rodney Square landscape project on Nov. 27 at noon. For more information, call the Delaware Center for Horticulture at 658-6262.

Article by Margo McDonough

Photo by Danielle Quigley

This article can also be viewed on UDaily.

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