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Keep Your Trees – They’re BMPs!

March 17th, 2014 by Eileen Pannetier

Guest Blogger – David Nyman, P.E.

Did you know that mature trees provide a valuable environmental service by reducing stormwater runoff during every storm?  In central Massachusetts, a 12-inch red maple will capture – or “intercept” – about 1350 gallons of rainfall per year, which represents an annual reduction in stormwater runoff equal to 8-10% of the total annual rainfall over the area of the tree’s leafy canopy.  Not only do trees intercept significant quantities of rainfall, but they also contribute to runoff reduction and to water quality treatment through enhancing infiltration at ground level, and through the biological uptake of water and nutrients from the ground.

Unfortunately, current design practices and regulatory programs only indirectly recognize these important stormwater functions, and do not fully account for the significant runoff reduction that can be achieved through preserving and planting trees and maintaining mature tree cover.  Municipal stormwater managers could benefit from a science-based method to quantify the runoff reduction function of trees as an integral component of their stormwater programs, with credit for this reduction accounted toward compliance with the EPA’s MS4 permitting requirements, as well as state regulatory standards.

Development practice often involves clearing large areas of woodland cover in order to provide space for installing stormwater management facilities to meet regulatory standards.  Ironically, this results in a permanent loss of the stormwater reduction function – not to mention other ecological benefits -  offered by mature tree canopy.  In other words, to mitigate for the development impacts that result from clearing trees, we clear more trees to install stormwater management practices!  Instead, we should be exploring ways to preserve, replace, and enhance mature tree canopy, as an integrated component of stormwater management design for new and redevelopment projects, and also for stormwater management retrofits for existing infrastructure.

The prudent use of street trees and strategic retention of woodland cover should be considered as Best Management Practices (BMPs), and included in our BMP “tool boxes” along with the whole variety of management practices currently being implemented to control stormwater.  Individual trees and wooded areas should be credited for the runoff reduction that they afford.  And trees should be more seriously considered in the landscaping of other stormwater BMPs, to enhance the treatment of runoff by those facilities.

CEI is currently embarking on a program to develop design and regulatory strategies that more fully recognize the value of trees in the management of stormwater.  The goal is to integrate tree canopy into the design of roadway and development projects, as well as the stormwater management facilities supporting these projects.

If you would like further information about this program or about the stormwater benefits of trees, please contact Dave Nyman at 508-281-5160 Ext. 320 or at dnyman@ceiengineers.com.

Posted in the categories Flood Management, Stormwater.