Engineering Urban Forests for Stormwater Management
The U.S. Environmental Protection Agency (EPA) offers an informative brochure titled Stormwater to Trees. It discusses how to engineer urban forests for stormwater management.
The brochure primarily targets engineers, planners, developers, architects, arborists and public officials. However, residents concerned about the loss of trees to new development may find it a useful tool to begin discussions with all of the above.
The 34-page brochure is about using trees to augment existing stormwater management systems and improve water quality while beautifying cities. It contains four major sections, briefly summarized below.
Section One: Urban Stormwater Runoff
By design and function, urban areas are covered with impervious surfaces such as roofs, streets, sidewalks, and parking lots. Rain falling on impervious surfaces cannot infiltrate the ground. Instead, it creates runoff: a problem for everyone. The runoff collects pollutants on its way to storm sewers that discharge into ditches, streams, bayous, lakes and bays.
This is how “non-point source pollution” starts. It can contaminate water supplies and affect the health of plants, fish, animals, and people. Excess runoff can also erode and damage property.
Section Two: The Role of Trees in Stormwater Management
In cities, trees can play an important role in stormwater management by reducing the amount of runoff that enters storm sewers. Trees act as mini-reservoirs that reduce and control runoff by:
- Transpiration: Drawing water in through their roots and gently releasing it back into the atmosphere in the form of water vapor.
- Interception: Leaves absorb rainfall, reducing the amount that hits the ground, and delaying/reducing peak flows.
- Reduced Erosion: Tree canopies diminish the volume and velocity of rainfall, lessening its erosive force.
- Increased Infiltration: Roots increase both the rate and absolute level of stormwater infiltration.
- Phytoremediation: Trees can take up trace amounts of harmful chemicals and transform them into less harmful substances.
Trees have proven value in reducing runoff and mitigating the costs of stormwater management, but their innate ability to absorb and divert rainfall has been underutilized, according to the EPA.
Therefore a major focus of this section is how to design sites for successful planting. It offers strategies for dealing with impervious surfaces and compacted soils that can stunt tree growth and shorten trees’ lifespans. For instance, pavements can be supported by pillars, piles, and structural cells, allowing for large volumes of uncompacted soil below ground.
Section Three: Stormwater Management Systems with Trees
The next section goes into more detail on each of those strategies. It discusses pros and cons of each and design considerations in various locations and applications. It also provides illustrations that help the reader quickly grasp the concepts.
This is the meat of the brochure. There’s too much in this section to summarize, but you can quickly scan it.
Major subsections include: suspended pavement and structural cells; structural soil; stormwater tree pits; permeable pavements; forested bioswales; and green streets.
Section Four; Case Studies
The final section of Stormwater to Trees contains illustrated case studies from cities across the country. Several describe 10% reductions in peak flows, a percentage consistent with academic studies elsewhere.
While that may not sound huge, it’s important if stormwater is lapping at your doorstep during heavy rains. It’s also important to remember that stormwater management is just one of the many benefits of trees. They also help clean the air, reduce energy needs, raise property values, and mitigate urban heat-island effects.
To review the complete brochure, click here.
Posted by Bob Rehak on 3/4/2023 based on EPA information
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