Successful LID – case study: Casey Tree’s Washington DC headquarters

  

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 Washington DC is a dense urban area, where the city must balance economic and environmental costs of overloaded storm drain systems. But Washington DC has moved away from traditional man-made infrastructure and is now looking for natural ‘’green’’ utilities solutions.

A new draft regulation would require every property owner to retain at least 90% of their property's rainfall runoff or 1-2 inches during a 24-hour rainfall. The Department of the Environment (DDOE) and D.C. Water would like to introduce charges and a stormwater fee discount program to foster the implementation of stormwater runoff management measures.

        Casey Trees, a Washington DC – based non –profit, has a simple solution for the City to be sustainable and to efficiently manage stormwater runoff. The organization fiercely believes in stormwater benefits of large trees. Their goal is to protect the existing trees of the City and to encourage it to add new ones. By planting more trees and nourishing a large urban tree growth, Washington DC could mitigate its infrastructure problems. This is part of a successful Low-Impact Development – project (LID) that Casey Trees experienced for its new headquarters on 12th Street.

Casey Trees wanted to have a building that promotes the efficient role of trees in stormwater runoff management, encourages the city to plant more trees and could be a model for LID. With this building, Casey Trees now belongs to the SITES Pilot program (Sustainable Sites Initiative), with only 150 other projects in the United States.

Wiles Mensch Corp. (WMC) was hired by Casey Trees for the project. This sustainable practice civil engineering firm strives to encourage LID approach to stormwater management. They designed a significant number of rain garden and bioretention areas in and around Washington DC.

Rain garden and bioretention planters are efficient systems in terms of stormwater management and have many advantages:

• They help avoid overloaded sewers by reducing the flows discharging to the public sewer system
• The help with a reduction in the potential for combined sewer overflows
• They encourage groundwater discharge

However, people have been skeptical about the possibility to meet the standard in an urban area. But Casey Trees’ headquarters are a perfect example of how possible and how successful it can be. Their building includes 4 LID strategies:

• A roof with 25 % vegetation
•  A raingarden
• A street tree bioretention system
• A cistern to collect roof runoff and overflows

Here is some more detail about the LID practices implemented :

THE STREET TREE BIORETENTION SYSTEM:  A bioretention system under paved surfaces.

This is an innovative means that enables a dense urban area to reduce stormwater runoff by reducing the amount of open space needed for bioretention. 

The key of this system is the Deep Root Silva Cell, which is an underground soil and stormwater management system installed under the sidewalk and connected to the rain garden on the other side of the sidewalk. It enables the City to store more water and to extend tree rooting volume.

TREE MECHANICS IN THE INFRASTRUCTURE

Tree bioretention systems are useful and efficient in terms of stormwater quantity and rate-control. This efficiency comes from three processes:

• Soil water storage : bioretention with trees helps direct stormwater runoff from nearby impervious surfaces into soil under suspended pavement
• Tree interception : this is  a natural stormwater treatment. Rainfall that remains temporarily on tree leaves and stem surfaces then flows down to the ground.
• Tree evaporation : this is a natural stormwater treatment as well, that helps reduce stormwater volume stored in the soil with evaporation of water from soil.

Thus, trees can provide several stormwater benefits.

Casey Trees’ headquarters are a tangible example of these benefits :  they planted many different sorts of trees in the rain garden and in the retention planters. The system reached its goal because it is now able to manage 5 inches of water runoff and the more the trees will grow, the more efficient the system will be thanks to interception and evaporation. 

FIGHTING POLLUTION WITH TREES

A tree bioretention system can consequently improve water quality of stormwater. The bioretention soil filters stormwater and removes some pollutants. It enables the soil’s sorption capacity to recharge in between rain events.

Bioretention is one the best stormwater control measures for pollutants removal. Trees can significantly help to remove nutrients – nitrogen and phosphorus. Everyone benefits from this situation. The trees uptake the nutrients and the City has less nutrients and stormwater flow to deal with.

The innovative LID- project carried out by Carey Trees is a model project: by planting trees and implementing LID strategies, they put forward a new way to efficiently mitigate stormwater in a dense urban environment.

If you have any question about LID-projects and stormwater runoff management, please contact John McAllister at jmcallister@norfolkram.com or at (508) 747 - 7900 x 117.

Download our complimentary 'Low Impact Design (LID) Executive Overview' here.

Information in this article taken from Sept 2011, article by Al Key and Nathalie Shanstrom , published in www.cenews.com.