Like many international cities, Washington, D.C., is grappling with the economic and environmental costs of overloaded storm sewers. The city serves as an example of how municipal civil engineering evolved over several decades from man-made infrastructure technology to natural green utility solutions.
Casey Trees, a Washington, D.C.-based non-profit, believes municipal infrastructures need to incorporate more trees for urban sustainability. The organization aims to restore, enhance, and protect the tree canopy of the nation's capital. To fulfill this mission, Casey Trees plants trees, engages thousands of volunteers, provides year-round continuing education courses, monitors the city's tree canopy, develops interactive online tree tools, and works with elected officials, developers, and community groups to protect and care for existing trees and to encourage them to add new ones.
When Casey Trees planned its new headquarters at 3030 and 3015 12th Street NE in Washington, D.C., the organization wanted to construct a building that respected the character of the historic Brookland neighborhood, highlighted tree planting, and promoted education efforts. It aimed to serve as a model for innovative low impact development (LID). In doing so, Casey Trees incorporated several external green features that qualified them to be invited in the Sustainable Sites Initiative (SITES) Pilot Program (www.sustainablesites.org). Casey Trees' headquarters is now one of only 150 projects in the United States and abroad to participate in the program.
Still, the SITES designation was only a part of the goal. "We wanted a truly exemplary project which shows leadership in both arboriculture and hydrology" said Mark Buscaino, executive director of Casey Trees.
Challenges and leadership in hydrology
In some Washington, D.C., watersheds, even a 0.1-inch rain event can trigger a combined sewer overflow (CSO) event. The city is heavily dominated by impervious surfaces, and meeting environmental management requirements is no easy task. New draft regulations in the district require property owners to retain at least 90 percent of their properties' rainfall runoff, or 1.2 inches during a 24-hour rainfall. That is almost double the current requirement. In addition, both the District Department of the Environment (DDOE) and D.C. Water charge for water runoff by calculating impervious surface area on a property. However, both departments are working together to establish a stormwater fee discount program that offers incentives to property owners who implement measures to manage and reduce stormwater runoff.
The Casey Trees and partner organizations showcase several ways to mitigate infrastructure problems while nourishing large urban tree growth. Marcelo Lopez – project manager for Wiles Mensch Corp. (WMC), a sustainable practice civil engineering firm – worked closely with DDOE to encourage a LID approach to stormwater management. WMC designed hundreds of rain garden and bioretention areas in and around Washington, D.C. The rain garden and bioretention planters help to reduce the flows discharging to the public sewer system, thereby reducing the potential for combined sanitary and storm sewer discharges into streets and waterways. They also help to maintain groundwater recharge. They are not only practical, but the impact of these LID practices can help to address the major impacts impervious areas have to the quality of the Chesapeake Bay, according to Lopez.
Casey Trees' property combines the following LID strategies to demonstrate how to meet the standard, even in densely developed urban areas:
- roofs with 25 percent vegetation;
- a rain garden planted with trees, shrubs, and herbaceous vegetation to treat runoff from the roof and parking lot;
- a street tree bioretention system with uncompacted rooting/water storage volume extended under the sidewalk to collect runoff from the sidewalk and street; and
- a cistern to collect roof runoff and overflows into the tree bioretention system.
The street tree bioretention system demonstrates an innovative way to build green infrastructure even in urban areas with limited open space. An underground soil and stormwater management system, called the Deep Root Silva Cell, is installed under the sidewalk along 12th Street to extend water storage and tree rooting volume under the sidewalk and connect to the rain garden on the other side of the sidewalk.
Silva Cells are fiberglass-reinforced polypropylene modular support structures designed to support pavement with up to HS20 loading. At the same time, the Silva Cell protects the soil from compaction. The modular design enables flexibility to size the rooting/bioretention volume as needed for each site. By allowing for bioretention volumes under paved surfaces with up to HS20 loading, the underground system can greatly reduce the amount of open space needed for bioretention, which is especially valuable in ultra urban areas such as Washington, D.C.
Tree mechanics in the infrastructure
Tree bioretention systems can provide stormwater quantity and rate-control benefits through three processes: 1) soil water storage, 2) tree interception, and 3) tree evapotranspiration.
Soil stores rainwater during and after a storm, making it available for plant growth. Stormwater runoff from nearby impervious surfaces can be directed into soil under suspended pavement using a number of different techniques – for example, pervious pavement installed over the cells or via a perforated pipe off a trench drain or manhole.
Stormwater calculations for bioretention with trees typically account only for soil water storage. However, as trees grow, they also provide greatly increasing stormwater treatment through interception and evapotranspiration. Interception is the amount of rainfall temporarily held on tree leaves and stem surfaces. This rain then drips from leaf surfaces and flows down the stem surface to the ground or evaporates. The volume of rain intercepted depends on the duration and rate of the rainfall event, tree architecture (leaf and stem surface area, roughness, visual density of the crown, tree size, and foliation period), and other meteorological factors.
Evapotranspiration is the sum of water evaporated from soil and plant surfaces and the water lost as a result of transpiration, a process in which trees absorb water through their roots and transfer it up to the leaves, where it evaporates into the environment through leaf pore transpiration. Evapotranspiration continues to reduce stormwater volume stored in the soil long after a rainfall event ends. Transpiration rate is influenced by factors such as tree species, size, soil moisture, increasing sunlight (duration and intensity), air temperature, wind speed, and decreasing relative humidity.
Potential evapotranspiration exceeds precipitation during the growing season in much of the United States. Even tree transpiration can exceed precipitation where it is sustained by irrigation. Transpiration uses heat from the air to change the water in the vegetation into water vapor, so in addition to providing stormwater benefits, transpiration also decreases ambient air temperature and reduces the urban heat island effect.
Fighting pollution with trees
Casey Trees planted a diverse set of trees including bald cypress, Jefferson American elm, river birch, sycamore, and sweet gum in the rain garden and in the retention planters along 12th Street. The system of rain gardens and retention planters along 12th street can already manage 5 inches of water. Unlike grey infrastructure, they will become capable of handling even greater quantities of runoff over time as the trees increase in size and can more meaningfully contribute to the performance of the system through interception, evapotranspiration, and soil infiltration.
The soil and its microbes, together with the trees in a tree bioretention system, also work together to improve water quality of stormwater that is filtered through the bioretention soil. Some pollutants are held or filtered by soil, others are taken up or transformed by plants or microbes, and still others are first held by soil and then taken up by vegetation or degraded by bacteria, "recharging" the soil's sorption capacity in between rain events.
Bioretention has been shown to be one of the most effective stormwater control measures for pollutant removal. High concentration and load reductions consistently are found for suspended solids, metals, polycyclic aromatic hydrocarbons, and other organic compounds. Nutrient (dissolved nitrogen and phosphorus) removal has been more variable.
However, trees can help nutrient removal significantly. Healthy vegetation has been found to be especially crucial for removal of dissolved nitrogen and phosphorus. Trees also seem to benefit from the nutrients in the stormwater. A study comparing the growth of trees irrigated with stormwater to trees grown with tapwater found that the trees irrigated with stormwater had greater height growth and root density compared with those irrigated with tap water.
Using innovative technology, Casey Trees has created a headquarters that not only showcases the potential of mature trees to thrive and mitigate stormwater in a dense urban environment, but also fulfills a mission where everyone benefits.
Al Key, owner of DeepRoot Green Infrastructure LLC (www.deeproot.com), has been involved in the green industry for 20 years and is a published author in the Journal of Arboriculture. He and his partners have received one of several forthcoming patents for their inventions that address trees and stormwater management in the urban setting. Nathalie Shanstrom, a landscape architect for The Kestrel Design Group (www.kestreldesigngroup.com), specializes in providing sustainable landscape architecture design services. She focuses on landscape genealogy, preservation and restoration of native plant communities, plant composition and distribution of native plant communities, and planning and design for sustainable stormwater management.