Irrigation vs. Bioretention for Trees

By Len Phillips

Is irrigation of street trees using municipal water the tried-and-true method and therefore the best solution for watering the city's trees? Or can the stormwater from a street be diverted to the soil around a street tree and might it be used to water the tree? If the contaminated water from a heavily traveled street is allowed to flow through the soil containing roots of a street tree, will the water become cleaner and what impact might the contaminants have on the tree? Suppose there is a heavy rain and the soil is saturated. How long will it take before tree roots begin to show signs of rotting?

As I began my search for answers, I came across a recently completed action plan that might answer some of these questions. This action plan is from the Province of Ontario, Canada.

Ontario's Climate Ready Action Plan

Ontario’s, Climate Ready Adaptation Strategy and Action Plan indicated that climate change is predicted to result in extreme flooding (but hopefully not as bad as what happened in Texas, Florida, and Puerto Rico recently). The extreme floods will be placing more strain on everyone's aging water, wastewater, and stormwater infrastructure. When combined with increased runoff from urbanization, the Great Lakes could also experience a strain on water quality that will likely damage the fish and wildlife habitats.

Mentioned in this action plan is the growing interest in innovative stormwater management techniques including low impact development (LID). For example, the action plan calls for distribution pipes to be installed within the LID facility to send stormwater through perforated pipes into a bioretention soil media containing the perfect soil for growing the trees planted as part of the LID. As the stormwater percolates through the soil column, the tree will use as much of the water and nutrients as it needs while the excess water and nutrients migrate to the bottom of the bioretention system. The water discharges through an under-drain connected to the City’s stormwater sewer system. This design dictates that there is no permanent water storage within the soil volume, so there will be no rotting roots and the soil volume will act as a filter to clean the water.

Stormwater treatment and water benefits are provided for smaller rainfall events as the capacity of the system allows. Larger rain events that exceed the capacity of the bioretention system bypass the system through overflow pipes within the existing catch basins and discharge directly into the existing storm sewer system.

Usually in most cities, stormwater is handled by drains and underground pipes instead of open land containing streams and trees. The open land can absorb most of the water during a normal rain event. However, because there are buildings and pavements in the city, stormwater management must be conducted entirely underground. The system needs to be designed to handle a 100-year rain event (5 inches or 121mm per 24 hours).

Examples

The examples below, will illustrate projects that are similar to the Climate Ready Action Plan and are working well under the following city streets.

City of Boise

In the past, Boise, Idaho struggled to grow trees to maturity in its urban core. The trees were typically planted in small 4ft. by 4ft. (1.2 m.) square wells. The lack of soil space in combination with extremely hot and dry summers, resulted in poor health and long-term damage to the city's trees. In addition to this, the storm drain system drained largely untreated runoff directly into Boise River through a series of large pipes that are now under-sized.

This presented a set of challenges for Boise's urban foresters, city planners, and construction companies. These diverse groups decided to join forces and form a strategic partnership to create a long-term plan for their urban forest and stormwater drainage system. The group decided to try using soil cells because they have been proven to provide conditions to grow large trees and treat rainwater on-site. To date, over 100 trees have been planted in soil cells totaling about 100,000 cubic feet of soil, which will nurture the tree's long-term growth and also provide runoff reduction and improved water quality. The design includes connecting the existing catch basins to a 4”(10 cm) perforated pipe for water distribution. The under-drain sits at the base of the soil cell system and is connected to the city storm drain system. The catch basins have not been over flowing and are performing quite well in controlling runoff. The trees are rapidly growing and the soil provides improved water quality.

Mississauga, ON

Another example of this stormwater treatment system is located on Central Parkway in Mississauga, Ontario. This system was designed like the others, however, the city conducted a study to determine the performance of the system. Findings from 2015 show a 97% average stormwater volume reduction and a 96% peak flow reduction. In addition the system helped to replicate a natural water balance in an urban setting, contributing to erosion control, improved water quality, and protection of the natural aquatic habitat, downstream. The system minimizes infrastructure and maintenance costs while allowing runoff to filter through the soil, where it can be cleaned and cooled before re-entering the storm sewer.

Copenhagen, Denmark

The City of Copenhagen's tree planting program was designed to provide a 30% stormwater volume reduction.

In addition, the integration of urban trees into LID practices improves runoff water quality, while improving root growth and tree vitality leading to increased canopy cover.

Belmont, Perth, Australia

The City of Belmont, Perth, Australia, is a pioneer in placing a monetary value on their trees. They have proof that the benefits of soil cells and the perforated pipe drainage systems described in these previous examples really work and work well. More information on this subject will be published in Online Seminars #77 starting on January, 1, 2018.

Cleaning Stormwater

Many cities apply salt on their streets in winter and this salty run-off could kill the trees. Besides the benefits of soil cells described above, there are three alternatives to prevent winter groundwater damage to the trees.

1. Switch to calcium chloride instead of sodium chloride to treat roads in winter.
2. For more highly trafficked and treated retail streets the city should assist the spring rains to flush the system of sodium chloride before the trees start new growth every spring.
3. Design a winter by-pass into the drainage system to close off the water in-take during winter.

Soil and trees remove pollutants and hydrocarbons from stormwater through several processes:

• Sedimentation is slowing water flow so the contaminates will settle to the bottom and clean water is available at the surface level.
• Filtration is the physical removal of solid contaminates when stormwater passes through soil.
• Adsorption is the attachment of contaminate ions to the surface of clay and organic particles in soil.
• Microbial degradation is the breakdown of contaminates by microorganisms in the soil.
• Phytoremediation is tree uptake of solid contaminates via bio-degradation in the tree root rhizospheres.

The best soil mix for urban tree planting is specified in items 10 and 11 in Online Seminars. The bioretention soil will slow the discharge of stormwater runoff as well as provide a soil suitable for tree and plant growth. For hydrocarbon removal, the soil blend will need a higher clay and organic content level. The organic content supports good microbial growth which supports bio-degradation of hydrocarbons and good plant growth which also supports microbial degradation and phytoremediation processes.

Stormwater can contain many contaminants that are important to consider. As far as hydrocarbons are concerned, studies are clear that green infrastructure solutions that incorporate soil and plants are very effective at treatment and removal.

Sources

• Special thanks to Deeproot staff members for providing several examples of projects that dealt with stormwater retention systems.
• Buchanan, Patty, ”Treating Hydrocarbons with Green Infrastructure”, Deeproot, August 28, 2017.
• Credit Valley Conservation, “Central Parkway – Low Impact Development Infrastructure, Performance and Risk Assessment”, May 2016
• Gooden, Ben, “Tree Valuation”, Online Seminar #77.(to be published in Jan. 2018)