Bioretention Basins and the Urban Forest

December 9, 2008 1 Comment

by Doug Johnston,

2607 Backyard Forest. Photo: Mike Repkin.

Clearly, trees are an important component of our urban spaces. From beautification to air quality, they provide us with many benefits in simply being alive. This is, however, a great hurdle in the impervious delineated world of the city. On average, an urban tree has a life expectancy (yes, even trees have these) of around 30 years. An inner city tree, though street wise, on average only sees the age of 12. Yes, urban trees make gangs look safe.

This is a major issue when we begin to ask, why are we planting trees? If shade, carbon sequestration, air quality, lower air temperatures, beautification, or storm water retention are any of the answers, then there is a serious problem here. Mature, healthy trees are needed to accomplish any of these tasks. What we are left with is the obvious need to alter building and urban design to allow for the presence of trees and other vegetation, other than as space fillers on building plans. We also begin to see that management practices, the species that we plant, and the locations of planting become issues of importance as well.

A good read on the importance of mature tree growth is an article written by David Nowak, et al.. If you are wondering about best practice for tree planting, you can find that on the Morton Arboretum website. This applies mostly to residential planting. When the inner urban environment is encountered, much more drastic measures are needed.

Major research has been done on alternative building methods for urban tree health at Cornell University, among many other places (it, Cornell, is the focus of this entry though). Nina Bassuk has pioneered efforts there with methods of urban tree planting and alternative materials usage to aid in tree health. One major outcome has been CUSoil. CUSoil is a structural soil that may be used in place of heavily compacted sub-grades to provide structure for roads, sidewalks, buildings, and various other constructed forms.

It is made by spraying a rock aggregate with a polymer that allows mineral and nutritional elements to adhere to them. These structural soils (of which there are many types and can be made by anyone) engineer support while still providing rooting ability for trees and other plants along with water/air infiltration and retention and a nutritional element. They do not however, take the place of mineral soil. More plainly put, it would take three times as much structural soil to support a tree than mineral soil (that found in your yard). This is a vastly large amount when speaking of a mature tree. What they do is extend the rooting area that a tree otherwise would not be able to penetrate due to compaction.

A possibility that has begun being exploited for structural soils, is the construction of bioretention basins. Papers by both S. Day and A. Thompson (paper not available online) illustrate beneficial aspects of these devices. They aid in reducing peak storm water runoff, storm water treatment, and sediment runoff. They are also source controls, treating storm water at the source. As noted by Thompson:

“Examples of source controls include bioretention basins,
infiltration basins, porous paving systems, and wetlands. Local
systems offer several advantages over regional controls
(e.g., detention/retention ponds) because they typically receive
smaller runoff volumes, velocities, and pollutant loads.
This reduction lessens the potential for, and consequences of,
design failure. Moreover, smaller runoff loadings associated
with distributed source control can increase pollutant removal
efficiency and groundwater recharge (Butler and Parkinson,
1997) compared to larger systems that may provide less
efficient infiltration and increased groundwater mounding
(Morel-Seytoux and Miracapillo, 1989).”

Susan Day’s paper points out that the use of bioretention basins with certain bottomland tree species (red maple, black oak, green ash) planted over them, may offer the possibility of soil sub-grade compaction remediation. Thusly, higher water infiltration rates and groundwater recharge. Which, if you keep looking down the line, reduces stresses on the storm water infrastructure, increases source storm water treatment, and replenishes a very thirsty urban landscape. It also greatly aids in urban tree health and subsequently urban tree life expectancy.

This works in a multi-step mechanism. The structural soil primarily provides the ability for the tree to grow at the given site. Secondly, the retention basin greatly increases moisture in the soil directly below it in the profile. This increased moisture allows the tree roots (vigorous in these species) to infiltrate into the otherwise inaccessible soil sub-layer. Think of it like dried out bread. Throw it in a sink filled with water for awhile then take a bite of it. Not so hard anymore. Not all trees are suited to grow in these environments just as not all humans are suited to eat soggy bread, so tree selection is important. As the roots infiltrate through the geo-textile fabric and into the lower soil profile, they provide space for water preferential flow into the same space. This is what is allowing for groundwater recharge and greater water infiltration.

Voila, we have a complex web of storm water treatment, groundwater recharge, urban tree/vegetation health, engineering, urban planning, architecture, and land management all coming together. Much more research and practice is needed to address what mixtures of structural soils work best with what plants, along with installation and overall design, but with plenty of Wal-Marts going up every hour, there should be plenty of places to start though, right?

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