Illustrated Guide To Using Trees and Soils to Manage Stormwater

I’ve discussed using trees and soils (green infrastructure) to manage stormwater in two recent webinars. This is a rich and complex topic, so to help break it down into digestible pieces I organize it in to what I call the First Principles of Urban Stormwater Systems.

Those first principles are listed in the illustration above. They are:

1)      Soil/water dynamics: what kind of soil is being used? What is the ratio of clay, silt, and sand? What is the soil density and structure? How much soil is being provided? What is the expected water holding capacity? How slowly will water infiltrate, and how does water move through the system?

2)      Water to and into the system: what method is being used to direct water in to the system? There are many options – drains, curb cuts, and pavers are some of the most popular – but not all designs are equal. Make sure the design is functional, not just aesthetic.

3)      Sediment and debris: most urban stormwater carries floatables and detritus, things like coarse sediment, trash, and leaves. A pre-treatment bay to remove sediment and debris is necessary to prevent clogging, and essential for long-term function of the system.

4)      Water distribution: water distribution throughout the whole soil volume is essential for the efficiency of the system and health of the tree. Designs must ensure that water doesn’t all collect in one spot but moves, infiltrates, and drains.

5)      Water out of the system: just as getting water in is essential, so is getting water out. Standing water is bad for tree and soil health. Is there a contingency plan for overflow or to channel excess water out of the soil after a certain amount of time? Can the drains be inspected and cleaned? Does too much water drain out too fast?

6)      System maintenance: all systems require upkeep and attention in order to maintain their function. Annual maintenance is an essential and often overlooked aspect of successful design. Green infrastructure solutions are no different. Make sure you know what maintenance is required and who will be performing it. The maintenance department must be a part of the design team.

So where is the tree in this discussion? I have an illustration for that, too!

Six Critical Requirements to Grow a Successful Tree. Image: James Urban

Six Critical Requirements to Grow a Successful Tree. Image: James Urban

Here is what you should remember about growing healthy, successful trees in tight urban conditions where stormwater management is also a consideration:

1)      If you get the soil and water equation correct tree will grow well.

2)      If designed correctly, these are dry (not wet) environments most of the time in most climates.

3)      The volume of soil required to treat the first inch to inch and a half of water usually provides sufficient soil to support large trees.

4)      Trees will grow better in soil mixes with greater amount of unscreened soil with less sand.  Drainage rates of 2-3” per hour at installation is great for water treatment and trees.

5)      Locate the tree in slightly mounded areas of the open soil zones so the tree can adapt to the inundation periods.

If you consider these points carefully throughout the design process, bringing in trees, stormwater, or soils experts if necessary, then you will have a strong design that should be as successful as how it was envisioned.

James Urban, FASLA, is an expert in urban trees and soils. He is the author or the book Up By Roots: Healthy Trees and Soils in the Built Environment. All images courtesy of the author.

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