If a tree falls in Wilmette, do nearby basements flood?

If your neighbor chops down just one single tree, would this cause more flooding in your basement?

This issue was reported during a Wilmette Village Board meeting, during a discussion on an appeal for a zoning variance, so we decided to see what Google had to say about how this works…

Clearly, the facts support local experience. Cutting down a tree will absolutely increase runoff, especially if the removed tree was large, in good condition, with summertime full-sized leaves or evergreen needles. After the tree is removed, more runoff can mean more basement flooding.

How Do Trees Prevent Runoff?

Trees create a canopy of leaves, supported by a root system that together can intercept rainfall and reduce runoff. The process works in a number of ways:

  • Leaves and bark collect rain.
  • The collected rain drips off leaves slowly, decreasing the intensity of rainfall below the tree.
  • Some of the collected rain evaporates.
  • Rain is funneled along twigs, branches, and down the trunk, where it slowly infiltrates the ground.
  • Fallen leaves can help retain moisture below the tree.
  • Tree roots absorb water.
  • The absorbed water sustains the tree or evaporates off leaves.
Your hardworking backyard tree (source: Arbor Day Foundation).

Your hardworking backyard tree (source: Arbor Day Foundation).

How Much Do Trees Reduce Runoff?

Finding exact metrics on runoff reduction is difficult, but some estimates are available.

For example, a mature deciduous tree may intercept 500 to 700 gallons of water per year, an evergreen as much as 4,000 gallons
per year, according to research cited in a Virginia Tech bulletin.

What About Wilmette Trees?

Luckily, the Village of Wilmette values trees, and claims to maintain over 17, 500 parkway trees. The village also supports maintenance of existing trees on private property. For example, property owners must file a tree removal permit to get permission before chopping down any tree over 10 inches in diameter.

But I wonder how many trees are removed without permits, and how many developers and property owners get variances to remove unwanted trees?


Links

A good overview of runoff issues and landscaping solutions:
Freeborn, John (2011, July 5) Urban Water Quality Management, Residential Stormwater: Put It in Its Place: Decreasing Runoff and Increasing Stormwater Infiltration.

Latest research on benefits of urban trees:
US Forest Service, Pacific Southwest Research Station Urban Ecosystems and Social Dynamics Program.

Comprehensive slideshow on using trees to manage urban storm runoff:
Using Trees To Reduce Stormwater Runoff

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