Stormwater Library

resources for stormwater professionals

Archive for Urban Forestry

Developing a Stormwater Credit System for Using Trees

The following four municipalities have developed a stormwater credit system for developers who incorporate trees into their stormwater plans:

The issue of quantifying reduction rates can be a difficult one due to the different capacities of tree species. For more information on use of trees as a stormwater BMP, please refer to the recent study on Trees and Structural Soils by the Department of Urban Forestry at Virginia Tech University. Developers and design professionals can also refer to the Green Values Calculator, which allows you to create hypothetical designs using trees and other green interventions and quantifies the results in terms of costs savings and reduction in stormwater discharge.

Over the next several weeks, I will be presenting a series on how the municipalities of Pine Lake, Sacramento, Portland, and Indianapolis each went about changing the local building code to include stormwater credits for trees.

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Using Trees to Manage Stormwater – (Green Street Series)

Trees have an integral part in the green street stormwater strategy. Essentially, they help to collect rainwater in the canopy and then direct that water to the ground – allowing the water to infiltrate and recharge the groundwater table in a controlled manner. Water stored in the canopy is transpired back to the atmosphere.

The key factor impacting the tree’s ability to peform these tasks effectively is inadequate soil volume. Trees often have their root movement restricted within tree box installations. Highly compacted soils from pavement installation also impedes the roots from growing to full potential. The compacted soil acts as a deterent to stormwater infiltration, as well. The net effect is that the trees are smaller with reduced canopies.

Research by the Urban Forestry Department at Virginia Tech focused on the use of structural soils in the pavement surrounding the trees. The structural soil was shown to provide necessary strength for support of the pavement. More importantly, the porosity of the soil allowed for more infiltration, water storage, and provided sufficient soil volume to allow the trees to grow.

A .pdf of the presentation on how trees and structural soils work together to effectively manage stormwater can be found here.