Cities That are Pioneers in Developing Stormwater Credit Systems for Trees

Cities That are Pioneers in Developing Stormwater Credit Systems for Trees

Recognizing the many benefits trees provide, more and more jurisdictions are providing incentives to both preserve existing trees and plant new trees. Examples of such incentives include grants, tax rebates, development incentives, stormwater fee discounts, and stormwater credits. Today I’ll be focusing on stormwater credits for trees, particularly stormwater credits applicable to individual trees on urban sites.

What is a credit? For the sake of clarity, I want to define our terms from the outset. Stone Environmental (2014)defines stormwater credits as follows: “A ‘credit,’ when considering a site-scale development or re-development project, is most commonly applied to the amount of stormwater that an applicant would otherwise be required to treat in exchange for—and to encourage—the use of alternate practices that reduce the volume of runoff generated… The use of stormwater credits can result in cost savings to the applicant, by reducing the size of otherwise required structural stormwater practices.”

A previous blog discussed the challenges involved in quantifying stormwater credits for trees. Today’s article will give examples of how some jurisdictions have addressed these challenges.

Trees provide stormwater benefits primarily through the following mechanisms:

  • Interception
  • Soil storage and infiltration
  • Evapotranspiration
  • Pollutant reduction through adsorption and biological processes

How jurisdictions quantify these benefits in developing stormwater credits varies considerably.

Impervious Surface Reduction Credits

The most common approach is to grant an impervious surface reduction credit: the area of impervious surface that has to be treated on the site is reduced if trees are planted or preserved. Cities that have adapted this approach include Austin TX, Portland OR, Sacramento, San Jose and Santa Clara Valley CA, Indianapolis IN, Seattle WA, Philadelphia PA, and Minneapolis MN.

Several of the cities on this list allow a 100 square foot impervious area reduction for deciduous trees, a 200 square foot reduction for evergreen trees, and a reduction of half the area of the canopy for existing trees. Others, like Austin and Seattle, give much less credit. Seattle, for example, gives a 20 square foot impervious area reduction for deciduous trees, and a 50 square foot reduction for evergreen trees. Impervious surface reduction credits can generally only be applied to the impervious area adjacent the tree. Many also state that total tree credit cannot exceed 25 percent of the total ground level impervious surface area requiring mitigation, though the maximum percentage allowed varies from jurisdiction to jurisdiction. Some make an exception to this for narrow impervious areas, stating that up to 100 percent of narrow impervious areas, such as sidewalks, may be disconnected (e.g. Philadelphia, Indianapolis) by the application of tree credits.

Because trees over impervious surfaces will reduce runoff more than trees over pervious surfaces, most, but not all, specify a maximum distance trees can be from impervious surfaces to get credit. Seattle gives credit to all trees regardless of proximity to impervious surfaces, but gives more credit to trees close to impervious surfaces. Many jurisdictions require specific tree material standards, minimum size requirements, as well as design, siting and planting requirements to be met in order to be eligible for stormwater credit. Most also require species to be selected from an approved species list.

Volume Reduction Credits

Another approach to providing stormwater credits for trees is to give a volume reduction credit. For example, Pine Lake, GA, provides 10 gallons of credit per inch of the diameter at breast height (DBH) for preserving existing trees under 12” DBH, and 20 gallons of credit per inch of DBH for preserving existing trees over 12” DBH. (DBH is tree diameter measured at 4.5 ft. (137 cm) above ground level).

Washington DC provides a volume credit of 20 cubic feet for each preserved tree, and 10 cubic feet for each planted tree. Trees that are planted as part of another best management practice (BMP), like bioretention, also receive 10 cubic feet credit. A minimum soil volume requirement of 1,500 cubic feet of rootable soil volume per tree must be met to be eligible for the credit (1,000 cubic feet per tree where trees are sharing rootable soil volume). Rootable soil volume must be within 3 feet of the surface. Providing adequate volumes of rootable soil is crucial for a tree to be able to attain its mature size, and therefore also crucial to maximize stormwater benefits, as larger trees provide greater stormwater benefits than smaller trees.

Minnesota’s Minimal Impact Design Standards (MIDS) calculator provides still a totally different approach to quantifying stormwater volume runoff credits for trees.  MIDS includes performance goals for new development as well as for redevelopment and linear development. The goal for new development, for example, is:

“For new, nonlinear developments that create more than one acre of new impervious surface on sites without restrictions, stormwater runoff volumes will be controlled and the post-construction runoff volume shall be retained on site for 1.1 inches of runoff from impervious surfaces statewide.”

The MIDS calculator is a tool to coarsely quantify how much BMPs contribute to achieving the performance goals. Trees are given credit for infiltration, interception, and evapotranspiration based on rootable soil volume and tree canopy size. A detailed description of how credits are quantified, as well as an example calculation, can be found here.

The calculator also calculates phosphorus credits for each BMP, including trees, as discussed at the above link. In order to receive full credit, trees must be provided 2 cubic feet of rootable soil per square foot of canopy. Trees with less soil volume still receive credit but the credit it reduced. As seen in the example calculation at the above link, for a red maple with 1000 cubic feet of rootable soil the total credit is the sum of the infiltration, evapotranspiration and interception credits and equals (310 + 28.2 + 2.5) or 340.7 cubic feet. By accounting for the soil volume as well as canopy size, and quantifying infiltration, interception, and evapotranspiration, this approach seems to be perhaps the most applicable to incentivize individual urban tree Stormwater Control Measures (SCMs) of any of the credit systems developed to date.

What does this all mean?

These are just a few examples of the ways that jurisdictions are dealing with the changing landscape of providing stormwater credits for urban trees. None of these approaches are perfect, and modeling will always just be modeling, but it is nonetheless encouraging to see trees starting to be recognized as powerful stornwater control measures in stormwater regulations. If you know of any other approaches that have been used to quantify stormwater credits, please let us know!

References

Stone Environmental. 2014. Tree Credit Systems and Incentives at the Site Scale Draft Report January 6, 2014. Prepared for Urban and Community Forestry, Vermont Department of Forests, Parks and Recreation.

Nathalie Shanstrom is a sustainable landscape architect with The Kestrel Design Group.

Image: starfive

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