Trees and Stormwater Management
Trees and the Clean Water Act
The EPA’s Clean Water Act mandates that certain communities apply for a stormwater discharge permit under the National Pollutant Discharge Elimination System. While Phase I and Phase II requirements add a new challenge in reducing polluted stormwater runoff from the nation’s waterways, the EPA also allows communities to use trees as part of the solution. Cities are turning to CITYgreen to help model the effects of planting programs and tree ordinances. CITYgreen analyses increasingly help guide similar long-term planning efforts for large urban areas.
What’s in the Stormwater Management Plan
All affected Phase II municipalities were required to file a stormwater management plan in 2003. The plans incorporated six required control measures as outlined in Phase II, including construction-site runoff control, post-construction runoff control, pollution prevention, public education and outreach, public participation and involvement, and illegal discharge detection and elimination. The plans included a narrative for measurable goals for each of the control measures.
More Trees Means Less Stormwater Runoff
Trees function as nonstructural stormwater management facilities. Here’s how they work and why they should be part of any stormwater management plan:
Trees slow stormwater flow, reducing the volume of water that must be managed in urban areas and decreasing the amount of runoff that containment facilities must store.
Trees intercept rainwater on leaves, branches, and trunks, slowing its movement into channelized drainage areas.
Stormwater volume is diminished when some intercepted water evaporates into the atmosphere and some soaks into the soil. The net reduction in total volume and peak flow lessens the potential for flooding, a critical concern during heavy rains.
Trees provide their greatest benefit during light rains by increasing soil permeability, which facilitates groundwater recharge. Reducing impervious surfaces and increasing tree cover promotes the movement of water into the water table.
Long-term studies document trees’ ability to reduce the movement of stormwater and cut peak flow rates that cause flooding and overtax storm sewers. The Natural Resources Conservation Service has measured the effects of stormwater movement across various landcovers over the last 50 years. Based on these studies, engineers have developed predictive models that calculate the volume of water produced from a given rainstorm and landcover (TR-55: Urban Hydrology for Small Watersheds). Stormwater management facilities’ construction costs are calculated based on these models.
Urban areas could reduce their stormwater runoff and save millions of dollars by increasing their tree cover. In Fayetteville, AK, increasing tree canopy from 27% to 40% would reduce stormwater runoff by 31%, a value estimated to be worth an additional $43 million in capital improvement savings (represents $2-per-cubic-foot cost to contain stormwater runoff).
What You Can Do
Include these provisions in your community’s stormwater management plan:
Incorporate trees into planning and management policies and programs.
Set a tree canopy target goal.
Measure your community’s tree canopy’s effect on stormwater runoff.
Tree Cover as a Measurable Goal
Hundreds of communities have tree ordinances and tree programs in place to conserve and improve their urban forest and other natural resources. The intent of many existing ordinances is to maintain and improve forest cover through provisions that ensure a specified density or number of trees on a per-acre basis. However, this density requirement is subjective, and it would be difficult to quantify the ordinance’s benefits. Now that green infrastructure can be measured with computerized technology and aerial and satellite imagery, communities can measure the value of their urban forests for stormwater benefits. The best way to ensure urban forest protection is to establish tree-cover goals. This measurable and defensible approach can be used in the development of stormwater management plans.
Setting Tree Canopy Target Goals
The average urban tree cover for the 20 states in the northeastern quarter of the country is 30%. Setting a tree-canopy goal will help protect a community’s green infrastructure and maximize the environmental benefits it can provide. American Forests offers generic tree-canopy guidelines for specific zoning categories. While these percentages are a good starting point, they should be used only as a guide. Each community must consider local climate and other ecological conditions in setting its own targets.
For metropolitan areas east of the Mississippi and in the Pacific Northwest:
Average tree cover for all land-use zones 40%
Suburban residential zones 50%
Urban residential zones 25%
Central business districts 15%
For metropolitan areas in the Southwest and dry West:
Average tree cover for all land-use zones 25%
Suburban residential zones 35%
Urban residential zones 18%
Central business districts 9%
How to Develop a Stormwater Plan With Trees
Once plans are filed, communities have up to five years to detail those plans for funding, identify measurable benefits, and develop guidelines and other regulations. Technical expertise will be especially helpful in developing measurable goals, conducting stormwater controls monitoring, and measuring success, as well as assisting with training, developing educational materials, and conducting public education programs for citizens and local officials.
For further information:US Environmental Protection Agency (EPA)
For information on NPDES Phase II:
For EPA’s regional contacts:
For state stormwater contacts: