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Treating urban stormwater

Some sustainable stormwater management approaches have the potential to actually treat the water to remove pollutants as well as control its volume and rate of discharge. These strategies include constructing wetlands and planting trees. Wetlands have proven to be very effective in both temporarily storing stormwater runoff and reducing flooding risks, while also reducing the pollutant load carried to the wetland (because of its high biological activity that can capture and degrade the contaminants). As a result, the federal government has adopted a "no net loss" policy with respect to protecting existing wetlands. Section 404 of the federal Clean Water Act requires that the U.S. Army Corps of Engineers (under U.S. EPA oversight) review any proposals to fill or damage any wetlands that are directly hydrologically associated with navigable waterways. Any actions affecting existing wetlands will need a Corps 404 permit in addition to any local or state approvals.

Besides preserving existing wetlands, new wetlands can also be designed, created and maintained as part of a "green" stormwater management strategy ( NRC, 2008 ). The constructed wetland can be designed and used to intercept, temporarily store and treat stormwater runoff before it is released to a stream or river. Water control structures are also usually installed to ensure that the constructed wetlands remain flooded for long enough periods of time to support wetland vegetation. If appropriate plants are selected, they can also provide important habitats. Wetland maintenance involves the control of invasive plant species (e.g. Purple Loosestrife) and the management of any sediment that can be carried by stormwater runoff into the wetland, since the sedimentation of wetlands can fill them in, impairing their ecological and treatment functions.

The planting of trees is an especially valuable strategy to manage urban stormwater, especially when the trees become mature. Tree canopies break rain velocity, reducing runoff flow rates, while tree roots can stabilize soils against being eroded by urban runoff. Tree canopies reduce temperatures, mitigating urban heat island effects, by providing shade and through their transpiration processes. Their leaves and roots can also capture some stormwater contaminants and provide carbon sequestration to reduce climate change impacts. Moreover, trees provide a valuable soil amendment as their fallen leaves decay into mulch, improving the infiltration rate and biological activity of surrounding soils, while larger broken branches falling into urban streams can slow stream velocities and provide improved riparian and aquatic habitat. The shading of streams by riparian trees is particularly important in ensuring that a stream's ecological functions remain resilient in the face of rising temperatures caused by global climate change.

Conclusions

All of the green infrastructure and Low Impact Development techniques that provide interim on-site stormwater storage to reduce flood risks can also provide some pollution removal capabilities, as well. The American Society of Civil Engineers and U.S. EPA maintain an International Stormwater BMP Database of development projects using green infrastructure. This on-line resource reviews the effectiveness of various stormwater management practices and makes these sustainable techniques more accessible to local officials and municipal public works departments charged with managing stormwater runoff in their communities.

There is increasing public interest in using sustainable stormwater management techniques to replace or supplement conventional stormwater facilities. The U.S. federal government, for example, is now requiring that green infrastructure be used in all federal projects above a certain size to manage urban stormwater runoff. Local officials are also showing a greater interest in these sustainable approaches, since they are often less expensive to install and maintain over their life-spans than conventional stormwater sewer systems and detention facilities. Finally, state governments are beginning to set aside money in their revolving loan funds for public infrastructure that is earmarked for green infrastructure projects. It is likely that this interest in sustainable urban stormwater management will continue to grow.

Review questions

Which of the sustainable urban stormwater management practices can best be used in existing neighborhoods, and which are best suited for new development?

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The performance of many of the green infrastructure practices often depends on how well they are maintained over their life-spans. What are some effective strategies that local officials can consider in order to ensure that the green infrastructure being used to manage urban stormwater in their communities is adequately maintained and continues to perform as designed?

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Resources

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References

Gulliver, G.S.&Anderson, J.L. (eds.). (2008). Assessment of Stormwater Best Management Practices. Stormwater Management Practice Assessment Study . Minneapolis: University of Minnesota.

Jaffe, M., Zellner, M., Minor, E., Gonzalez-Meler, M., Cotner, L., Massey, D., Ahmed, H., Elbert M., Wise, S., Sprague, H.,&Miller, B. (2010). Using Green Infrastructure to Manage Urban Stormwater Quality: A Review of Selected Practices and State Programs . Springfield, IL: Illinois Environmental Protection Agency. Retrieved June 23, 2011 from (External Link)

National Research Council. (2008). Urban Stormwater Management in the United States . Washington, DC: National Academies Press. Retrieved June 23, 2011 from (External Link)

U.S. Environmental Protection Agency. (2000, October). Street Storage for Combined Sewer Surcharge Control: Skokie and Wilmette, Illinois (Factsheet). (EPA Publication No. EPA-841-B-00-005C). Washington, D.C. Retrieved May 17, 2011 from (External Link)

U.S. Environmental Protection Agency. (2007, December). Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices . (EPA Publication No. EPA 841-F-07-006). Washington. D.C. Retrieved June 23, 2011 from (External Link)

U.S. Global Climate Change Research Program (USGCCRP). 2009. Global Climate Change Impacts in the United States . Cambridge: Cambridge University Press. Retrieved May 18, 2011 from (External Link)

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Source:  OpenStax, Sustainability: a comprehensive foundation. OpenStax CNX. Nov 11, 2013 Download for free at http://legacy.cnx.org/content/col11325/1.43
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