Question: My city just started putting in rain gardens. Do rain gardens really save a city money?
Answer: As explained in the last Soils Matter blog post, cities are using green infrastructure techniques like rain gardens and green roofs. They help reduce the amount of water going into the storm water management system under our cities. Cities built storm water management systems to manage rainfall and snowmelt, carrying the water away from the city.
Because they also act as filters, rain gardens can even reduce the amount of pollution getting to natural bodies of water.
Rain gardens are receiving greater attention as an effective approach to reduce storm water runoff. They cost a fraction of the installation and maintenance cost of permanent “best management practices”—such as retention ponds.
A rain garden is simply a planted depression in the landscape. It can be either natural or engineered. Cities place soil in the depressions, and plants that can survive in both wet and dry conditions. Chicago has planted rain gardens along their streets, and even in the middle of parking lots.
For engineered rain gardens, soil selection is important. The soil needs to control the water absorption ability of the rain garden and support the growth of vegetation. The best “recipe” to maximize water absorption is 25-40% sand, approximately 50% compost and 15-25% native soil.
Rain gardens function by retaining and reducing storm water runoff through percolation and evapo-transpiration. Recent studies from Burnsville MN, Haddam CT, Greensboro and Charlotte NC, Wooster OH and College Park MD show that rain gardens receiving runoff from roofs, shopping centers, or parking lots effectively reduce runoff volumes from 42 to 99%. They also reduce the peak flow from 70 to > 96%. One study from College Park MD reported that 18% of the storm events generated zero flow into the storm water system.
The main benefit of using rain gardens to filter and reduce storm water runoff is that they require little maintenance. The plants don’t typically need pruning, mowing, or mulching.
In some urban rain gardens, sediments from the street can build up and require removal. The overall cost to maintain a rain garden equals the salary of maintenance and supervision. However, a single person can easily do all these tasks for several rain gardens. The extent of maintenance required depends on the characteristics of the catchment area but should be minimal such as removing the surface layer of compost and replacing with fresh material.
Rain gardens not only hold vast amounts of water, they are aesthetically pleasing, can clean water and reduce pollution in the surrounding areas. Rain gardens present a unique solution to the stormwater runoff mitigation problem because they offer a cost–effective and efficient option while improving aesthetics and beauty of the landscape.
To learn more about green infrastructure, including rain gardens, visit https://www.soils.org/discover-soils/soils-in-the-city/green-infrastructure.
-Answered by Kuldip Kumar and Lakhwinder Hundal, Metropolitan Water Reclamation District of Greater Chicago
To view SSSA’s Soils Support Urban Life video, visit: https://www.youtube.com/watch?v=vkJ7H9DMEX4
More educational materials can be found on various SSSA websites:
http://soils4teachers.org/ (K-12 Lesson Plans and Activities)
http://soils4kids.org (Just for kids!)
http://soils.org/iys (International Year of Soils, with a coloring book and monthly ideas for teachers and scientists!)
Subscribe to SSSA’s Soils Matter blog posts to get monthly answers to common soils-related questions: https://soilsmatter.wordpress.com/
Become a Friend of Soil Science (no charge) at: https://www.soils.org/membership/friends-of-soil-science/
Dig in further with a free trial membership at https://www.soils.org/membership/become-a-member/trial/
Further Reading For Runoff Volume Reduction Quantification Case Studies:
Barr Engineering Company. Burnsville Stormwater Retrofit Study: Prepared for City of Burnsville. Minneapolis, MN. June 2006.
Davis, A.P. 2008. Field performance of bioretention: hydrology impacts. Journal of Hydrologic Engineering, 13, 90-95.
Dietz, M.E., and J.C. Clausen. 2005. A field evaluation of rain garden flow and pollutant treatment. Water, Air, and Soil Pollution, 167, 123-138.
Dietz, M.E., and J.C. Clausen. 2006. Saturation to improve pollutant retention in a rain garden. Environmental Science and Technology, 40, 1335-1340.
Hunt, W.F., A.R. Jarrett, J.T. Smith, and L.J. Sharkey. 2006. Evaluating bioretention hydrology and nutrient removal at three field sites in North Carolina. Journal of Irrigation and Drainage Engineering, 132, 600-608.
Hunt, W.F., J.T. Smith, S,J, Jadlocki, J.M. Hathaway, and P.R. Eubanks. 2008. Pollutant removal and peak flow mitigation by a bioretention cell in urban Charlotte, NC. Journal of Environmental Engineering, 134, 403-408.
Yang, H., D.C. Florence, E.L. McCoy, W.A. Dick, and P.S. Grewall. 2009. Design and hydraulic characteristics of a field-scale bi-phasic bioretention rain garden system for storm water management. Water Science and Technology, 59, 1863-1872. 64