Kristen Balschunat
Groundwater Hydrology Final Paper
12/3/13
Rainwater Gardens for Urban Stormwater Quality Improvement
INTRODUCTION
Rainwater gardens are a non-point source urban groundwater quality
improvement strategy that filter urban contaminants from surface water runoff and
decrease the flashiness of floods during minor rainstorm events. This paper reviews
current research on rainwater gardens, analyzes their effectiveness and discusses the
community-wide impacts of grassroots conservation efforts like rainwater garden
installation campaigns.
HISTORY
Early water management strategies in the US focused mainly on flood prevention
and peakflow mitigation and efforts to prevent pollution focused mostly on point source
pollution, like industries disposing of waste (Roy-Porier, 2010). In 1996 the EPA
recognized the need to also focus on water quality and the prevention of non-point source
pollution, stating, “Stormwater runoff from urbanized land is the leading cause of
impairment to lakes and estuaries in the US” (in Davis, 2008). Runoff from urban areas
contains higher levels of heavy metals, sediments and organic compounds than runoff
from less developed areas since urban areas have more impermeable surfaces like
rooftops, driveways and roads, over which water quickly runs over, picking up
contaminants and decreasing the lag time between the start of a storm event and peakflow
(cite).
Low impact development (LID) is a stormwater management strategy that reduces
non-point source pollution through techniques like permeable pavements, rainwater
collection barrels, green roofs, cut curbs, bioretention basins and rainwater gardens
(Hood, 2006, Chang, 2010). According to Roy-Porier (2010), “Bioretention systems
consist of small areas which are excavated and backfilled with a mixture of highpermeability soil and organic matter designed to maximize infiltration and vegetative
growth and are covered with native terrestrial vegetation.” Rainwater gardens are usually
smaller than bioretention basins, but employ the same principles. Runoff is directed from
a rooftop into a sunken garden that is filled with gravel, sand, organic material and native
plants so that it can slowly percolate into the groundwater system and be filtered of
contaminants by the plants and sediments. Rainwater gardens in residential areas should
be built at least 10 feet away from the house and away from any underground
infrastructure like pipes. Rainwater gardens are usually built into a slightly sloped hill, so
the ground is leveled and then dug 6-8 inches deep to collect water. Small-scale rainwater
gardens sometimes have a gravel or sand layer below the plants, but it is often
unnecessary (Native Plant Society, 2005). Larger bioretention basins always have
additional filtration layers below the soil. Determining the plants to put in the garden
depends on the local soil, sun availability and types of native flora. Plants that thrive in
wetter environments are recommended. A rainwater garden can be a beautiful
landscaping feature as well as a tool for water quality conservation. It can even increase
biodiversity in urban areas and promote native vegetation (Kazemi & Beechman, 2010).
RESEARCH
Much research has been done in the last 5 years to evaluate the effectiveness of
rainwater gardens and bioretention basins in the field and on a larger landscape scale.
Trowsdale and Simcock (2011), found that a bioretention pond build next to a busy road
smoothed out storm hydrographs in all 12 cases of rainfall events that took place over the
study period. It effectively filtered TSS and zinc and was most effective during small
rainstorm events during which water had more time to infiltrate. Davis (2008) also found
that bioretention systems worked best during small rainstorm events. Endreny and Collins
(2008) used a model to determine the long-term impacts of rainwater gardens and found
that over time groundwater mounding can interfere with subsurface infrastructure like
sewers, especially if they are not properly planned. James (2012) used GIS to model the
impact of bioretention basins on a watershed scale instead of just a single site. The author
determined that although bioretention basins do not decrease annual outflow volumes
significantly, they do reduce peak flow volumes for single storms, sometimes even lower
than predevelopment conditions and should be part of an integrated strategy for water
management. The author suggested creating bioretention basins and rainwater gardens in
only the most impermeable urban areas to concentrate and save resources. Hunt (2012)
found that, over time, rainwater gardens effectively change runoff to ET, filter
phosphorus and nitrogen and reduce the impact of heat pollution through infiltration.
CHALLENGES & CRITICISMS
Roy-Porier (2010) identified many challenges associated with rainwater gardens.
First, it is difficult to motivate homeowners to invest in the creation of rainwater gardens
and to properly maintain them. There can also be issues with construction methods,
especially if there is too much compaction. Different garden sizing regulations exist in
each state, so properly designing the garden can be a challenge. Also, concerns exist
about the accumulation of oils, heavy metals and other contaminants in the plants and
sediments in a rainwater garden. In highly polluted areas, the plants and soils may need to
be periodically removed and properly disposed of. In areas with high TSS, grains can
block infiltration pores over time and decrease the permeability and efficiency of a
rainwater garden.
COMMUNITY BENEFITS
Perhaps the greatest benefit of promoting rainwater gardens is engaging
community members in the conservation of their local water quality. Building a rainwater
garden involves developing watershed awareness, understanding local soils and climates,
researching native plant species and collaborating with neighbors and experts to put the
project into place. It also gives people a sense of pride in their local environment. As one
publication asserts, “You are about to embark on a journey that can improve the future of
your state!” (Chesapeake Bay Foundation, 2003). Other publications describe how to
plan and build rainwater gardens in New Jersey (Native Plant Society, 2005), Wisconsin
(Bannerman, 2003) and Washington (Hinman, 2003). These publications tend to over
exaggerate the benefits of rainwater gardens on regional hydrology, but also provide
accessible knowledge for the public about hydrologic systems and the role that
individuals play in maintaining overall water quality.
CONCLUSION
In conclusion, rainwater gardens cannot solve all urban water quality and
peakflow issues, but they should be part of a multi-faceted LID water management
strategy in urban areas.
BIBLIOGRAPHY
Bannerman, R., & Considine, E. (2003). Rain gardens: A how-to manual for
homeowners. University of Wisconsin Extension. Retrieved from:
http://learningstore.uwex.edu
Chang. N. (2010) Editorial: Hydrological conncetions between low-impact development,
watershed best management practices and sustainable development. Journal of
Hydrologic Engineering, June 2010, 384-385.
Chesapeke Bay Foundation. (2003). Student BaySavers Projects: Build Your Own Rain
Garden.
Davis, A. P. (2008). Field performance of bioretention: Hydrology impacts. Journal of
Hydrologic Engineering, 13(2), 90-95.
Endreny, T., & Collins, V. (2009). Implications of bioretention basin spatial
arrangements on stormwater recharge and groundwater mounding. Ecological
Engineering, 35, 670-677.
Hinman, C. et. al. (2013) Rain garden handbook for western Washington: A guide for
design, installation and maintenance. Washington State University Extension.
Retrieved from: https://fortress.wa.gov
Hood, M., Clausen, J.C., & Warner, G.S. (2006). Low impact development works!
Journal of Soil and Water Conservation, 61(2), 58a-61a.
Hunt, W.F., & Davis, A.P., Traver, R.G. (2012). Case study: Meeting hydrologic and
water quality goals through targeted bioretention design. Journal of
Environmental Engineering, 138(6), 698-707.
The Native Plant Society of New Jersey. (2005). Rain garden manual for New Jersey.
Retrieved from: www.npsnj.org
James, M. B., & Dymond, R. L. (2012). Bioretention hydrologic performance in an urban
stormwater network. Journal of Hydrologic Engineering, 17(3), 431-436.
Kazemi, F., Simon, B., & Gibbs, J. (2011). Streetscape biodiversity and the role of
bioretention swales in an Australian urban environment. Landscape and Urban
Planning, 101, 139-148.
Roy-Porier, A., Champagne, P., & Filion, Y. (2010). Review of bioretention systems
research and design: Past, present and future. Journal of Environmental
Engineering, 136 (9), 878-889.
Trowsdale, S.A., & Simcock, R. (2011). Urban stormwater treatment using bioretention.
Journal of Hydrology, 397, 167-174.