Research Paper
Demonstration
Rain Gardens: Their
Influence and Value
O. Jackson
Cascadia
College
Abstract
Rain gardens are an example of
green infrastructure. They help reduce the need to replace expensive stormwater
infrastructure by slowing and filtering runoff. Rain gardens absorb pollutants as
well as reduce flooding. The proposed site for a new rain garden is the Cascadia
College campus, which is sited near a restored wetland. The campus is deeply
involved with measuring and managing polluted stormwater inflows from
impervious surfaces such as roads and rooftops, which often carry nutrients,
bacteria, and car exhaust pollution downstream into bodies of water used by
species such as salmon. The sensitive ecology and the many people who visit the
campus make Cascadia College a good location for a demonstration rain garden. An
attractive rain garden could inspire the public to use green infrastructure on
their own properties. Widespread adoption of bioremediation methods would reduce
the impact of urban development on waterways. In order to be an effective
educational tool, the rain garden must be prominently sited and include
interpretive signage so that passers-by will learn what rain gardens are and
how they function.
Keywords: impervious surface, green
infrastructure, rain garden, bioremediation, wetland
Demonstration
Rain Gardens:
Their
Influence and Value
Research has shown
that people are most motivated to construct their own rain gardens when
provided with financial incentives by local government agencies (Newburn &
Alberini, 2016). But that research reveals little about the way viewing
attractive demonstration rain gardens might inspire passers-by to install
biological stormwater solutions on their own properties. Until we understand
the influence of modeling a socially desirable behavior on the choices of
homeowners regarding making property improvements, we can't know whether there
are people who would invest in their own rain gardens if they had a chance to
see, learn about and examine a successful one. This paper examines the benefits
and challenges of rain garden installation in an educational environment. It
will describe the ways that building a demonstration rain garden at Cascadia
College could increase the likelihood that passers-by will take an interest in
creating a rain garden of their own, particularly if the garden is
prominently-sited, attractive, and provides educational signage.
Engaging the Public
Aesthetics
Surveys reveal
that Washington residents who already have a rain garden were most motivated to
install it to reduce water pollution, but only weakly motivated by the desire
to beautify a landscape (Bertolotto, 2018, p. 3). Logically, this should imply
that in order for a demonstration rain garden to influence passers-by it is
most important that information be provided about its role in pollution
abatement. Filtering stormwater by promoting infiltration, evaporation, and
evapotranspiration is, after all, the key function of a rain garden (Jennings,
Berger & Hale, 2015). Educational information about the rain garden's
utility seems like an appropriate way to catch the public's interest,
particularly since lack of understanding of these benefits is a major reason
why environmentally-useful behaviors are not adopted by the public (Brown et
al., 2014, p. 80). On the other hand, it seems logical that people will be more
likely to spend time learning about something that is eye-catching and
attractive. Does research support this assumption?
In their analysis
of a survey on public response to rain gardens conducted in 2018, Kim, Lee, Lee
and An concluded that "to successfully apply a rain garden policy in the
future, basic consideration should be given to aesthetics in order to meet
visitors’ visual expectations prior to education and publicity of rain gardens"
(p. 1). As this study points out, the support of the community is crucial if
rain gardens are to be adopted successfully (p. 3). The survey revealed that
when respondents rated a rain garden chosen for the study as visually attractive,
they were more likely to be in favor of rain garden implementation generally. On
the other hand, when they did not support rain garden implementation as a
policy, they also evaluated the demonstration rain garden as aesthetically
unappealing or poorly-maintained (p. 5). This finding, though based on research
conducted in Korea, is confirmed by community reactions to controversial rain
garden implementations in Australia, where foliage becomes dry and unsightly in
the summer months (Leonard et al., 2014, p. 55) and the United States, where
rain gardens capture urban trash (Nir, 2017). This shows that aesthetic
considerations are crucial to draw not just attention to, but also public
support for – rather than rejection of – highly visible rain garden projects.
Aesthetics is
important in teaching people about ecology for another reason: engaging with
the natural world through face-to-face "experiential knowing" is one
method to inspire people toward "deep commitment to working for
change" (Reason, 2007, p. 36). Spending time outside in a flourishing rain
garden can open people's senses to a more personal appreciation of the natural
world. For Reason, beauty is a gateway to the sacred experience of natural
rhythms (p. 39). Aesthetic appreciation is therefore likely to be a powerful
motivator to change behavior and could inspire a lasting appreciation for the
ecological services provided by the plants, the rocks and soil, and the water
that compose a healthy rain garden.
Siting for Visibility
Since the primary
purpose of a demonstration rain garden is public education, it is important to
know the factors other than aesthetics that make it most persuasive. According
to Obropta, DiNardo, & Rusciano, a "demonstration rain garden should
be constructed in a highly visible place such as a public library, town hall,
or school. It is also desirable to select a location where public education
programs can be easily held" (2008, p. 1). As they so succinctly point
out, "why should someone expect you to invest in a rain garden without
seeing one?" (p. 1). A rain garden near an institution of higher learning
facilitates educational tours, which can be offered to interested parties by
students of the Bachelor of Applied Science in Sustainable Practices program in
much the same way that tours of Cascadia's restored wetland are offered.
Interpretative Signage
Interpretive
signs are effective at increasing the ecological understanding of visitors to
natural areas, which could include rain gardens. According to Davis and
Thompson, such signs are particularly effective if they combine large print
with images, and if people spend a long time interacting with them (2011, p.
56). They mention that repeat visitors often get something new from signs
they've already glanced at in passing during previous visits. They are viewed
most often when placed at natural rest stops and ignored most often by people
running or bicycling (p.64). This implies that if a rain garden wants people to
get the most from interpretive signage, the rain garden should be placed in a
location where people naturally stand and linger rather than rush on by. If
signage includes images of plants and animals, or three-dimensional interactive
features, they will be even more engaging to passers-by. It was noted that
signage is often the most expensive feature of natural places designed for the
enjoyment of the community (p. 1); for this reason, it makes sense for signage
to be designed to be durable and long-lasting so replacement is seldom
necessary even though this will require a larger initial investment.
Rain Garden Best Practices
Native Plants
Native flowering
foliage recommended for rain gardens in the Pacific Northwest include "Autumn
Brilliance" serviceberry (Amelanchier
x grandiflora), Oregon grape (Mahonia
aquifolium), beach strawberry (Fragaria
chiloensis), “Blizzard” mock orange (Philadelphus
lewisii), Pacific ninebark (Physocarpus
capitatus), redosier dogwood (Cornus
sericea), tufted hair grass (Deschampsia
cespitosa), and daylilies (Hemerocallis
spp.) (Recommended Plants, n.d.). Many of these species are already used in
Cascadia's wetland restoration project. Rushes, sedges and flags are a good
choice for frequently-saturated ground. Native plants are the best choice
because they are so well-adapted to local conditions that they require minimal
irrigation, fertilizing or other maintenance. The use of native species is also
thought to contribute to the biodiversity of the insects that rely on them,
such as pollinators (Chaffin et al., 2016). Plants that tolerate wet soils best
should be placed in the center of the garden.
Siting for Hydrology
Siting needs to
take the type of soil and the flow of water into account so that its filtration
effectiveness can be maximized. A rain garden should be sited to intercept
runoff from streets, roofs, or agricultural areas, and should be placed at
least ten feet from any building's foundation. It should not be sited in an
already-boggy area since its purpose is to absorb and filter excess water.
Impervious soil layers below the site, such as clay, are better avoided (Siwiec, Erlandsen,
& Vennemo, 2018). Many critics of rain gardens are concerned that
they will become dangerous or ugly standing pools of water, so soil percolation
tests are advisable in order to make sure that the quantity of rain from the
area's average storm event will drain within 48 hours (Obropta, DiNardo, &
Rusciano, 2008, p. 1). A good guideline is that water should infiltrate soil at
least one-half inch per hour. Do not place rain gardens over septic systems
(Groundwater Foundation, 2019).
In the Puget Sound
area, peak discharges of stormwater in developed areas are 0.15 cubic feet per
second (cfs) during 10-year floods. This is comparable to the rate in
pre-developed areas. However, this rate jumps sharply to 0.8 cfs during
200-year floods, which is twice the rate of peak discharges in pre-developed
areas. This shows the powerful impact of impervious surfaces such as roofs and
paving on the increase of stormwater runoff. The more the Puget Sound area is
developed, the more surface runoff discharge rates will increase. This is a
reason to build new features that handle stormwater now, since the area has
already experienced problems with Combined Sewer Overflow (CSO) events; without
action, these are only projected to get worse (City of Seattle Stormwater
Manual, 2015, Appendix F). Rain gardens can be sited uphill of locations that
are flood-prone and downhill of impervious surfaces for maximum benefit. If
draining runoff from a specific impervious surface like a roof, the rain garden
should be about 20% of its size (Groundwater Foundation, 2019).
Rain Garden Substrate
It
is important to use the proper rock and soil to both nurture plants and to maximize
filtration. Turk et al. tested three different soil substrates to see which had
the best filtration of different pollutants and to see how the differences
affected plant growth (2014). There was little difference noted between the
types for shoot or root growth, but soil-based substrate, slate-based substrate
and sand-based substrate did have significant differences in nutrient
absorption. Slate retained nitrogen and phosphorus best; soil was poorest for
remediating phosphorus, but about the same as slate for remediating nitrogen;
and sand had good retention of all pollutants except for nitrogen.
A rain garden's
depth should be from three to twelve inches. A rain garden should not be sited
on overly-steep slopes, since water will flow through it so quickly that it
will not have time to be filtered properly. Soil compaction and surface
clogging are factors that can reduce infiltration rates (Jennings, Berger,
& Hale, 2015) to the detriment of the garden's effectiveness. To avoid
compaction, do not drive excavation equipment directly over the rain garden
site. Use the smallest equipment possible. Compacted soils can be broken up by
hand prior to planting, but heavy equipment can compact soil up to a depth of
three feet (Guttman, n.d.), so it is better to be careful from the outset.
It is a good idea
to use washed rocks along the swale so that soil isn't washed into the rain
garden, since siltation would reduce filtration. Make the edges of the rain
garden slope gently so they are not a trip hazard and so they can be planted
more easily. Compost should be used sparingly since it can be a source of
excess nutrients that can be washed out of the garden into bodies of water,
thus to some extent defeating the purpose of the installation. Coarse mulch
should be applied thickly between plants and along the edges of the garden;
make sure it is free of weed seeds and debris (Guttman, n.d.).
Conclusion
Cascadia College
has the potential to teach ecological principles to both students and the
community through practical projects and immersive experiences. Though it
already has rain gardens and bioswales to its credit, many of these are
visually subtle or tucked away and unobtrusive. A demonstration rain garden
with an interpretative sign and high aesthetic value could catch the attention
of the many community members who use the area for running and bicycling when
they stop for a break. Students will learn more about how biological stormwater
features work. Local residents are more likely to build their own rain gardens
if they have a positive experience with a bioremediation project that has
already been installed. Cascadia College has already been awarded a grant
dedicated to stormwater projects; using some of these funds to build a rain
garden would be an appropriate way to utilize this opportunity. A new rain
garden would educate the community, help remediate stormwater flowing into the
wetland restoration site, and give students a chance to appreciate the
dedication of Cascadia College to ecologically progressive projects.
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