The Right Spot for a Rain Garden
Author: Sarah Pounders
Overview
 Students
can make "mudshakes" to explore soil texture
and drainage capacity.
|
The first step in rain garden design is scouting an appropriate site.
This activity challenges students to explore one aspect of this process:
soil drainage assessment. (For full background
information about the purpose
of
rain gardens
and
basic
design principles,
please
read
Rain
Gardens to the Rescue.)
Objective: Students will explore soil drainage
to determine if a site is appropriate for installation of a rain garden.
Standards (Microsoft
Word document)
Materials:
Shovel
Water
Small amount of laundry detergent
Plastic containers - one for each potential rain garden site
Background
Rain gardens are created to attract, absorb, and filter storm runoff.
It’s important for the garden to contain well-drained
soil so that
water dissipates within two to four days. If water sits for too long,
plant roots will suffocate and mosquitoes may breed there.
By definition, soil is made up of sand, silt, and clay particles derived
from rock broken down over thousands of years by climatic and environmental
conditions (rain, glaciers, wind, rivers, animals, etc). Defining
soil components as sand, silt, or clay is about sizing the particles,
not determining
their original source. So, how do they size up?
-
Sand particles are between 2.00 - 0.05 mm in diameter
and feel gritty in your fingers.
-
Silt particles are between 0.05 - 0.002 mm and feel
similar to flour.
-
Clay particles are particles smaller than 0.002
mm and feel sticky in your fingers when wet, and clump to the point
that
you can’t see
an individual particle without a microscope.
Soil also contains nutrients, organic matter/compost (decaying plant
and animal material), and pore space (open space that holds air and
water). The amount of each of these components characterizes your soil.
For instance, if you have a lot of sand, your soil will drain quickly;
a lot of organic matter helps boost moisture retention; and if you
have a lot of clay particles, your soil can easily become compacted
and may drain poorly.
The “recipe” for an ideal rain garden
soil:
Although you can amend the soil for a rain garden if your site’s
soil doesn’t match this ideal composition, experts recommend avoiding
locations with extremely poor drainage for maximum efficiency and success.
Laying the Groundwork
1. Introduce students to rain gardens and their benefits. (See Rain
Gardens to the Rescue for this information.) Start a class discussion
by asking, Should soil drainage -- how quickly moisture is absorbed
by the soil -- influence where we locate our rain garden?
2. Take students out to the schoolyard and identify a few potential
sites for your rain garden. Dig soil samples at each location. Allow
students to handle the soil from the hole as you explain the differences
among sand, silt, and clay. Ask, Can feel that there are
particles of
different sizes?
3. Next, explore each sample's
components
by
creating "mudshakes" and watching the different components
settle out.
-
For
each soil sample, have students fill a clear
plastic container about two-thirds full of water, then add enough
soil to nearly
fill it to the top. Add a pinch of laundry detergent to help
the
soil components separate well.
-
Shake each container vigorously
and observe them over the next couple of days as the particles
settle into layers. Ask students to hypothesize about the composition
of
the
different layers. Based on the Which are heaviest? (gravel
and sand). Which, followed
by silt, with
the last
full layer being clay. The clay may stay suspended and cloud
the water
for a long time. Organic matter will float on or just below
the water surface.
-
Measure the height of each layer and then
translate that into percentages for each component (height
of each component divided
by height of the sample).
-
Ask students to explore the qualities
of the various layers. Allow them to observe and feel the differences
among clay,
silt, and sand.
-
Based on these explorations, ask, How do you
think the various elements affect drainage?
-
Then, based
on the percentages they recorded, have them predict how well
the site where the sample was collected
is likely
to drain. The next step allows them to test their predictions.
Exploration
Test the drainage of the soil in each of the potential rain
garden locations.
1. Dig a hole 6 inches wide and 18 inches deep in each location
2. F fill each hole with water and measure depth with a ruler.
3. Check water depth every hour and record results.
- If all the
water drains within a few hours, the site has excellent
drainage.
- If the water drains within 24 hours, then the
site is
acceptable.
- If the water doesn’t drain within 48 to 72 hours,
then you will either need to remove or amend the existing
soil or
choose a
different location.
Making Connections
1. Use the data collected to determine water absorption rate in
inches per hour for each potential rain garden site. (Use the next
lesson, The
Right Size for a Rain Garden, to decide if this is adequate
for
the volume of water you expect the garden to receive.)
2. Compare the results of the drainage experiment to the soil
texture characteristics discovered through the "mudshakes." Do
you see any relationships between the texture and drainage? Do
the results match your predictions?
3. Lead a class discussion on which site is the best for your
rain garden. Remember to introduce factors other than drainage:
landscape appeal, convenience, and so on. For instance, a site
with good drainage may be better than one with excellent drainage
if it is more protected from foot traffic or more convenient
for classroom observations and experiments.
Branching Out
- Continue the rain garden design process for
your chosen location. The Virginia Department
of Forestry's Rain
Garden Guide offers for step-by-step instructions.
(You may find rain garden design information specifically for
your state or region by searching the Internet.)
- Ask students
to create educational brochures about rain gardens to distribute
to parents and community members.
- Bring in magazine and newspaper
articles focused on water quality or supply. Ask students
to evaluate and debate current
issues related to water.
- Lead students in researching the
Clean
Water Act.
Discuss the role of government in protecting natural resources.
Also, discuss what role citizens, including themselves, play
in the bigger picture.
- Read about the history of the development
of rain gardens at the Environmental
Protection Agency's Web site.
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© 2009
National Gardening Association