The Right Size for a Rain Garden
Author: Sarah Pounders
Overview
 Turf
and paved areas shed water at different rates. Measure how much of each your drainage area contains to determine how large your rain garden must be.
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This is one of two lessons we've developed to guide
students through the process of rain garden design (the other is
The
Right Spot for a Rain Garden).
For full background information about the purpose
of
rain gardens
and
basic
design principles,
please
read
Rain
Gardens to the Rescue.
Objective: Students will use math skills to determine
how large their rain garden must be to effectively
manage
stormwater
runoff
at a
chosen
location.
Standards (Microsoft
Word document)
Materials:
Measuring tape
Graph paper
Pencils
Calculators
Background
The size of a rain garden determines how much runoff it can collect
and absorb within 2 to 4 days (standing water must be absorbed quickly
to discourage mosquito breeding). Experts have devised formulas to
help estimate runoff rate and the size a rain garden must be in order
to handle it. This lesson takes students through this exercise. Please
note that there may be other constraints that limit the ultimate size
of your rain garden.
Laying the Groundwork
1. Introduce students to rain gardens and their
benefits. (See Rain
Gardens to the Rescue for this information.) To inspire thought
about stormwater runoff, ask students to consider the stage of the
water cycle where raindrops fall onto the earth’s surface, especially
in our built environment: buildings, roads, parking lots, and drainage
systems.
Ask students to recall where they’ve seen water on rainy days –
pooling on pavement or compacted soil, coursing down gutters, sheeting
off the eaves, gushing out of a culvert into an eroded gully, or
maybe dripping softly from the leaves of a tree onto vegetation below.
They might even go for a rainy day walk and make direct observations
of how water behaves around roads and buildings versus trees and
turf. Just be sure to observe high-volume runoff from a safe location.
Exploration
There are a number of different formulas for estimating
the size of garden necessary to manage your stormwater volume. This one
is from the Virginia Department of Forestry's Rain
Garden Technical Guide.
1. A rain garden should be about 7 percent of the size of the
surface area that will produce stormwater runoff. Begin by measuring
all surfaces covered with vegetation (grass, gardens, etc.) that
will produce runoff and add them together.
2. Next measure all impervious surfaces (roofs, sidewalks, driveways,
roadways, etc.) that will produce runoff and add them together.
Keep these numbers separate — impervious surfaces produce
more runoff than pervious surfaces (those that absorb moisture).
Multiply
them by an estimated runoff value:
3. Multiply the surface area measurements
by the runoff value, and then by .07 (since a rain garden should
be 7 percent
of total area producing runoff):
- Impervious surface area total x 0.9 x 0.07 = size of rain
garden needed to manage runoff from impervious surfaces
- Turf/vegetative
area total x 0.25 x 0.07 = size of rain
garden
needed to manage runoff from turf areas
4. Add these two numbers together for the total recommended size
of your rain garden.
Here is a full example from the Rain
Garden Technical Guide:
Step 1. Calculate the square footage of the
impervious surfaces.
roof - 50 ft. x 50 ft. = 2500 sq. ft.
driveway - 12 ft. x 20 ft. = 240 sq. ft.
concrete Patio - 15 ft. x 12 ft. = 180 sq. ft.
total impervious surface = 2920 sq. ft. = runoff surface area Step 2. Calculate the square footage of the pervious surfaces.
lawn area 50 ft. x 30 ft. = 1500 sq. ft.
Step 3. Multiply the square footage obtained in steps
1 and 2 by the appropriate runoff coefficient, and by the
7 percent of runoff.
2920 sq. ft. (impervious surfaces) x 0.07 (percent of runoff) x 0.9 (runoff
value) = 183.96 sq. ft.
1500 sq. ft. (lawn area) x 0.07 (percent of drainage) x 0.25 (runoff
value) = 26.25 sq. ft.
Step 4. Add both impervious and turf areas together to
get total rain garden size.
183.96 sq. ft. + 26.25 sq. ft. = 210.21 sq. ft.
In this example, you'll need to install a rain garden
that is 210.21 sq. ft. in order to accommodate 100 percent
of the runoff from the property.
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5. Keep in mind that this formula provides only an estimate, and
that other experts may recommend the garden be a different percentage
of total surface area. There are also other factors that can affect
the ultimate size. One is soil type (e.g., with sandy soil your
garden can be smaller because it will absorb the water more quickly;
clay soil will drain more slowly, so you’ll need to expand area).
In addition, you’ll need to consider the amount of rain your area
typically receives during a rain event – a lot of rain in a short
time can flood and overflow a small rain garden.
Making Connections
-
Use different formulas to calculate the size of your rain garden
and compare the results. You can find other formulas at:
Rain
Gardens: A How-to Manual for Homeowners (PDF)
-
Build a rain garden
model using a 2-liter bottle to demonstrate the filtering process.
Instructions can be found on the
Rodale's
Kidsregen.org Web site
-
Visit a local water treatment facility to learn
more about the sewer and storm drains in your community. If
a field trip isn't
possible, invite a staff member from the facility to make a
presentation. Have students prepare questions for their guide/visitor
related to how gardens and greenspace affect water
quality.
Branching Out
- Learn about other ways to protect water resources in
your area. To get started, visit the EPA’s
water resources site for kids.
- Research appropriate rain garden plants for your area.
Native plants adapted to varying amounts of water are
the best choice.
- Search the Web to find out if there’s a rain garden
group in your area or state, and see if they can offer
advice, resources…or even a grant!
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