How to Build a Rain Garden
Today’s How-To comes
from the Rain Gardens
And Stormwater.
Where to Locate a Rain Garden
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Place that runoff can be collected or piped
Avoid underground utilities
At least 10’ from the foundation of your home
Avoid septic systems and fill lines
Avoid areas that already pond water
Place in area that receives at least partial sun
Gently sloping land reduces excavation efforts
Source of water is handy for establishment
Rain Garden Design Calculations
Percolation (Perc) Test
Hole should be about 12” deep and 6” diameter (post
hole digger size)
Saturate the soil by filling up the hole with water 1-3
times depending on soil moisture
After soil is saturated, fill the hole again and measure the
time it takes for the hole to completely drain.
If the hole drains in less than 12 hours, no amendments
If the hole doesn’t drain within 12-24 hours, amend soil
If the hole doesn’t drain within 24 hours, amend soil
deeper or find a new location
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Calculations
How to Size a Rain Garden?
The amount of water draining to your rain garden
is a result of drainage area. Calculate the
impervious area that will drain to your rain garden.
Rain Garden Design Calculations
How to Size a Rain Garden?
Length (red line) x Width (yellow line) = Area
Since there are two downspouts, we need to divide
the area by two.
The garden should be 10-30% of the impervious area.
30% is ideal
Drainage Area x 0.3 = Ideal Garden Area
Rain Garden Design Calculations
Measure the slope of your proposed location
Rain Garden Design Calculations
Determine the depth of your rain garden
Slope
1-4%
4-8%
8-12%
Ideal ponding depth
4”
6”
8”
Rain Garden Design Calculations
Rain Garden Design Calculations
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Soil Amendments (4” Minimum)
Rain Garden Design Calculations
How much soil amendment do I need?
Soil amendments
depth (ft) x area (ft2) = volume (ft3)
ft3 ÷ 27 = yd3
1/3 Compost
1/3 Coarse Sand
1/3 Topsoil
Rain Garden Design Calculations
Rain Garden Calculation
Example
Rain Garden Design Practice
Impervious Area Calculation
(Length = 140 ft) x (Width = 44 ft) = (AREA = 6,160 ft2)
Rain Garden Design Practice
Impervious Area Calculation
(Roof Area = 6,160 ft2) ÷ 4 (Downspouts) = (Drainage Area = 1,540 ft2)
1,540 ft2
Rain Garden Design Practice
Rain Garden Size Calculation
(Drainage Area = 1,540 ft2) x .30 = 462 ft2
462 ft2
1,540 ft2
Rain Garden Design Practice
Rain Garden Size Calculation
Slope = Rise ÷ Run
8% = 1.5 ft ÷ 20 ft
Garden Ponding Depth should be 6”
Percolation Test Results
Bad…hole didn’t drain in
first 24 hours
Rain Garden Design Practice
Rain Garden Soil Amendments
We plan amend the soil 12” deep.
depth (ft) x area (ft2) = volume (ft3)
1 ft x 462 ft2 = 462 ft3 soil amendments
462 ft3 X 1/3 = 154 ft3 of each type (soil, sand, compost)
1 yd3 = 27 ft3
154 ft3 ÷ 27 = 5.7 yd3 of each type (soil, sand, compost)
Rain Garden Design Practice
6” Ponding Depth
12” Soil Amendments
18” Excavation
Rain Garden Design Practice
Rain Garden Mulch
We need 4” of mulch.
depth (ft) x area (ft2) = volume (ft3)
4/
12
ft x 462 ft2 = 154 ft3 mulch
1 yd3 = 27 ft3
154 ft3 ÷ 27 = 5.7 yd3 mulch
Rain Garden Design Practice
Garden area ÷ 2.25 = # of plants
We used large grasses, therefore
we used a lot less plants!
Rain Garden Design Practice