Bioretention,
Raingardens &
Improved Swales
Frank Henning
fhenning@uga.edu
What is a
Rain Garden?
• A shallow depression in a landscape that
captures water and holds it a short time
• Runoff water is captured & infiltrated into soil
• An attractive addition to a landscape
Benefits of Rain Gardens
• Attractive landscape feature
• Increases infiltration of rainwater in landscapes
with impervious surfaces
• Reduces runoff volume and temperature
• Creates habitat in the landscape
Rainfall
Rice Creek Watershed District
Storage
Infiltration
Basic Rain Garden
Components
Overflow
Evaporation
Ponding Area
Engineered soil
Infiltration
Runoff In
Water Quality - Expected BMP Performance
Georgia Stormwater Management Manual
ARC, 2001
Credit: GA CSS
Nutrients – Chapel Hill, NC
0.7
0.6
0.5
Mass 0.4
(Kg) 0.3
0.2
0.1
0
In
Out
TN
NO3-N
TKN
TP
Hydrology
Burnsville, MN
Neighborhood
Rain Garden Study
Barr Engineering
Locating the Rain Garden
• Where does water come
from ?
• Where is it going or
collecting?
Garden Location
– Between runoff source &
destination
– Avoid wet locations
– >10 feet from house
foundation or well
– > 25 ft from septic system
components
– Avoid TV, phone, utility
lines
– In full to partial sun is best
Quick Soil Infiltration Test
• Hole about as
deep as garden
(>4 feet)
• Fill with water
• Allow infiltration
& refill hole
• Calculate
infiltration rate
(in/hr, ft/hr)
Beware of Clay Soils
• Gray soil colors indicate extended
saturated soil conditions
•
Typical rain garden plants may not
establish well
Options:
1) Try another location.
2) Consider a wetland
3) Install an under drain.
Rain Garden Design
Steps to sizing a rain garden:
1. Estimate watershed drainage area (<5ac).
2. Estimate area of imperviousness.
3. Choose a ponding depth, mulch depth, media
depth and gravel depth (ft).
4. Determine media and native soil hydraulic
conductivity, or K (in/hr)
5. Determine media & gravel porosity.
6. Determine required size for rain garden.
Estimate area of imperviousness
Impervious areas include roof
tops, sidewalks, and
driveways.
www.nifty-stuff.com
thedailygreen..com
Thehomegarden.blogspot.com
Choose a rain event, ponding
depth & media
Design Rain Event:
The rainfall that you want the
rain garden to treat; usually 1.2
inches for Georgia
Ponding depth:
the depth to which water will pond in
the rain garden before overflowing;
usually 9 inches or less
© NCSU, 2009
Media
pore space & infiltration rate
Raingarden Size
Bioretention Ponding Zone - Full
Drain Time <48hrs
2’ below surface
Draw - Down Time?
24 – 48 Hours After the Storm
Weir (10 year peak flow)
-Use TR55
-Generate Q(cfs) for 10 year storm
-Calculate weir length
Rain garden Installation
& Construction
Construction Timing:
• Install rain garden
last – Sediment!
• Site must be
stabilized and
erosion-free!
• Use a forebay:
runoff filtered and
slowed before
entering garden
Underground Utilities???
Call Your State’s One Call Center 811
Digging Your Rain Garden
• Necessary when under-drains are required
• Don’t forget about spoil
Pretreatment Inlet
(forebay, verge, etc.)
Rain Garden – Underdrain
Runoff
Engineered Media Primarily Sand
Under-drain pipe
Under drain System
Clay - Underdrain
Installation
Inlets
Outlet
• Under-drains are a necessity where clay soils or
compaction cause water infiltration rates to be low
• Increases cost - $5 sq. ft
Underdrain Installation
Outlet
Inlets
• Backfilling with an engineered soil (mineral material)
and a perforated pipe with protective ‘sock’
• Drain is about 1 foot off bottom to allow ponding and
encourage infiltration
Media
Spoil
Upturned
Elbow
2-3 inches of hardwood mulch
Cover the berm
Stable Weir or Spillway
Choosing Plants
• Tolerant of moisture
and drought
• Adapted to site
conditions
• Moisture levels within
rain garden vary
depending on location
Consider Native Species!!
• Accustomed to the climate
• No invasive issues
• Beautiful selections
http://www.bae.ncsu.edu/programs/extension/wqg/srp/riparian.html
Maintenance
Irrigate
-First Year
-During Drought
Lime
-Soil test
Fertilization
-Never
Weed
-often
www.bae.ncsu.edu/topic/raingarden
http://stormwater.uga.edu/
Cistern to Raingarden
Improved Swales
Infiltration Trench
- Similar to RG
-Gravel is Media
Enhanced Swales
* Convey 25-year event with >6” freeboard
-Dry swale: maximum infiltration rate = 0.75 in/hr , max
ponding depth = 1.5’, filter WQv within 48 hours
(linear bioretention with constraints)
-Wet swale: Must hold the WQv (linear wetland)-