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Raingardens
Managing Stormwater Runoffin Urban Areas
Julie Gibson
Leslie Newton
December 4, 2001
What is a ‘raingarden’?Definition: A constructed depressional area that is used as a
landscape tool to reduce direct drainage into streams and rivers using vegetation and soils to treat stormwater and remove pollutants.
-Also called a “bio-retention” area
- Can be thought of as a miniature, artificial riparian buffer
Problems with stormwater runoff
• More flooding during storms—less base flow during dry periods
• Increases in:– erosion & sedimentation– chemicals & metals in streams– nutrients– harmful bacteria & other pathogens– potential harm to plants & animals
Why do we need rain gardens?
• Avoid direct drainage• Remove water
pollutants – Chemicals (fertilizers)
– Gasoline/oil
– Sediment
• Aesthetics
Direct drainage(the ‘ugly’ picture)
StormwaterStreams & Rivers
Indirect drainage(the ‘pretty’ picture)
Stormwater
Streams & Rivers
Raingarden
Pollutant removal mechanisms
Pollutant Removal Mechanism Pollutants
Absorption
Plant uptake
Dissolved metals & soluble P
Small amounts of nutrients including N & P
Microbial processes Organics, pathogens
Exposure to sunlight & dryness Pathogens
Infiltration of runoff Lessens flooding; increases base flow; leaching of nutrients
Sedimentation & infiltration Traps solids, debris, trash, soil-bound P & pathogens
Source: NCCES 6/01 Hunt.
Location• Strategically placed to intercept water runoff• Engineered on a graded slope no greater than the angle
which can be retained by vegetative cover or other adequate erosion control devices or structures
• Size based on area of drainage
Process
• Water collects & ponds in rain garden
• Water infiltrates & percolates through mulch, sand layer, and through root system
• Filtered water drains through outlet into stream or groundwater
Design Elements
• Grass buffer strip• Surface mulch layer• Planting soil• Sand bed• Gravel underdrain
system• Overflow storm drain• Plants
Grass buffer strip• The surface of the grass buffer strip slows water as it
enters the rain garden and filters particulates from the runoff
Summary of design parameters
Design parameter Sandy soils Clayey soils
Infiltration rate > 1” per hour 1 – 6” per hour
Max depth of H2O 6 – 12 inches 6 – 12 inches
Relative size 3 – 8% of contributing watershed
3 – 8% of contributing watershed
Mulches > 2” – hardwood > 2” – hardwood
Rock for gravel layer Washed stone
Drainage pipes Design to carry 10 times max inflow from soil layer
Source: NCCES 6/01, Hunt.
Soil Mulch layer
– Provides area for plant growth– Maintains moisture & avoids surface sealing– Filter for fine particles in suspension– Allows for microbial community breakdown of pollutants
Planting soil– Provides region for water & planting material above– Soil pores provide additional storage for water volume– Soil particles adsorb pollutants through cation exchange
Sand Bed – optional (12” deep)– Keeps finer soil particles from washing out through the
underdrain system– Provides aerobic sand filter as final “polishing” treatment media
Gravel Underdrain System – Collects & distributes treated excess runoff– Helps keep soil from becoming saturated
Planting Soil Bed
– Minimum 4 feet deep
– non-hydric top soil (A horizon)
– uniform composition
– At least 10 to 25 % clay
– Sandy loam or loamy sand texture
– Objects > 1 inch removed for plant growth
– Free of invasive/exotic plant parts
– Tested to meet NCDA recommendations for hardwoood trees
– Lime & fertilizer tilled into top 6 inches as needed
Gravel Underdrain System• Gravel layer 8 inches deep
• 4 to 6 inch perforated piping system
Overflow storm drain
• Drains stormwater that overflows ponding limits
Plants
• Endure prolonged direct sunlight• Quick transpiration rates• Withstand ponded periods for up to 3 days• Provide shade for herbacious ground cover• Native species – adapted to site• Dependent on season of planting• Diversity will help to increase resistance to pests
& diseases
NC Plants for Raingardens
Trees Shrubs Herbaceous
Red maple
Black alder
Persimmon
Sweetgum
Black gum
Loblolly pine
Water oak
Willow
Summersweet
Euonymus
Rose-of-Sharon
Leucothoe
Waxmyrtle
Sweet azalea
Elderberry
Blueberry
Astilbe
Joe Pye weed
Rose mallow
Siberian iris
Cardinal flower
Gooseneck Loosestrife
Water forget-me-not
Pitcher plant
CostConstruction Element
Coastal Plain Piedmont
Unit Cost Total Unit Cost Total
Excavation Cubic Yard $100 $9.50/cubic yard $1,600
Hauling Included in above price
Importing rock and sand
N/A N/A $0.40/cubic foot $1,280
Piping and filter fabric
N/A N/A $2/linear foot $800
Mulch $0.30/sq.ft. $80 $0.30/sq.ft. $250
Vegetation $2/sq.ft.(mature plants, somewhat dense
$400 $0.30/sq.ft. (young plants and free transplants)
$250
Total $580 $4,180
Total/sq.ft. $2.32 $4.65
Source: NCCES. 2001.Hunt
Why build a rain garden and not a treatment wetland?
• Smaller in size• Less expensive• Higher success rate• Low maintenance• Safe for children• Easy and effective way to meet EPA regulations
requiring communities to improve the quality of storm water runoff
Local Raingardens
• Carpenter Village – Morrisville
• Environmental Demo Showcase House – Morrisville (Carpenter Village)
• Rocky Branch Project – NCSU (Motor Pool)
• Unnamed raingarden – NCSU Library
Environmental Demo House – Carpenter Village
“Nonpotable water”
Other Resources
• Design of Stormwater Filtering Systems. Chesapeake Research Consortium. (December 1996). Chapter 6: Key Design Elements of Bioretention Systems.
• Earthtech. (January 29, 2001). Preliminary design of bioretention area for the Rocky Branch restoration and greenway project. Raleigh, NC.
• Ferlow, Donald L. “Development-Related Storm Runoff Renovation.” Constructed Wetlands for the Management of Stormwater Runoff. Cornell Cooperative Extension. August 20, 1997.
• House, C.H.1995. “Engineered Environments: Mimicking Nature to Clean Our Waters.” WaterWise, NC Sea Grant College Program. Vol.3, No.1.
• Hunt, William F. (2001). Designing Rain Gardens (Bio-Retention Areas). North Carolina Cooperative Extension Service. Raleigh, NC.
• Hunt, William F. (2001). Designing Stormwater Wetlands for Small Watersheds. North Carolina Cooperative Extension Service. Raleigh, NC.
• Rain Garden/ Bio-Retention Areas. (2001). Port Towns Community Development Corporation. http://www.porttowns.com/special/rain.html
Questions?
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