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?