Rain Gardens & Wetland Gardens

& Friends

What is a Rain Garden?

• A stormwater feature that has plants - or - a garden that captures and holds water ??? • A shallow depression in a landscape that captures

stormwater runoff and holds it a short time • Allows stormwater to infiltrate into the soil • An attractive addition to any landscape !

Example in Parma

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Benefits of Rain Gardens • Reduces water pollution downstream • Reduces flooding downstream • Recharges groundwater • Low maintenance, low water use • Can increase water infiltration 30-40% • Creates habitat in the landscape • Beautiful landscape feature!

Rain Gardens Improve Water Quality

• Handles stormwater at its source. • Holds water on the surface aiding infiltration to shallow

groundwater • Decreases the velocity of water flowing from impervious

surfaces. • Improves water quality before it enters the stream or ditch. • Reduces nutrients, heavy metals, sediment, fecal coliform • Cost effective (average $2-$5 per sq ft. – w/o underdrain)

Rain Gardens are like. . .

Basic Rain Garden Components

Disturbed soil

Infiltration

Runoff In Overflow

Ponding Area

Evaporation

Overview: Steps for Choosing RG Location • Identify potential spots (may have 2-3) using existing

landscape • Dig 1-ft-deep hole for preliminary infiltration test at

each potential site • Pick best site based on infiltration rate • Dig a one-ft-deep hole at best site and fill with water to

test infiltration potential

‘Priority’ Downspouts

YES NO

‘Priority Downspouts’ • Look first for downspouts that discharge onto

impervious surfaces – stormwater from these downspouts frequently goes directly to storm drains and streams

• Try to divert them to a rain garden • Diversion may require pipes or berms • Consider changing gutter and downspout

configuration to discharge onto grass or rain garden

Priority Downspouts

YES YES

Rain Garden Location

Rain Garden Certification

Rain Garden Location

“Pinch Point”

Rain Garden Certification

Rain Garden Location

Potential Collection

Points

Rain Garden Certification

Locate Rain Garden with Topography in Mind to Minimize Digging and Berm Construction

Rain Garden Location: Site Constraints

SITING GUIDELINES: • > 10 ft from house crawl

space or basement • NEVER uphill

• > 10 ft from wellhead • NEVER uphill

• > 25 ft downhill or laterally from septic system drain field • NEVER uphill

• In full to partial sun if possible

The Infiltration Test

To choose the best location, dig 2-3 holes to a depth of 1’ at each potential spot and fill with water.

Repeat 2-3 times. Take the longest draining time as the representative time for the site.

Rain Garden Certification

Infiltration Test: The Results

Drain Time Appropriate BMP < 12 hours Quick-Draining Rain Garden 12 – 3 days Standard Rain Garden

> 3 days Wetland Garden

Note: These are small-scale rain gardens and wetland gardens. Larger projects designed for

regulatory purposes require design and approval by a P.E. or RLA.

Rain Garden Certification

Important! The landowner/homeowner MUST know that at

times, rain gardens will be very wet. They can also be very dry.

Rain Garden Certification

Steps to sizing a rain garden: 1. Determine the watershed boundaries.

2. Estimate impervious areas.

3. Determine runoff capture depth and ponding depth.

4. Determine required rain garden size.

© NCSU, 2009

Rain Garden Certification

Estimate impervious areas in square feet Impervious areas include roof tops, sidewalks, and driveways.

www.nifty-stuff.com

Thehomegarden.blogspot.com

thedailygreen..com

Rain Garden Certification

Choose a runoff capture depth and a rain garden ponding depth.

runoff capture depth: the amount of rainfall that you want the rain

garden to treat; usually 1 inch for N.C. (1.5 inch in Coastal Plain)

rain garden ponding depth: the depth to which water will pond in the rain garden before overflowing;

usually 10 inches

www.andersonvalley.net

© NCSU, 2009

Rain Garden Size Equation:

Total impervious area

X 10%

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Rain Garden Size • Example: • Roof area: 10X30 = 300 sq. ft. • Driveway: 12X30 = 360 sq. ft. • Total = 660 sq. ft. • Calculation: 660 X .10 = 66 sq ft

• Rain garden size is 6.6 X 10 feet

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Overflow Weirs – a way “out” for excess water

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Call before you excavate!

• Locate wells, septic systems, and utilities

• Ask the homeowner (verify, verify, verify!)

2. Excavation

Suggested Tools • Backhoe, shovels,

rakes • Soil Tamper • Wheelbarrow • Sight Level, Level,

Line Level, Ruler • Stakes and String • Tarps • Soil Test Kit, Clean

Bucket • Labor!

Two basic choices

Flat site: dig a hole Sloped site: dig hole and make a berm

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‘Harden’ the inflow… • Rocks slow and still the

incoming stormwater

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Mulching: hardwood bark is less prone to float away

ambroselandscapes.com

Why Upside-down Pots?

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Planting –lay them out…

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Plant – but not too deep

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Lime? Yes, follow soil test. Fertilizer? NO

Durham Rain Garden Gothic

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Green St., Durham….

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Collecting downspout water

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Routing downspout water into rain garden

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Excavation depth measurement

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Measuring elevations of inflow and outflow

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Outflow weir measurement

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Stilling inflow water

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Inflow and outflow

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Finished Product…..

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What if your site dictates a wetland garden?

• Dig 1-foot deep test hole and fill with water • Standing water > 3 days later indicates poor

drainage and wetland preference • Ponded water on surface for extended time? • Do you see mottled soils when you dig? • Do you intercept water when you dig? • Under these conditions a wetland is called for

Evaluate for Wetland Soils Wetland soils –

grey matrix mixed with

areas of brown color

Wetland Garden vs. Rain Garden Two major differences: 1. “Wetness” 2. Plant selection