Rain Gardens Rain Gardens Gone Wild!Gone Wild!

ByBy

Frank ReillyFrank Reilly

Prince William Prince William County Master County Master

GardenerGardener

Design PrinciplesDesign Principles

• Near the drainage areaNear the drainage area

• Must empty within ~2 daysMust empty within ~2 days

• May Need an overflow structureMay Need an overflow structure

• Porous soilsPorous soils

• Suitable plantings (“Bio”-retention)Suitable plantings (“Bio”-retention)

• Acceptance related to aestheticsAcceptance related to aesthetics

Possible locationsPossible locations

Where can I put my rain garden? Where can I put my rain garden? • Any of the problem areas mentioned in step 1 Any of the problem areas mentioned in step 1 • 10 feet away from the foundation of your 10 feet away from the foundation of your

house! house! • 25 feet from a septic system drainfield 25 feet from a septic system drainfield • 25 feet from a well head 25 feet from a well head • Avoid underground utility lines Avoid underground utility lines • Partial to full sun Partial to full sun • Water table is at least 2’ below the surface of Water table is at least 2’ below the surface of

the soil. the soil.

Locating Rain Locating Rain GardensGardens• Between area to be drained Between area to be drained

(roof, driveway or yard) and (roof, driveway or yard) and storm drain or streetstorm drain or street

• Where downspout or other Where downspout or other area can drain to it across a area can drain to it across a grassy area, or “filter strip” grassy area, or “filter strip”

• At least 10 feet from your At least 10 feet from your house’s foundationhouse’s foundation

• An oval or oblong shape An oval or oblong shape approximately 5-7% of the approximately 5-7% of the size of the area draining to size of the area draining to it (roof, etc.) it (roof, etc.)

Building Rain Building Rain GardensGardensCall Miss Utility!Call Miss Utility!

CCall before you digall before you dig

AAllow time for markingllow time for marking

RRespect the marksespect the marks

EExcavate carefullyxcavate carefully

One Number for all Virginia : One Number for all Virginia :

1-800-552-70011-800-552-7001

Size mattersSize matters

• Determine your roof or drainage areaDetermine your roof or drainage area

• Determine the amount of rain water Determine the amount of rain water that you want it to hold. that you want it to hold.

• Consider the soil typeConsider the soil type– Tighter soil gardens require more Tighter soil gardens require more

volumevolume– Or overflow structureOr overflow structure

Washington, DC - Reagan National

2001 Daily RainfallFrequency (inches)

90%

4%4% 2%

0.25

0.5

1

> 1

Size matters (Continued)Size matters (Continued)

• Estimate the drainage area of your roof:Estimate the drainage area of your roof: (Length x Width)(Length x Width)Example: Length of roof = 40' Example: Length of roof = 40' Width of roof = 20' Width of roof = 20' (Calculate the area of the roof that is draining to one (Calculate the area of the roof that is draining to one

gutter.)gutter.)

• Estimate the depth of rain:Estimate the depth of rain: (In VA, use an (In VA, use an estimate of 0.25" of rain per event)estimate of 0.25" of rain per event)Example: event depth of rain = 0.0208' (0.25"÷12"per Example: event depth of rain = 0.0208' (0.25"÷12"per

foot = 0.0208')foot = 0.0208')(You can also use a depth of rain from a specific rainfall, (You can also use a depth of rain from a specific rainfall,

just be sure to convert it into ‘feet’ units.)just be sure to convert it into ‘feet’ units.)

• Calculate the Volume:Calculate the Volume: (Length x Width x (Length x Width x Depth)Depth)Example: 40'L x 20'W x 0.0208'D = 16.6 cubic feetExample: 40'L x 20'W x 0.0208'D = 16.6 cubic feet

Size matters (Continued)Size matters (Continued)

• Calculate the Garden Size:Calculate the Garden Size: (Length x Width x Depth to contain (Length x Width x Depth to contain 16.6 cubic feet)16.6 cubic feet)Example: Depth of 6 inches need 33 Example: Depth of 6 inches need 33

square feet. (16.6 Cubic feet of water/ square feet. (16.6 Cubic feet of water/ 0.5 feet = 33.2 square feet)0.5 feet = 33.2 square feet)

• Poor drainage = bigger area Poor drainage = bigger area (resources at NC State web site)(resources at NC State web site)

DesignDesign

• Shape of the depressionShape of the depressionfor homes consider shallower depths to alleviate for homes consider shallower depths to alleviate

drainage problemsdrainage problems• Consider type of plants and arrangement (Put the more Consider type of plants and arrangement (Put the more

aquatic plants where most standing water will be.)aquatic plants where most standing water will be.)• Grass on berm to avoid erosionGrass on berm to avoid erosion

What can go wrong?What can go wrong?

• SizeSize

• Blow outBlow out

• Not enough waterNot enough water

• Too much waterToo much water

• Bad drainageBad drainage

• Your neighbors could find out!Your neighbors could find out!

Too small or steep = too much flow

NOTE CHANNELING, MULCH WASHOUT

Make sure water can get out

It doesn’t always rain the “average amount”

FLOW IN

FLOW OUT

GREEN GABLES RAIN GARDEN

GREEN GABLES – RAIN GARDEN

FLOW PATH

OVERLAND RELIEF

Make sure that some rain can get in

Chinn Rain Chinn Rain GardenGarden

If it isn’t flat the neighbors WILL know!

LOT 4, RG 4NOTE SHAPE AND STANDING WATER

HOPEWELLS LANDING SEC 1

FLAT SURFACE GOOD, SLOPES TOO STEEP

HOPEWELLS LANDING SEC 1

GRADING, NOTE BOWL SHAPE

HOPEWELLS LANDING SEC 1 GRADING ISSUES

NOTE: STANDING WATER, BOWL SHAPE, STEEP SLOPES

Poorly drained soils

NOTE STANDING WATER

Too small – no relief

Questions?Questions?• www.AdvancedMasterGardener.orgwww.AdvancedMasterGardener.org

follow the water buttonsfollow the water buttons