Rain Gardens Welcoming and filtering rainwater naturally Asad Rouhi Northern Virginia Soil and Water Conservation District
Urban Development Impact Inadequate drainage, erosion, flooding Northern Virginia Soil and Water Conservation District
Urban Development Impact Inadequate drainage, erosion, flooding
Urban Development Impact Inadequate drainage, erosion, flooding Conventional solutions
Urban Development Impact Inadequate drainage, erosion, flooding Conventional solutions
Conventional solutions Concentrate the flow No quality, quantity benefit Ineffective in dealing with large volumes Maintenance intensive Outfall limitations
High maintenance Conventional solutions
Outfall limitations Conventional solutions
Outfall limitations Conventional solutions
Rain Gardens Welcoming and filtering rainwater naturally
Definition: A landscaped depression that has the ability to capture, temporary retain, and filter rainwater 6-12 ponding depth Source: LID Center
Benefits Improves runoff water quality 6-12 ponding depth
Benefits Recharges groundwater Decreases runoff volume 6-12 ponding depth
Benefits Improves surface drainage, and reduces surface erosion and flooding potentials
Benefits Acts as a healthy habitat for plants and birds,
Benefits Could be beautiful and fun to build!
How Rain Gardens Work Detains runoff Allows runoff to pass through a media (compost and soil) Physical effect Microbial effect Plants effect 6-12 ponding depth A typical backyard rain garden
The main components of a rain garden Inflow, outflow Ponding area Amended soil Gravel filter and under drain Runoff G surface 6-12 ponding area 3 mulch 1.0-2.0 planting soil
The main components of a rain garden Inflow Outflow
The main components of a rain garden Inflow Outflow
The main components of a rain garden Ponding Area 6-12 ponding depth
The main components of a rain garden Amended soil 6-12 ponding depth Amended soil
The main components of a rain garden Runoff 6-12 ponding area 3 mulch 2-3 soil media 1-2 gravel sump
The main components of a rain garden Runoff Outflow str Max. WSE Ground surface Pretreatment Area 6-12 ponding area 3 mulch 2-3 soil media underdrain 4-6 Pea Gravel 1-2 gravel sump (VDOT # 57)
How to design and build a rain garden 1. Choose the right location 2. Size your rain garden
Choose the right location Close to the source of runoff At least 10.0 from building foundations Where there is no underground utilities Where there are no easements Where ground is not steep Not under big trees Not over the septic system Not where there is standing water Where there is suitable soil
Close to the source But not this close! Choose the right location
Choose the right location Suitable soil Rain garden is an infiltration facility Find out whether The soil is suitable for building a rain garden
Choose the right location How to find out whether soil is suitable for a rain garden Start with the County s soil maps Soil Number Soil Name Problem Soil Foundation Support Drainage Potential Septic Drainfield Infiltration Trenches Erosion Potential 55 Glenelg C Good Good Good Good High 21 Manor C Good Good Good Good High 20 Meadowville B Fair Marg. Poor Poor Mod.
For Soil Information: http://www.fairfaxcounty.gov/gisapps/pdfviewer/default.htm http://soildatamart.nrcs.usda.gov/manuscripts/va013/0/arlington.pdf or Northern Virginia Soil & Water Conservation District Suite 905, Herrity Building 12055 Government Center Parkway Phone: 703-324 1460 or http://websoilsurvey.nrcs.usda.gov/app/
You can measure soil infiltration yourself GS h 0 h 1 I rate = h 1 h 0 I rate < 0.5 in/hour not suitable for rain garden except using under drain
Size your rain garden Basic rules for designing your rain garden Limit the size of your rain garden to between 100 to 300 sf. Maximum area that drains into a single rain garden (DA) should not exceed 0.5 acres (on residential lots)
How to size your rain garden? Example Problem: erosion, drainage Source of runoff: roof, driveway (impervious), grassed slope (pervious)
Size your rain garden Step 1: Determine the source of runoff Step 2: Determine the area that will flow into the rain garden (drainage area) Step 3: Determine the pervious and impervious area within the drainage area Step 4: Determine the surface area of the rain garden
Step 1 Determine the source of runoff
Step 1 Determine the drainage area The drainage area = 6,000 sf
Step 3 Determine the pervious and impervious area within the drainage area Impervious: 850 sf (roof) + 550 sf (driveway) + 100 sf (sidewalk) = 1500 sf Pervious: 4500 sf (lawn) Impervious areas: roofs, paved roads, paved sidewalks, patio Pervious areas: any other surface cover
Step 4 Determine the rain garden s surface area Surface area = (Impervious drainage area x 0.072) + (Pervious drainage area x 0.028)
Step 4 Determine the rain garden s surface area Surface area = (Impervious drainage area x 0.072) + (Pervious drainage area x 0.028) Surface area = (1500 x 0.072) + (4500 x 0.028) = 234 sf
Rain garden sizing working sheet 1 2 3 4 Drainage area (square feet) = Impervious drainage area (square feet) = Pervious drainage area (square feet) = Surface area = (Impervious drainage area x 0.072) + (Pervious drainage area x 0.028) = Note: If (6) is more than 300 sf divide the area into two rain gardens
How to build a rain garden A- Ground surface is relatively flat 20.0 1.0
How to build a rain garden A- Ground surface is relatively flat 20.0 1.0
How to build a rain garden A- Ground surface is relatively flat 20.0 Amended soil
How to build a rain garden B- Ground surface is slopped 20.0 Flow direction 1.0
How to build a rain garden B- Ground surface is slopped 20.0 Flow direction 9.0
How to build a rain garden B- Ground surface is slopped 15.0 Flow direction 9.0
How to build a rain garden B- Ground surface is slopped Berm 15.0 Flow direction Ponding area Amended soil
How to build a rain garden B- Ground surface is slopped Berm 20.0 Flow direction Under-drain drain Ponding area Amended soil Gravel filter Under-drain drain outfall
How to build a rain garden
Before starting building your rain garden please ensure that all the required permits are obtained For information regarding permits contact: Fairfax County DPWES, LDS Engineer of the Day at 703-324 324-1750or or NVSWCD at 703-324 324-1460
Contact information: Asad Rouhi, PhD Urban Conservation Engineer NVSWCD Asad.rouhi@fairfaxcounty.gov Phone: 703-324 324 1460