Rain Gardens: Are They the Answer? Holmdel Homeowner Education Program Holmdel, NJ October 9, 2010
Jeremiah D. Bergstrom, LLA, ASLA Water Resources Program Senior Project Manager Rutgers Cooperative Extension Phone: 732-932-9800 ext. 6126 E-mail: jbergstrom@envsci.rutgers.edu www.water.rutgers.edu
New Jersey Regulations NJDEP Stormwater Management Rules NJDEP Municipal Separate Storm Sewer System (MS4) Permitting Rules Municipal stormwater management plans and ordinances Residential Site Improvement Standards - Stormwater Management Total Maximum Daily Loads (TMDLs)
Nonstructural Low Impact Development Strategies Minimize impervious cover Minimize disturbance Maximize vegetation, minimize lawns Pollutant Reduction Minimize vegetation that needs fertilizers Trash racks and receptacles
Groundwater Recharge Groundwater Recharge: Infiltrated water that is not evapotranspired Groundwater shall not be recharged in areas with high pollutant loading Waiver available for urban redevelopment Groundwater Criteria: Maintain existing (100%) average annual groundwater recharge OR Infiltrate the increase in runoff volume for the 2-year storm event
Critical Issue Question: How can we develop property without increasing Non-Point Source (NPS) loads while maintaining groundwater recharge? Answers: Advanced Best Management Practices (BMPs)
The Traditional Approach Design Dry Detention Basin that: Treat Water Quality Storm (1.25 rain over 24 hours) Detain for 18 hours (residential) or 36 hours (commercial) Minimum outflow orifice = 3 inches Use Concrete Low Flow Channels to Minimize Erosion
The Traditional Detention Basin
The New Approach Combines settling of detention basin with physical filtering and absorption processes Provides very high pollutant removal efficiencies More aesthetically pleasing than conventional detention basins Can be incorporated into the landscapes of individual homes
Rain Gardens (Bioretention Basins) Landscaped areas that treat stormwater runoff. Designed to merge two important goals: aesthetics and water quality. Can be blended into the landscape and made to look natural. Water is directed into them by pipes, swales, or curb openings.
Rain Gardens (Bioretention Basins)
Rain Gardens (Bioretention Basins)
Rain Gardens/Bioretention Systems NJDEP. 2004. NJ Stormwater BMP Manual.
Pollutant Removal Mechanisms Absorption to soil particles Removes dissolved metals and soluble phosphorus Plant uptake Removes small amounts of nutrients Microbial processes Removes organics and pathogens Exposure to sunlight and dryness Removes pathogens Infiltration of runoff Provides flood control, groundwater recharge, and nutrient removal Sedimentation and filtration Removes total suspended solids, floating debris, trash, soil-bound phosphorus, some soil-bound pathogens NOTE: 90% of all storm events produce less than 1 inch of rain. Therefore, the key to reducing pollutant loads is to treat the runoff associated with the first 1 inch of rain (Claytor & Schueler, 1996).
Cross-Section of Rain Garden - Bioretention Basin BASIN SLOPE NATIVE PLANTINGS 4 IN. MULCH LAYER 2.5 3 FT. PLANTING SOIL BED (MIN. PERMEABILITY 0.5 IN. PER HOUR) 12 IN. SAND BED 12 IN. GRAVEL BED 4 IN. PERF. PVC PIPE FILTER FABRIC
Bioretention Basin Expected Infiltration 0 Hours 12 ponding depth 6 Hours 9 ponding depth 3 infiltration 12 Hours 6 ponding depth 6 infiltration
Bioretention Basin Expected Infiltration 1 Day No Standing Water 2 Days 12 infiltration No Standing Water 24 infiltration 3 Days No Standing Water 36 infiltration
Pollutant Removal Rate Comparison Detention Basin (Detain 1.25 storm for 12 hours) Bioretention Basin - (exfiltrate 1 runoff volume/impervious acre) RAIN GARDEN/ PARAMETER DETENTION BASIN BIORETENTION BASIN PROBABLE RANGE PROBABLE RANGE Suspended Solids 70% to 90% 90% Total Phosphorus 10% to 60% 70% to 83% Total Nitrogen 20% to 60% 68% to 80% BOD 30% to 40% 60% to 80% Lead 20% to 60% 93% to 98% Zinc 40 % to 60% 93% to 98% Hydrocarbons 60% to 77% 90%
Bioretention Basin
Design of Backyard Rain Gardens
Rain Garden Placement The rain garden should be at least 10 feet from the house so infiltrating water doesn t seep into the foundation. Do not place the rain garden directly over a septic system. Do not put rain garden in places where the water already ponds. Place in full or partial sunlight. Select a flat part of the yard for easier digging. http://clean-water.uwex.edu/pubs/raingarden/rgmanual.pdf
http://clean-water.uwex.edu/pubs/raingarden/rgmanual.pdf Rain Garden Placement
Depth Between four and eight inches deep Depth depends upon lawn slope If the slope is less than 4%, it is easiest to build a 3 to 5-inch deep rain garden. If the slope is between 5 and 7%, it is easiest to build one 6 to 7 inches deep. If the slope is between 8 and 12%, it is easiest to build one about 8 inches deep. http://clean-water.uwex.edu/pubs/raingarden/rgmanual.pdf
Other Considerations Is the soil type suitable? percolation test/infiltration test texture test/soil type test Is the rain garden able to handle the drainage area? if not, consider multiple rain gardens http://clean-water.uwex.edu/pubs/raingarden/rgmanual.pdf
Size of the Rain Garden The size of the rain garden is a function of volume of runoff to be treated and recharged. Typically, a rain garden is sized to handle the water quality design storm: 1.25 inches of rain over two hours. A typical residential rain garden ranges from 100 to 300 square feet.
Example in Sizing Problem: How big does a rain garden need to be to treat the stormwater runoff from my driveway?
25 50 Driveway House 25 50 10 Driveway Area 50' x 15' = 750 square feet 25' x 10' = 250 square feet Total Area = 1,000 square feet 15 One-Quarter of the Roof 25' x 12.5' = 312.5 square feet
Example in Sizing Drainage Area = 1,000 square feet 1.25 inches of rain = 0.1 feet of rain 1,000 sq. ft. x 0.1 ft. = 100 cubic feet of water for the design storm Let s design a rain garden that is 6 inches deep Answer: 10 ft wide x 20 ft long = 200 square feet
Rain Garden Sizing Table for NJ s Water Quality Design Storm Area of Impervious Surface to be Treated (ft 2 ) Size of 6 deep Rain Garden (ft 2 ) or [w x d] Size of 12 deep Rain Garden (ft 2 ) or [w x d] 500 100 or 10 x10 50 or 10 x5 750 150 or 15 x10 75 or 10 x7½ 1,000 200 or 20 x10 100 or 10 x10 1,500 300 or 30 x10 150 or 15 x10 2,000 400 or 20 x20 200 or 20 x10
How much water does this treat? 90% of rainfall events are less than 1.25 New Jersey has approx. 44 of rain per year The rain garden will treat and recharge: 0.9 x 44 = 40 /year = 3.3 ft/year The rain garden receives runoff from 1,000 sq.ft. Total volume treated and recharged by the rain garden is 1,000 sq. ft. x 3.3 ft. = 3,300 cubic feet, which is 25,000 gallons per year Build 40 of these and we have treated and recharged 1,000,000 gallons of water per year!
Rain Garden: Maintenance Issues Repair planting soil bed if erosion occurs. Core aerate or cultivate unvegetated areas annually if surface becomes clogged with fine sediments. Apply mulch twice per year until groundcover establishes. Replace dead or diseased plant material. Inspect/remove any sediment buildup/trash/leaves at inflow and outflow devices on monthly basis. Do NOT fertilize unless you do a soil test!
Maplewood, MN Example Rain Gardens Offer Stormwater Control Community choose rain gardens to improve drainage in older neighborhoods instead of curb and gutter drainage systems Two block pilot project resulted in no runoff from the neighborhood 100% stormwater control City offers three standard garden sizes: 12 x24, 10 x20, and 8 x16 And seven different garden themes: Easy shrub garden, Easy daylily garden, Sunny garden, Sunny border garden, Butterflies and friends gardens, MN Prairie garden, and Shady garden. Gardens designed to catch 1/2 of rain. Gardens cost 75 to 85% of traditional curbs and gutters.
Rain Gardens in NJ? Gardens should be designed to catch 1.25 of rain. Maximum water depth should range from 6 to 12 Size should be 3 to 10% of contributing watershed (e.g., a 1,250 sq. ft. house footprint 125 sq. ft. garden that has a maximum water depth of 1 ft.) Install an underdrain system where soils are not suitable for infiltration Double shredded hardwood mulch 4 thick
Rain Garden Plantings Swamp Milkweed Bee Balm Soft Rush Photos by Linda Brazaitis
Rain Garden Plantings Blue Flag Iris Cardinal Flower Bald Cypress Shasta Daisy
Conclusions Rain gardens (bioretention basins) provide extremely high levels of stormwater treatment. These systems design will detain runoff from 1 inch or smaller storms (90% of all rainfall) for several days. These designs will provide mechanisms for stormwater temperature decreases. The plant material (trees and shrubs) within the basins will provide diversity in the landscape and wildlife habitat. New stringent stormwater treatment requirements may be satisfied using rain gardens and bioretention basins. They are aesthetically pleasing and easily incorporated into the landscape.
Frequently Asked Questions: Q: I have a wet spot behind my house; would this be a good place for a rain garden? A: No, a rain garden is suppose to drain after 24 hours. Q: Can I put plants in my berm? A: Yes. Q: Will RCE come out and build my rain garden for me? A: How much money do you have? Only, kidding. We will provide some level of technical support. Q: What about deer? A: We recommend purchasing an SUV (Hybrid of course) and driving very fast on the back roads at night. Or you can use fencing or deer resistant plants or deer retardant sprays.
Frequently Asked Questions: Q: Do I need to fertilize? A: You should only fertilize during the initial planting and only according to the county soil test kit results. Q: Do I need to water? A: Yes. Frequently during the first year. Especially the first few months after planting (particularly if it is in the middle of the summer). Q: What if some plants die? A: Younger plants will be more susceptible to drowning if they are immersed in standing water for too long. As a rule of thumb, place the most water tolerant plants near the water s entrance to the garden. Replace the dead plants with larger cuttings or a species that you know has survived.
Frequently Asked Questions: Q: How do I know if my soil is suitable to for a rain garden? A: Do a perc test or an infiltration test. If the infiltration rate is at least one inches per hour, you are good to go! Q: Can I build my rain garden in clay soils? A: Yes, but there are some additional measures to take: increase the area and decrease the depth make sure the clay layer is dry (otherwise shovels will seal pores) Add soil amendments (using your rototiller) Replace the first 2 feet (below the ponding area) with sandy loam soil. You can create your own by obtaining bank run sand from a nursery. DO NOT USE MASON OR BALL FIELD SAND. Mix in 3 inches of compost at the top
Frequently Asked Questions: Q: How do I slow down the speed of water as it enters the garden? A: (1) Attach a perforated plastic diffuser to the end of your roof leader (2) Use river rock at the entrance point (3) Site your rain garden with a 10 15 foot grass buffer between it and the roof leader. Use native grasses and let them grow tall. Q: What to I do with my existing grass before starting the rain garden? A: A two options: (1) Kill the grass with herbicide beforehand or (2) Remove the turf as sod pieces. You can reuse it on your berm or another area of your yard.
Jeremiah D. Bergstrom, LLA, ASLA Water Resources Program Rutgers Cooperative Extension Phone: 732-932-9800 x6126 E-mail: jbergstrom@envsci.rutgers.edu www.water.rutgers.edu