Choose the right Rain Garden

Why Should I Use This Tool?

We hope this tool raises your awareness of siting considerations that can lead to both a less costly and more environmentally friendly rain garden. This “Choose the Right Rain Garden” decision tree (i.e. wizard) helps users arrive at and implement the rain garden configuration that best suits their site conditions. Additional information and tips are offered below each question and many pages have illustrative graphics to help you choose the answer that reflects your conditions.

Download a flow chart of the entire rain garden decision tree

While rain gardens alone will probably not fully protect or restore your watershed, the goal of this wizard is to help you avoid the most complicated and therefore most costly to build and maintain configuration, the lined filtration rain garden. Throughout Oregon, these facilities don’t mimic pre-developed hydrology; additional runoff volumes (aka hydromodification ) flowing in and out of a lined facility are likely to damage downstream waterways, impacting water quality, habitat, and availability. In addition, they cost more to build and maintain.

What if I Don’t Understand or Know the Answer to a Question?

We recognize that you may not have answers to all the questions, but only questions truly relevant to water quality and cost have been asked. If you need more information, footnotes below each question provide important general information. You may also want to experiment now, first answering conservatively, to see what configurations the decision tree takes you to.

 

 


Sensitive Areas: Question 1

Are EITHER of the following conditions true?

Rain garden will be located:

  • in floodplains1 or other sensitive areas such as wetlands2, riparian areas and buffers3, & habitat4
  • over septic systems5

Yes | No

If any of the above conditions are true, answer Yes. If all of the above statements are false (preferred), answer No. Answering yes means that this location is unsuitable for a rain garden. A lack of information from any one of the following sources does not necessarily mean that your site is without any sensitive areas. The ultimate decision will fall to those with jurisdiction, so be sure to contact any agencies mentioned for assistance if in doubt.

1 Use the 100-year floodplain area as a minimum. The 25-year floodplain may also be appropriate depending on the development standards and regulatory environment in your jurisdiction. More info.

2 Wetlands may or may not be recognized and mapped as "jurisdictional". If a wetland has not been mapped, it may still fall under multiple jurisdictions. Rain gardens in wet or seasonally wet areas could cause damage to your property. More info.

3 A riparian area can be simply defined as land adjacent to a waterway that either influences or is influenced by that waterway. More info.

4 Numerous animal species provide important hydrologic benefits to our watersheds. There may be sensitive uplands that provide important habitat to some of our specialized native animal species. More info.

5 Avoid installing rain gardens over a septic field or other infiltration septic system; this could ruin your septic system in a variety of ways. More info.

 


Sensitive Areas: Question 2

Are ANY of the following conditions true?

Rain garden will be located:

  • in contaminated soils or groundwater1
  • so that it will drain or overflow to a known landslide area2
  • for runoff from vehicular areas, in wellhead protection areas3
  • for runoff from vehicular areas, within a horizontal distance of 2x the depth of any nearby wells4

Yes | No     

If any of the above conditions are true, answer Yes. If all of the above statements are false (preferred), answer No. Answering yes leads to Filtration facilities, which prevent infiltration with a liner or are located in native soils infiltrating less than 0.5 inch/hour. While rain gardens alone will probably not fully protect or restore your watershed anyway -- evaporation generally plays an important role in healthy watersheds – lined filtration facilities in Oregon don’t mimic pre-developed hydrology. The additional runoff volumes (aka hydromodification) flowing in and out of a lined facility still cause damage to downstream waterways, impacting water quality, habitat, and availability. In addition, they cost more to build and maintain.

A lack of information from any one of the following sources does not necessarily mean that your site is without any sensitive areas. The ultimate decision will fall to those with jurisdiction, so be sure to contact any agencies mentioned for assistance if in doubt.

1 Infiltrating into areas of contaminated soils or groundwater can expand the area of contamination via transport through the soil. More info.

2 Infiltration rain gardens should not be installed in known landslide hazard areas. More info.

3 A wellhead protection area is a designated area of land where a jurisdiction might draw groundwater to supply public drinking water. Infiltration of runoff from vehicular areas is prohibited to protect public groundwater drinking supplies.  In some jurisdictions, infiltrating roof runoff may be acceptable. More info.

4 If runoff is not from vehicular areas, but is instead from roof or landscape areas, then automatically answer "No" to this. Groundwater can move twice as fast horizontally as vertically, so infiltrating too close to private drinking wells can contaminate them.  Locate these wells in relation to your site and infiltrate runoff in a rain garden no closer than 2 times the completed depth. For instance, for a well with a completed depth of 200', an infiltration rain garden should be at least 400' away. More info.

 


Setbacks: Vertical Separation

Are both conditions true? The distance from the bottom of the facility to the top of:

  • the seasonal high groundwater table is at least 3 feet1.

     AND

  • bedrock, fragipan2 (an impervious subsurface layer), or other impermeable material is at least 2 feet3.

Yes | No    

If all criteria are met answer “Yes” to this question. If only one of the criteria is met, answer “No”

1 Do not rely on groundwater modeling or maps; these are usually done at too large a scale and don’t apply to the site scale practices such as a rain garden. To  look for bedrock or other impermeable subsurface layer that may impede infiltration, after infiltration testing has confirmed that soils are suitable for infiltration (additional info is included in this wizard and the fact sheet “Infiltration Testing”), dig a hole 2 feet of depth from the bottom of the proposed rain garden. If the soil is pretty consistent all the way down, then you don’t have a fragipan or bedrock problem.  Now look for a seasonal high groundwater table by digging a hole to an additional one foot of depth. If the hole doesn’t fill up with water, then groundwater levels are sufficiently deep. Ideally, this will be done during the rainy season.

2 A fragipan is an impermeable soil layer. In Oregon, volcanic activity is often the source of this condition; ash deposited long ago has been compacted by additional soil deposited on top of it over time.

3 Vertical separation is the distance between the bottom of the rain garden ponding area and the top of a subgrade layer in question (i.e. bedrock, groundwater, or fragipan2). The soil is nature’s pipe that conveys water downhill to seep out to streams; adequate vertical separation provides enough voids and depth for that conveyance beneath infiltration facilities.

Vertical Separation


Setbacks: Horizontal Separation

Are all the following conditions true? Infiltration facility will be:

  • 10’ from a footing or foundation or the top of a wall AND
  • 10’ from underground tanks AND
  • 5’ from the property line1 AND
  • 100’ from down gradient slopes equal to or exceeding 10% plus 5’ of setback for each additional percent up to 30%2 AND
  • 5’ from underground pipes3 AND
  • 0’ from slabs footings or pavement4 AND
  • 0’ from pier or post footings5 or the bottom of a site (not building) wall

Yes | No    

All criteria must be met to answer “Yes” or if any one of these statements is false, but location has been approved by a geotechnical engineer, then answer Yes. Without adequate horizontal separation, infiltration facilities have the potential to flood basements, overturn walls, or otherwise damage other infrastructure.

1 Five feet from the property line assumes that the zoning requirements for your property require a minimum 5’ setback. If that’s the case, then this will provide the 10’ distance needed to prevent flooding.

2 Avoid installing infiltration facilities near slopes exceeding 30%.

3 Call 811 to locate utilities on private property. Infiltrating too close to pipes could polluted stormwater could contaminate the contents of the pipe. In addition, many pipes are installed on a bed of gravel, which provides a shortcut for infiltrating water that could cause damage to structures and pavements.

4 A vertical liner extending at least 6” deeper than the pavement section (including any base rock) should be installed so infiltrating water doesn’t undercut and cause a structural failure of the pavement.

5 Also known as an LID Foundation, which include any foundation types that don’t block the flow of groundwater the way conventional basements do.

Horizontal Separation

 


    Slow Soils: Question 1

    Is the infiltration rate of native undisturbed soils less than 12 inches/hour?

    Yes | No    

    The infiltration rate of the native soils should be discovered with an infiltration test in the field at a depth equal to where water will infiltrate when an infiltration facility is completed. For a simple infiltration testing method suitable for homeowners and others, see “Step 3: Assess Soil” section in the Oregon Rain Garden Guide. For more detailed information on testing methods, see “Infiltration Testing”, a fact sheet geared to jurisdictional staff and experienced designers. Do not rely on the NRCS soils data or any other mapping of hydraulic conductivity.

     


    Slow Soils: Question 2

    Rain garden sized to drain in time for next storm?

    Yes | No    

    Whether or not a facility will empty out and be ready for the next storm is a function of the storm distribution (i.e. Type IA, Type I, and Type II). Type IA is the most gentle, longest storm while Type II storms are the shorter, most intense storms. In general, jurisdictions within the Type IA distribution in Oregon require facilities to be empty in 30 hours. 

    To determine the answer to this question, you or your designer probably need to model the rain garden. You can use popular engineering modeling software such as EPA SWMM or HydroCAD or OSU’s open-source Excel calculator. Video tutorials are available at this link.

     


    Slow Soils: Question 3

    Are native soils suitable for plants?

    Yes | No    

    In general, native soils will be suitable for native plants since they evolved together, even if your native soils are clayey and you find soil guidance that says a plant needs well-drained soils. In the experience of many, clays are acceptable as long as the seasonal and/or year-round groundwater table isn’t too high. Having said this, there are some exceptions. Sometimes past construction has already disturbed or replaced soils with structural (rocky) fill. Compost amendment and mycorrhizal treatment of any soil can be beneficial for plant establishment.

     


    Structure Protection

    Are both these statements true?

    1. In a large storm or in the case of a clogging failure, when water flows over the berm and/or low point of the facility, it will NOT damage any structures downhill AND
    2. In a large storm or in the case of a clogging failure, water will NOT back up to flood nearby uphill structures.

    Yes | No    

    Both conditions must be true to answer Yes. If only one condition is met, answer No.

    Answer No if your site looks like either of the two graphics in A or B or if your LID facilities will drain over a public sidewalk. A No response means you need an area drain or similar overflow structure, which costs more to construct and maintain, so, consider alternative locations where both criteria can be met, although this may not be possible to avoid. Stormwater facilities that might drain over public sidewalks will always need an area drain.

    The way a site slopes (retrofit) or the way it will be sloped (development or redevelopment where sites will be re-graded) will impact the answer to this question. For instance, a rain garden located behind a house where the house is downhill of the rain garden  -- even if the rain garden is located 10’ from the house and meets other setback requirements – may flow down the slope and into the house during a large storm. In this and other possible cases, the answer is “No”.

    Even when horizontal setback and vertical separation criteria are met, a rain garden could be placed in such a way that damage to a house or other structure is possible. “A” in the graphic above illustrates the first question of whether water may flow downhill to damage a structure. “B” illustrates how finish grades could cause water to back up against a structure and cause damage.

     


    Final Step - Site is Unsuitable

    Floodplains, wetlands, riparian and buffer areas, habitat areas and septic systems are not suitable locations for any rain garden configuration. Consider using alternatives to rain gardens to manage stormwater.

     


    Final Step - LID 1.04 Lined filtration rain garden

    LID 1.04

     


    Protect Groundwater Resources

    To ensure adequate water quality treatment, healthy soil infiltrating not too fast and not too slow is needed for a depth of 18”. When the infiltration rate of your native soil exceeds 12 inches/hour, groundwater resources may be at risk, so native soils should be amended or replaced for a depth of 18”. Amending soils to drain slower can be tricky and potentially expensive. Many designers send a sample of the native soil to a laboratory and request a “recipe” of what to mix in and in what quantities. A more cost-effective approach to lab testing is to replace the first 18” of native soil with an engineered soil mix. (See “Amended Planting Soil for specifications on materials, mixing and placement.)

    Next »

    Amended Planting Soil Specifications

    Amended planting soil (aka engineered soil, 3-way mix, bioretention soil mix) is a mix of loamy soil, gravelly sand, and compost. There are a number of landscape material suppliers in Oregon that can provide you with a suitable amended planting soil (aka 3-way mix, bioretention soil mix). If your supplier has a mix that meets the City of Portland’s requirements, this mix will be equivalent to the mix that meets the specifications below. If your supplier does not have a mix that meets the City of Portland’s requirements, share the following specifications with your supplier to see if they have an equivalent product.

    Compost

    Care should be taken to ensure that compost is clean and free of weeds, pollutants, or other deleterious materials that may impact plant health and water quality.

    Organic compost should have the following properties:

    • Weed seed and pollutant free.
    • 100% should pass a 1/2-inch screen.
    • pH between 5.5 and 7.0. If the pH isn't quite right, it may be lowered by adding iron sulfate and sulfur or raised by adding lime or recycled, ground gypsum board.
    • Carbon nitrogen ratio of 35:1.
    • Organic matter content between 40 and 50 percent.
    • Fully composed. Earthy is good. Avoid compost that smells like ammonia.

    Organic compost may consist of the following:

    • Mushroom Compost. The used bedding material from commercial mushroom production.
    • US Compost Council Seal of Testing Assured (STA) compost. Visit the U.S. Composting Council site to find a participating supplier near you. The STA program is no guarantee of quality, only that the compost has been tested and those test results are available for the designer’s review.

    Organic compost may NOT be:

    • Composted Yard Debris. Excessive pollutants, mostly herbicides, pesticides, and fertilizers, have historically been found in these materials. “Cides” can kill beneficial soil life, reduce stormwater benefits, and increase maintenance.
    • Peat Moss. Peat moss is extracted from wetlands; this has negative impacts on the watershed from which the peat moss was removed.

    Gravelly Sand

    Gravelly sand should be free of organic material, contaminants, and hazardous materials, and should conform to the following gradation, which you can compare against the gradation provided by your quarry’s material:

    U.S. Sieve Size Percent Passing
    2-inch 100
    3/4-inch 70-100
    1/4-inch 50-80
    No. 40 15-40
    No. 200 0-3

    Mixing

    Mix soil and amendments to a homogeneous (i.e. all the same) consistency. Do not mix compost, sand, and native soil in the rain or wet conditions. Even in dry weather, soils and amendments themselves should not be overly wet.

    Storage

    Store stockpiles of organic soil mix in a manner that prevents them from becoming wet from rain, stormwater runoff, or other sources of water, or contaminated by fine soil or other undesirable materials. All stockpiles of mixed soil material should be protected and covered.

    Placement

    Place amended soil mix in thickness (i.e. lifts) between 9 and 12 inches in loose thickness. Spray water over the entire lift to accelerate settlement. After all lifts have been placed, grade soil to finish grades as specified on the plans and spray a final time to ensure that soil will not slump to an unacceptable finished grade after the first few rainfall events. Do not over compact soil mix with mechanical equipment after placement; following the construction steps above, soils have already been water compacted.

    Engineered Soil Mix

    Engineered Soil Mix (aka 3-way mix or bioretention soil mix) should have the following properties:

    • Free of contaminants & hazardous materials
    • 60% Loamy sand
    • 40% organic compost
    • Organic content matter from 8-10% by weight
    • Cation exchange capacity (CEC) greater than 5 milliequivalents/100 grams of dry soil
    • 2 – 5% mineral fines
    • Conform to the following gradation:
    U.S. Sieve Size Percent Passing
    3/8-inch 100
    #4 95-100
    #10 75-90
    #40 25-40
    #100 4-10
    #200 2-5
    • Minimum long-term hydraulic conductivity of 1 inch/hour per ASTM D2434 at 85% compaction per ASTM D2668
    • Meet specifications above for organic compost, mixing, storage, & placement.

    Post-Construction Facility Infiltration Testing

    To test a recently constructed or existing bioretention facility:

    1. Wet the surface of the rain garden with a sprinkler or hose until saturated.
      • Small rain gardens or cells separated by check dams (<100 square-feet in surface) area can be tested full-scale.
      • Large rain gardens, vegetated filter strips and swales can utilize isolated falling head tests (minimum 2 per 100 square- feet of area). For how to perform a falling head test, see “The Oregon Rain Garden Guide" under Step 3: Assess Soil.
    2. Fill the testing area to a depth of 4-inches and track the time it takes to completely draw down.
    3. Repeat test 3 times. If the water in any of the tests fails to draw down in less than an hour (i.e. infiltration rate = 4 inches/hour), add compost and gravelly sand to the mix and re-till. (If the facility is existing, remove plants before tilling soil, set aside, mix and re-till, then replace plants.)
    4. Repeat this procedure until favorable test results are achieved.

     

     


    Structure Protection

    Are both these statements true?

    1. In a large storm or in the case of a clogging failure, when water flows over the berm and/or low point of the facility, it will NOT damage any structures downhill AND
    2. In a large storm or in the case of a clogging failure, water will NOT back up to flood nearby uphill structures. 

    Yes | No   

    Even when horizontal setback and vertical separation criteria are met, a rain garden could be placed in such a way that damage to a house or other structure is possible. “A” in the graphic above illustrates the first question of whether water may flow downhill to damage a structure. “B” illustrates how finish grades could cause water to back up against a structure and cause damage.

    Both conditions must be true to answer Yes. If only one condition is met, answer No.

    A No response means you need an area drain or similar overflow structure, which costs more to construct and maintain, so, consider alternative locations where both criteria can be met, although this may not be possible to avoid. Stormwater facilities that might drain over public sidewalks will always need an area drain.

    The way a site slopes (retrofit) or the way it will be sloped (development or redevelopment where sites will be re-graded) will impact the answer to this question. For instance, a rain garden located behind a house where the house is downhill of the rain garden  -- even if the rain garden is located 10’ from the house and meets other setback requirements – may flow down the slope and into the house during a large storm. In this and other possible cases, the answer is “No”.

     


    Final Step – LID 1.06 Infiltration Rain Garden with Planting Soil

    LID 1.06

     


    Final Step - LID 1.03 Infiltration Rain Garden with Planting Soil and Overflow Structure

    LID 1.03

     


    Slow Soils

    Is over-excavation to a soil with an adequate infiltration rate possible so that the rain garden can be sized to drain within 30 hours?

    Yes | No

    Many regions of the Willamette Valley, even though surface soils are clay, are underlain by a faster draining gravelly soil. The depth to this soil layer varies. Where this condition is present and excavation to this layer can be achieved in a cost effective fashion and area for a rain garden or stormwater planter is limited, you may choose to replace the slower draining surface soils with an engineered soil mix. This approach could significantly reduce the area needed to infiltrate the required design storm. The design infiltration rate should be based on the slowest infiltrating soil that runoff will pass through. In the example shown here, that would be the “Slower” rate of the engineered soil mix. This rate may be obtained from laboratory testing per guidance under heading “2.2 Phase Two” in the technical memorandum “Bioretention Soil Mix Review and Recommendations for Western Washington”, published January 2009.

    Slow Soils

     


    Final Step - LID 1.05 Filtration rain garden without an impermeable liner

    LID 1.05

     


    Final Step - LID 1.02 Infiltration Rain Garden with Overflow Structure

    • Details as follows:

    LID 1.02

     


    Final Step - LID 1.01 Infiltration Rain Garden

    LID 1.01