Rain Garden Guide - Oregon

Building a Rain Garden: A Step-by-Step ApproachThe following steps will help you assess thecharacteristics of your site so you can design yourgarden to capture and treat water safely and effectively, build it correctly, and maintain it to be ahealthy and beautiful addition to your landscape.STEP 1: Observe and map your siteThe first step is to assess how water flows acrossyour property. We suggest you create a map ofyour site that includes the measurements of allstructures, with arrows to indicate where the waterflows after the rain lands on these surfaces. SeeFigure 5.LOWLOOWOW POINTO 24’22444’Sidewalk1. Walk through your yard and note any obviousslopes or low spots.2. Note areas where water might drain to yourneighbors’ or public property.3. Site your potential rain garden where water flowing into the garden will be higher than wherewater will naturally exit the garden.4. Look for areas nearby where overflow from a raingarden can be absorbed or safely directed intoan approved stormwater collection point (suchas a streetside gutter and storm drain).Determine how much area the rain gardenwill treatNext, decide which impervious surfaces you’d liketo manage using a rain garden. Measure the widthand length of these surfaces and multiply themtogether to give you the square feet of surface areato be treated. See Figure 6 for an example. The rainfalling on the part of the roof outlined in red drainsinto downspout H1 and could be directed into arain garden.Width of Surface Area x Length of Surface Area Area (square feet)An example: roof area draining to downspout H1 is30 feet x 12 feet 360 square feet of roof areaSTEP 2: Determine the location ofthe rain garden45’45’45Go with the ��X10’10 XX1010The easiest place to build a rain garden is relativelyclose to a gutter downspout. If you want to buildonly one rain garden, consider using the downspout that captures the largest portion of the roof.If necessary, you can route water from severaldownspouts to one location. It is also possible tore-hang your gutters to move the downspout to amore appropriate spot.Figure 5: Map of home site indicating measurementsof structures that shed water and the direction thewater flows. (Graphic: EMSWCD)OPEThe Oregon Rain Garden Guide: Landscaping for Clean Water and Healthy Streams

! Note: a rain garden should not be constructedin a location that stays wet throughout the rainyseason, since this is an indication of poorly drainedsoils.Respect the flowRain gardens must be designed to carefully captureand treat stormwater on-site and send the excessoverflow during a large rainstorm off-site, withoutdamaging structures and other property.! To prevent slumping and failing of the following under or immediately adjacent to trees, if diggingwill disturb their rootsDuring very large storms, rain gardens willoverflow because the soil becomes saturated andcannot hold all of the water. Special precautionsshould always be used to route the overflow to asafe location, away from structures, steep slopes,and neighbors’ property. Your rain garden shouldbe at least five feet away from property lines, andthe overflow should not be routed to the neighbor’s property unless it is an approved location,such as a ditch or swale in a right of way.Figure 6: Map ofhome site indicatingsurface area anddirection of water flowto downspout H1.structures, make sure the outer edge of your raingarden is at least: three feet from a sidewalk six feet from a basement two feet from a crawl space or slab ten feet from a retaining wall(Graphic: EMSWCD)LOWLOOWOW POINTO NTT14’14’4! To prevent landslides and surface erosion, donot place rain gardens on slopes steeper than 10percent. If the property does not have adequate flatareas and you are still interested in building a raingarden, contact a licensed landscape professionalor engineer for design modifications to safely storeand route water off-site without damage.10’10030’30220’024’22444’Divert the flowRain gardens are designed to drain water to the soillayers below the garden for treatment. To ensureadequate drainage and treatment while not polluting groundwater, rain gardens should not be placedin the following areas: on top of a septic drain field. When uphill of aseptic system, provide at least 50 feet between arain garden and the septic system. where a seasonal groundwater table is withinthree feet of the bottom of the rain garden (aboutfour to five feet below ground surface) areas that stay wet during the rainy season, suchas wetlands, natural springs, or seeps in soils that don’t have good drainage (minimumof 1/2 inch/hour infiltration) or on bedrock in soils that have been contaminated by chemicals or other toxic 0’X10’10 XX1010LOPEThe Oregon Rain Garden Guide: Landscaping for Clean Water and Healthy Streams7

Regulations, permits, and designmodificationsMeasure the slopeTools needed:Some cities and counties have specific regulationsregarding disconnecting downspouts, routing orpiping water off-site, and setbacks to structures,steep slopes, and property lines. They may alsorequire a permit. Always check with your city’sbuilding or planning department if you live withincity limits; otherwise, check with your countygovernment.Rain gardens may be designed with impermeableliners, rock trenches, and piping to safely routewater away from structures and off-site. These designs would allow you to build rain gardens closerto structures, on steep slopes, in soils that don’tdrain well, and in other challenging situations.However, these designs are beyond the scopeof this guide. We recommend that you contact alicensed landscape professional or engineer forassistance with these alternative designs. Two stakes Survey line or string Line level Measuring tape CalculatorSteps:Measure and calculate the slope of the site to assess how water will be routed to and from your raingarden. It is critical to make sure your property hasa slope of less than 10 percent before constructinga rain garden there.1. Place stakes in the ground at the top and bottomof the slope to be calculated.2. Attach a survey line (or any heavy-duty string) tothe two stakes. Be sure that the line touches theground on the uphill stake. Use the level to makesure the line is level.3. Measure the horizontal distance (along the line)between the two stakes.4. Measure the rise (vertical distance) from theground up to the line on the downhill stake.5. Calculate the slope by plugging your numbersinto the following formula:slope RISEHORIZONTAL DISTANCEx 100%Note: both measurements must use the sameincrements (for example, inches).Example:RISE 18"HORIZONTAL DISTANCE 233"7.7 % Graphic: EMSWCDSTAKEx 100%HORIZONTAL DISTANCE (233”)LINE LEVEL818"233"RISE (18”)STAKEThe Oregon Rain Garden Guide: Landscaping for Clean Water and Healthy Streams

STEP 3: Assess soilThe ability of soil to drain water is one of the mostimportant considerations for understanding thesite and properly sizing a rain garden. An infiltrationand texture test will help you determine the soil’scapacity to absorb and percolate water down intothe lower layers.Testing infiltration1. Dig a test hole in the area where the rain gardenwill be built. Try to site the hole in the middle ofthe planned rain garden. Dig a hole to the expected depth of the rain garden (from grade to the topof the rain garden’s base). Note that ideally, thistest should be done when soils are not frozen andwhen groundwater levels may be highest, such asin the spring.12. Fill the hole with water to just below the rim. Thisshould be the same depth of water expected if therain garden filled to the rim. Record the exact timeyou stop filling the hole and the time it takes todrain completely.3. Refill the hole again and repeat step 2 two moretimes. The third test will give you the best measure of how quickly your soil absorbs water whenit is fully saturated, as it would be during a rainyperiod of the year or during a series of stormsthat deliver a lot of rainfall in a short period oftime. Building a rain garden to handle theseconditions is a way to be sure you will not causedamage to your own or a neighbor’s property.2Photos: Robert Emanuel, OSU4. Divide the distance that the water dropped by theamount of time it took for it to drop. For example,if the water dropped 6 inches in 12 hours, then 6divided by 12 equals 1/2 inch per hour of infiltration. If the slowest infiltration rate measured ofthe three trials is less than 1/2 inch per hour, thenyou should dig another 3 to 6 inches deeper andrepeat the above steps. Repeat this process atvarious depths down to 2 feet, or until you have atleast 1/2 inch per hour infiltration.! Note: Soils with drainage of less than 1/2 inch perhour are not appropriate for rain gardens without significant modifications in design. Consulta licensed landscape professional or engineer forassistance in these circumstances.3The Oregon Rain Garden Guide: Landscaping for Clean Water and Healthy Streams9

Determining soil texture1. Take a handful of the soil you have excavatedfrom your infiltration test. Pulverize it in yourhand and remove any bits of organic matter orobvious rocks.Figure 7a: Conducting a soil feel test by making a ball with the moistened soil, tosee how easily it falls apart as well as what texture it has (sticky, silky, or gritty).(Photo: Gina Emanuel)2. Wet it with a small amount of water and rub itbetween your thumb and index finger. Don’tsaturate it until it is runny mud. You might feelstickiness, grittiness, or smoothness. The grittierthe feel, the more sand is present in your soil.The slicker the soil, the more clay in it. Smoothsoils are sometimes an indicator of a fine silt orloam. Discard the soil.3. Next, take another sample in your hand. Wet ituntil it has the consistency of dough. You shouldbe able to form a ball that holds together withthe soil in your palm. If you cannot get the ball toform, then your soil is very sandy. In most soils,however, you should be able to create a roughball.4. Knead the soil together between your thumb andfingers and attempt to form a ribbon. As youbuild the ribbon, it will either hold together orbreak off. If the soil breaks quickly in the process,then it likely has a high sand content. If the ribbon forms quickly and stays strong, it has moreclay.Figure 7b: Using a soil ribbon to test soil for clay, loam, or sand content. Thelonger and firmer the ribbon, the more clay content in the soil. (Photo: Gina Emanuel)10 If the soil forms a ribbon less than 1 inch inlength before it breaks, the soil is sandy or silty. If the soil makes a ribbon 1 to 2 inches in lengthbefore it breaks, the soil is clayey. If the soil makes a ribbon greater than 2 inchesbefore it breaks, it may not be suitable for a raingarden constructed without professional help,depending on how fast the soil drains during theinfiltration test.The Oregon Rain Garden Guide: Landscaping for Clean Water and Healthy Streams

Interpreting the infiltration testDrainage rateRecommendationLess than 1/2 inch per hourDo not build a rain garden on this site without professionalassistance.Between 1/2 and 1 inch/hourLow infiltration for a rain garden. Homeowners may want to builda larger or deeper garden, or likewise plan for additional overflowduring high-rainfall storms.Between 1 and 1 1/2 inches/hourAdequate infiltration for a rain garden. Plan for sufficient overflowduring high-rainfall storms.Between 1 1/2 and 2 inches/hourAdequate infiltration for a rain garden. Plan for sufficient overflowduring high-rainfall storms.Faster than 2 inches/hourHigh infiltration for a rain garden. Design should feature fewermoisture-loving and more drought-tolerant plants. The rain gardenmay also be sized to hold smaller amounts of water, have a deepermulch layer, or have denser plantings.Building a better soilAmending with compost is recommended toimprove initial plant and microbial health. Ifyour soil has high clay content, soil amendments may be needed to improve conditionsfor good plant health. Sites with very high claycontent and low drainage are usually inappropriate for the types of rain gardens describedin this guide, without significantly altering thedesign for better drainage (such as addingsubsurface drain pipes and drain rock).If you plan to amend the soil, a typical soilmixture contains 20 to 40 percent organicmaterial (compost); 30 to 50 percent clean,coarse sand; and 20 to 30 percent top soil.Note: do not add sand to a highly clayey soil.Soils are commonly amended to a depth of18–24 inches.STEP 4: Determine the sizeof the rain gardenAlways check with your local planning department,public works, or stormwater utility before designing your rain garden. If your local jurisdiction doesnot have a recommended size calculation for arain garden, then we recommend the size of therain garden be at least 10 percent of the impervious surface draining to the garden. Rain gardensshould ideally be between 6 and 24 inches deep.To use this number, your soil should drain at least1/2 inch per hour or greater. Compare the result ofyour soil infiltration test with the table above, fora good idea of how your soil influences the size ofthe rain garden. Rain gardens of 10 percent and 1/2inch minimum infiltration rate should treat a largemajority of storms in Oregon.Using the area of impervious surface that youcalculated in Step 1, multiply this by 0.10 (or 10percent). The result will be the area of the raingarden in square feet. The calculation is:The Oregon Rain Garden Guide: Landscaping for Clean Water and Healthy Streams11

Typical ponding depth6–24 inchesFigure 8: Pondingsurface is denotedby the dotted line.(Graphic: EMSWCD)(Length of surface area x Width of surface area)x .10 total rain garden areaAn example: 30 feet x 12 feet 360 square feetx .10 36 square feet of rain gardenThe more impervious area you want to treat, thebigger your garden. The size of your rain gardenwill also depend on the space available and yourbudget. If you don’t have enough space, you canbuild multiple rain gardens or build a smaller oneand plan for it to overflow more often.Note: Rain gardens should be a minimum of fivefeet wide to accommodate gentle side slopes thatwill host plants and minimize soil erosion.Rain garden depthMost rain gardens should be between 6 and 24inches in ponding depth with 2–4 inches of additional depth for safety. This means that raingardens range from 8 to 28 inches in depth. Thetable below is a general guide for rain garden ponding depths based on different drainage rates. Adddepth where overall size is restricted by setbacks,structures, vegetation, or other obstacles.12Note: remember to account for the addition ofmulch when you plan for your finished depth (see“Mulching” on page 23). For example, if you areadding 3 inches of mulch to your final planted garden and it needs to be at least 12 inches deep, youmust excavate to a depth of 15 inches from grade.RAIN GARDEN DICTIONARY:The size of a rain garden refers to the volumeof water it can hold before the water overflowsat the exit point. This volume is described interms of ponding depth and square feet ofsurface area (depth x width x length).Ponding depth is the depth at which the watercan pond before it flows out of the rain garden(see Figure 8). It is measured from the surfaceof the rain garden at its lowest point to theelevation of the outlet. Rain gardens generally should range between 6 and 24 inchesin ponding depth, adding 2–4 inches of extradepth below the outflow for safety.Drainage rateSuggested rain garden ponding depthBetween 1/2 and 1 inch/hour12–24 inchesBetween 1 and 2 inches/hour6–8 inchesFaster than 2 inches/hour6 inchesThe Oregon Rain Garden Guide: Landscaping for Clean Water and Healthy Streams

Dig safely!It’s the law in the state of Oregon to call the UtilityNotification Center by dialing 811 or 1-800-332-2344before beginning any excavation. The service isfree and convenient for homeowners, contractors,excavators, landscapers, etc. Anyone planning todig must contact the Center at least two businessdays prior to digging, in order for the undergroundutilities to be located and accurately marked usingcolor-coded paint. For more information ondigging safely, visit the OUNC Web site:www.digsafelyoregon.com.Delineate the rain gardenUse a garden hose, string, stakes, or marking paintto delineate the boundary of the rain garden on thesite. Before you dig, be sure to note any existingutilities or vegetation that might be damaged bydigging (see note above).Getting water to the rain gardenFigures 9a and b: Delineating rain garden boundaries with a garden hose (top)and marking paint (above). (Photos by Robert Emanuel, OSU)If not using a rock-lined trench, the outlet of therouted water and inlet of the rain garden should belined with rock (again, 3/4-inch diameter, washeddrain rock or pea-sized gravel) to prevent erosion.In addition, a 4-inch-wide strip of grass could beused to filter and settle sediment from your rainwater before it enters the rain garden.Figure 10: Moving water from a gutter to a gardencan be as simple as a piece of buried 4-inch drain pipe.Note that the pipe is buried at least 12” below thesurface as required by Oregon’s plumbing code. Seenote on page 19.The Oregon Rain Garden Guide: Landscaping for Clean Water and Healthy StreamsPhoto: Portland Bureau of Environmental ServicesYour garden will not actually be managing stormwater if it does not collect rain water from yourhome’s impervious surfaces. That means youmust find ways to get water from your collectionpoints to your garden, sometimes by diggingtrenches, running gutter extenders, or even building artificial streams that run only when the rainfalls. The water may be routed using a pipe, rocktiles, or other hard surfaces, or a small swale (ditch)lined with rock (3/4-inch diameter, washed drainrock or pea-sized gravel). If using a pipe, we recommend a 4-inch diameter ABS.13

Figure 11: Using a rock-lined trench to convey water across a walkway (Photo: Portland Bureau of Environmental Services [BES])Figure 12: Trench drain through a sidewalk at a building in Portland.(P

4 The Oregon Rain Garden Guide: Landscaping for Clean Water and Healthy Streams Introduction Figure 1: Cross-section of a typical rain garden. (Graphic by East Multnomah Soil and Water Conservation District [EMSWCD]) The purpose of this guide The Oregon Rain Garden Guide was written to help Oregonians learn how to design and build rain gar-