Research Reveals Problems and Potential of Rain Gardens
by Dan Schwartz, Soil Scientist
(Conservation Currents, Northern Virginia Soil and Water Conservation District)
Rain gardens are becoming a popular method of stormwater management both for their relative ease of installation and their environmental appeal as a low impact development practice. Yet little is known about the long-term functioning of rain gardens: several years after construction, do the rain gardens we’ve built filter the volume of stormwater for which they were designed? Are they able to cleanse that water to the anticipated degree?
The Northern Virginia Soil and Water Conservation District attempted to answer these questions by surveying twenty Fairfax County rain gardens. Now completed, this research shows that while many older rain gardens function well, the functionality of others is degraded due to poor design, poor maintenance, poor initial construction or a combination of some or all of these factors.
Stormwater best management practices (BMPs) are highly engineered facilities that are a legal requirement of new development. They treat the additional runoff created by new impervious surfaces such as roofs and parking lots. The rain gardens involved in this study are all stormwater BMPs.
Each BMP rain garden is designed to filter a specific volume of runoff. This volume is calculated based on the size and land use of the rain garden’s drainage area. Each facility goes through the county permitting process before being built to ensure that it will properly filter the new runoff volume for which it has been designed. BMP rain gardens should not be confused with the simple rain gardens that homeowners can design and install themselves on their own property. Homeowner rain gardens are relatively easy to construct and depending on their size or location may not need to be permitted.
How rain gardens work
All rain gardens work on the theory that infiltrating runoff through soil will filter out pollutants as well as lead to groundwater recharge.
From the surface down, BMP rain gardens consist of a thin layer of hardwood mulch, a thick layer of engineered soil, and a layer of gravel. Either the rain garden surface is depressed below the surrounding ground or an earthen berm is built around the rain garden to create an area where water can pool. This surface is planted with species that can tolerate flooding. A perforated PVC drainpipe typically runs within the gravel layer and an overflow structure allows excess runoff to escape the rain garden.
Rain gardens are placed in depressions on the landscape that naturally capture stormwater. During storms runoff flows into the rain garden where it puddles on the surface. An overflow structure determines the maximum volume of water that can be held by the rain garden. If this volume is exceeded, the excess stormwater flows out of the garden via the overflow structure.
Rain garden plants take up stormwater and pollutants such as heavy metals (such as copper, lead and zinc) and nutrients (nitrogen, phosphorous and potassium to name a few). The thin mulch layer and the engineered soil allow for quick infiltration of the stormwater. The mulch layer is exceptionally good at filtering out heavy metals from the stormwater. The soil layer filters heavy metals as well as nutrients, oil, grease and other pollutants.
Filtered stormwater percolates down to the gravel layer. The gravel stores some of the stormwater so that it may continue to flow downward through the natural soil to the water table. The remaining water is re-released into the stormwater system via the underdrain if present.
Evaluating the Rain Gardens
The district analyzed twenty BMP rain gardens throughout Fairfax County. With one exception, all rain gardens were two or more years old. A variety of data was collected from each rain garden to answer two general questions: does the rain garden still infiltrate water adequately, and does the rain garden meet the physical criteria set forth in its original design? If these two criteria are not met, the rain garden is not performing as intended.
If the soil does not adequately infiltrate water, most runoff—if not all—will accumulate on the rain garden’s surface until it can pass, untreated, through the overflow structure. If the rain garden and its structures do not conform to the original design, for example if the overflow structure has been set at the wrong elevation, the rain garden may treat less than the volume of stormwater for which it was permitted.
Infiltration Rates and Soil Texture
To determine infiltration rates and factors affecting infiltration, we performed infiltration tests and analyzed the soil texture (percentage of sand, silt and clay), density and structure (the arrangement of the soil particles). To achieve high infiltration rates, current Fairfax County standards (enacted after the rain gardens in question were built) call for a soil mix with greater than 71% sand and less than 3% clay by weight. Ponded water must infiltrate within 24 hours and the rain garden must completely drain within 48 hours.
Of the twenty rain gardens examined, three facilities failed infiltration tests. They have an infiltration rate of zero inches per hour, which means these rain gardens are not filtering stormwater. Textural analysis revealed that the soil has much lower percentages of sand and much higher percentages of clay than Fairfax County currently requires. Additionally, the structure of the soil is in poor shape. The natural soil pores, which act as passageways for infiltrating water, are damaged by compaction in two cases and the formation of a thin crust on the surface caused by erosion in the third case.
The remaining rain gardens have adequate infiltration rates. However, their soil texture does not meet the county standards. It appears that a well-textured soil with slightly less sand and significantly more clay than allowed by the current Fairfax County standard is still able to quickly infiltrate water. An example would be a loam containing >50 percent sand and up to 15 percent clay. There may be benefits to adding clay; a soil with more fine particles, such as clay or organic matter, can retain nutrients and water better than a very sandy soil. This can help the growth of rain garden plants.
Does the Rain Garden Match the Original Design?
To determine if a rain garden was constructed according to its original design, and thus, is treating the volume of stormwater intended, the district performed a physical survey of each facility. The surface area, ponding depth (depth of pooling area on the rain garden surface), soil depth, condition of the inlet structures, and any other relevant information were recorded. The ponding depth and surface area of a rain garden determine the volume of runoff it holds. The inlet structures ensure that the stormwater can enter the rain garden.
Many discrepancies were found between the original designs and what was actually constructed. Commonly, the constructed rain gardens lack adequate ponding depths or surface areas. Even in a large facility with fast infiltration, stormwater will puddle on the surface—especially during a large storm. If the ponding depth or surface area is less than what is specified in the design, unfiltered stormwater will flow out of the rain garden before the required volume is treated.
Four of the twenty studied rain gardens have a smaller surface area than required by design. However, inadequate ponding depth is the most common problem. Thirteen rain gardens do not have adequate ponding depths. In most cases, the overflow structures are at or near the surface of the rain garden—preventing almost all ponding.
Inlet structures can also limit rain garden function. Inlet structures include grassed swales, cuts in parking lot curbs and stormwater outlets to name a few. If the inlet or landscape grading does not direct water toward the rain garden, stormwater will bypass the facility. Two rain gardens exhibit improperly graded inlets.
The district also identified several rain gardens where the soil depth was less than had been specified in the design. The depth of the engineered soil available within the facility determines the amount of filtering the stormwater undergoes. Soil depth also influences which plants the rain garden can support. Current standards (enacted after most of these rain gardens were constructed) state that rain gardens should have at least 30 inches of filtering soil.
Rain Garden Maintenance
Maintenance of the rain gardens is also an issue. In many cases, homeowners and HOAs do not know how to maintain their rain garden or do not understand its proper function. In one rain garden, the berm has been breached by erosion so that the facility no longer collects water. In several cases, homeowners added extraneous overflow structures to reduce the volume and time of ponding on the rain garden surface. Ponding water within the rain garden may seem like a sign of malfunction to well-meaning property owners, but is actually a vital part of rain garden functioning.
Based on this research, recommendations to help improve rain garden performance in the future can be made. Inadequate infiltration rates in rain garden soils and a failure to build and maintain facilities as designed shows the need for better education of all parties involved: the county site inspectors who ensure that the rain garden is built to code, the contractors who install the rain garden, and the property owners who care for the rain garden after construction.
Educational workshops can be held to instruct contractors and inspectors on the proper design and functioning of rain gardens, possibly leading to certification of the attendees. A list of certified individuals or companies can then be compiled. Detailed construction and inspection guidelines can be created to guide construction in the field, and a post construction checklist can be created to ensure that the finished rain garden meets all design criteria. Information about how a rain garden functions along with a detailed maintenance checklist can be given to property owners to ensure that they can adequately maintain their rain garden.
While many problems were found with the rain gardens studied, pessimism about these facilities is unwarranted. In all but the worst cases, a rain garden that does not meet its design and infiltration requirements will still treat some stormwater. This is true of most of the rain gardens identified in this research. Of the twenty rain gardens studied, twelve can be considered to be providing considerable water quality improvement.
A rain garden that is precisely constructed and maintained is obviously preferable, however, and with a little more education of those involved and a little more time to perfect the craft, it is likely that many of the issues found in this research can be corrected.