I recently read an article from a journal called “Infrastructures”
To quote from the website:
Infrastructures (ISSN 2412-3811) is an international scientific peer-reviewed open access journal published quarterly online by MDPI. MDPI has supported academic communities since 1996. Based in Basel, Switzerland, MDPI has the mission to foster open scientific exchange in all forms, across all disciplines. Our 203 diverse, peer-reviewed, open access journals are supported by over 35,500 academic editors. We serve scholars from around the world to ensure the latest research is freely available and all content is distributed under a Creative Commons Attribution License (CC BY).
The important point is that the article was peer-reviewed, so one can have confidence that their methods and data analysis are in line with current accepted standards.
The article is entitled:
Factors Contributing to the Hydrologic Effectiveness of a Rain Garden Network (Cincinnati OH USA)
Authors: William D. Shuster, Robert A. Darner, Laura A. Schifman and Dustin L. Herrmann
The authors are scientists and post-docs from the EPA National Risk Management Research Laboratory and a hydrologist from the US Geologic Survey Michigan-Ohio Water Research Center.
Why did I read this article – I found it using a Google search meant to find studies of the effectiveness of rain gardens. Once I saw the system they had studied, I knew I had to somehow get through this article, even though my hydrology knowledge can fit in a drop of water. I’m not saying I understood everything in the article, so basically I’m going mostly by the authors’ summaries and interpretations of their data.
The focus of the paper was on infiltrative rain gardens – these are stormwater management practices meant to allow stormwater runoff to be redistributed back into the water cycle, while also detaining excess water so that its entry into the public stormwater system is delayed (thereby not exacerbating peak flow into the system).
I have long hoped that these practices would be used more widely in suburban residential and commerical development, especially infill development – the only kind of development left in most places around where I live. Right now, the “accepted” standard is to dig gigantic holes and bury large plastic chambers with open bottoms (often referred to as Cultechs) which accumulate the stormwater and allow it to infiltrate into the ground underneath and around the chamber. While this is definitely better than allowing the stormwater to go directly into the sewer system, directly into streams, lakes and rivers, or directly onto the neighboring property, it doesn’t allow for evapotranspiration and doesn’t support plantings. In other words, the water is buried - it doesn’t re-enter the water cycle as it would naturally. I have often wondered who decided, and why, that trying to infiltrate stormwater into the subsoil that is present when you dig a 5 foot deep hole should become an “accepted” practice. No offense to engineers, but they need to start thinking outside of the Cultech.
Whenever I’ve broached this subject of encouraging – yes, even requiring – landscaping (aka green infrastucture aka rain gardens) as part of the overall stormwater management practice in new development, it is said by folks like heads of Building Departments or Village Engineers that “rain gardens don’t work”. So I’m trying to find hard data that evaluates whether they work or not, and what factors affect their ability to infiltrate and detain stormwater.
Just a quick reminder of what we’re talking about – the term “rain garden” tends to mean different things to different people. In Stormwater Management Guideline-speak, it would be referred to as a Bioretention Cell.
A bioretention cell (rain garden) is an excavated area that is filled with a specialized soil media and plants. It is designed to temporarily store runoff volume in ponding areas, engineered rooting zone soils and gravel-ﬁlled underlayers. Rain gardens can re-direct excess water into other areas of the landscape, and rain garden vegetation returns water into the water cycle via evapotranspiration. Rain gardens are among the most versatile green infrastructure stormwater management practices: They can be installed in a variety of soil types from clay to sand and in a wide variety of sites. They are also quite effective for removing pollutants through a variety of different mechanisms, including infiltration, absorption, adsorption, evapotranspiration, microbial action, plant uptake, sedimentation, and filtration. Their overall goal is to combine inﬁltration and storage processes to manage at least the smallest and most-frequent storms.
Some important common elements of every rain garden practice:
• The garden is an excavated space that is filled back up with a specialized well-draining soil mixture.
• The space is shaped like a saucer – i.e. it has slightly bermed-up sides all around it so that the middle can fill up with water when it rains
• The depth and square footage of the garden, as well as those of the gravel layer underneath and the ponding area, are calculated based on the amount of rain to be expected versus the rate at which water can infiltrate into the underlying native soil beneath the special soil mixture. In other words, if the underlying soil drains slowly, the depth of the “holding” area needs to be larger. The math is a complex function how much water is coming in versus how much is draining out during the rain event. Eventually the rain will stop, so the underlying soil drainage rate will determine how long there will be standing water the garden. Luckily for us, there are computer programs that model this dynamic process, so you can change sizes and depths to accommodate the needs of your specific site.
• There needs to be a controlled overflow area that is defined within the design in case more rain enters the “holding” area than it can accommodate. Overflow from the rain garden should be directed onto a flow-attenuating surface– eventually this overflow may enter the storm sewer system directly or indirectly.
• The planted surface is covered with a fairly substantial layer of mulch – 3 inches or so – and the type of mulch used is important because you don’t want it to float away or wash away. The mulch is an important component because it prevents weeds as well as helping to retain soil moisture (since most of the time the soil in a rain garden will be dry). The mulch layer also protects the special rain garden soil from getting contaminated with mud or silt that might be present in stormwater runoff.
• There is generally a defined inlet into the rain garden – often a downspout extension is directed into the space or it may receive surface flow (or both). Wherever stormwater enters the garden there should also be some sort of flow attenuation mechanism to keep erosion within the garden to a minimum.
• There are obviously lots of other design considerations for rain gardens that you want to be ornamental and not just for mall parking lots. Like the space should be shaped so that water flows into the deepest parts first so that it is maximally effective. And often dry river beds are incorporated into the design so that when its “empty” it still has some structure. Sometimes weirs are also included to increase the ponding volumes and to manage sloped areas.
• When done well, a rain garden becomes an element of the landscape design that contributes to the overall beauty of the space as well as allowing stormwater to re-enter the water cycle in a natural manner without causing erosion or flooding the neighbor’s yard.
Each water-managing element contributes to the overall effectiveness of the rain garden. For example, the selection of rooting zone soil is a key part of the rain garden design process, as it regulates the movement of runoff volume into the gravel drainage layer. If the material is too permeable, retention time is short, possibly leading to immediate outﬂow conditions and adding to the stormﬂow burden in the sewer system. Alternately, if the soil proﬁle has low permeability, then drawdown times are increased, predisposing the rain garden to an overﬂow condition.
Due to mulching and washing through of organic matter and soil particles, and development of soil biotic communities, the soil proﬁle in a rain garden is in a constant state of development, inﬂuencing soil hydraulics.
Plants are integral to the success of rain gardens because their roots improve soil structure, thereby increasing infiltration rates. Plants used in rain gardens must be able to withstand widely varying soil moisture conditions, since rain gardens are often dry for long time periods, punctuated with periods of temporary standing water.
Back to the Research Paper
For this particular article, the authors followed a two-tier rain garden practice in Cincinnati OH for 4 years. The greater Cincinnati area has a humid continental climate pattern with approximately 40 inches of precipitation annually and average daily high temperatures of 28 degrees F in January to 75 degrees F in July.
Description of the site and rain garden network: Note that in many ways this site is the worst case scenario for building a rain garden.
• The area made into an infiltration garden used to be an asphalt parking lot associated with a residential apartment complex
• It is a two-tier system that receives run-off from a hillslope
• It accepts runoff from a roadway drainage system, from the forested hillside and from the asphalt parking lot at the southern end of the catchment
• All of these ﬂows combine and are piped to an inlet in the upper rain garden. As runoff volume ﬁlls the upper rain garden, if storage capacity in the upper rain garden is ﬁlled, the excess drainage volume is conveyed to the lower rain garden.
• If the capacity of the lower rain garden is exceeded, excess drainage volume is conveyed to the centralized combined sewer collection system that runs along the adjacent street
The rain garden system was built fall 2010 to spring 2011. It consists of of an upper rain garden (4,300 sf) and a lower rain garden (3,200 sf), and drains an area approximately 96,875 sf (2.2 acres) in extent. Each garden is bermed at its borders with the perimeter in turf slopes. This creates a bowl shape that has considerable surface storage capacity of ~9,430 cu ft and ~8,475 cu ft, for upper and lower gardens, respectively.
The rain garden soil proﬁle is composed of a 2 – 4 inch surface layer of chipped hardwood mulch placed over a 16-24 inch layer of engineered soil (texture: loamy sand in the upper garden, sandy loam in the lower garden)
The drainage layer is 14 inches of #57 gravel aggregate wrapped with a geotextile fabric
The native soil under the gravel is a very-slowly permeable, cohesive silty clay subsoil with trace shale parent material and limestone fragments.
Each garden includes PVC pipe underdrains that are wrapped in geotextile fabric and bedded into the gravel layer, and routed to drop box junctions. The upper garden drainage is conveyed along a pipe to the lower garden inlet. Lower garden underdrains are routed to its own drop box, and ﬂows from this box are conveyed to the city combined sewer collection system.
Each garden was planted with generalist (drought- and ﬂood-tolerant) perennials and grasses (plant list shown at the end).
Summary of Key Findings:
Based on 233 monitored warm-season rainfall events over 4 years, nearly half of total inﬂow volume was detained, with 90 percent of all events producing no ﬂow to the combined sewer.
Conclusion: 90% of all rainfall events were fully detained in the gardens
For a storm event that drove the rain gardens to release ﬂow to the sewer system (10% of all events), we found that the ﬂows into the local combined sewer system were delayed off-peak for an average of 5.5 h.
Conclusion: when rain garden capacity was exceeded, peak flow into the sewer system was delayed to avoid adding to the immediate burden on the sewer system cause by the rainfall event
With the exception of areas immediately around the inlets, the initial plantings quickly established in the construction phase from 2011–2012, with canopy coverage in 2016 (4 years in to the operational phase) estimated at 97%. Although the thick surface mulch layer likely restricted evaporative loss, the amount of transpiration may have increased due to increased vegetative cover and presumably increased removal of soil moisture through likewise expanded root systems. Our analysis suggests that total event rainfall depth (an input) and evapotranspiration (a loss) are primary factors regulating ﬂows through the rain garden network.
Conclusion: Chosen plants need to establish quickly because they are one of two major factors in regulating flow through a rain garden
We found that events with the largest ﬂows to the combined sewer system had high total rainfall depth delivered over longer durations (i.e., 24 h). This suggested that average event intensity was not as important as total event rainfall depth.
Comment: good to know that the high intensity events were fully detained
The infiltration rate of the rain garden is four-times greater than that of the surrounding turf areas.
Comment: this is why rain gardens are better than grass! especially sloped areas of grass.
Soil proﬁles developed over time, and the stratiﬁcation of the surface mulch layer was similar for both gardens. Serial, bi-annual mulching (2012, 2014) led to the development of a pronounced organic horizon in both gardens, which we attributed to the hardwood-chip mulch composting in place. Over the ensuing six years since construction, the surface horizons in both gardens stratiﬁed into the coarse, newer mulch layer that comprises the Oi horizon, which transitioned to the ﬁner, older layer of organic matter that deﬁned an Oa horizon. By 2016, the total organic layer thickness ranged from 4 to 13 cm, and 7 to 25 cm in the upper, and lower gardens, respectively.
Conclusion: Soil structure improved over the years due to mulch composting in place. Also, the garden was only mulched once every two years in the beginning. This probably helped to allow plants to spread and fill in as they became established.
We were particularly surprised that there was no evidence of degradation in upper garden inﬁltration rates, where the mass of sediment delivered ranged between 0.1 to 56 kg with a median of 8 kg per event.
Overall, the upper rain garden acted as a ﬁne sediment ﬁlter, protecting the lower garden from sedimentation, such that the study-wide, event-wise maximum suspended sediment load into the lower garden was only 2 kg. This 75% decrease in ﬁne sediment loading is in agreement with other ﬁeld studies which reported 68 to 90% reductions in suspended sediment loads in networked rain gardens. Jenkins et al. observed that although the texture of rain garden surface soils was changed by settling of ﬁne sediments over an eight-year study period, inﬁltration rates did not change. Taken in the context of the present study, the speciﬁc composition and thickness of the surface mulch layer may regulate the impact of sediment load on rain garden hydrology. Based on our data, we speculate that sediments were well-dispersed in the vicinity of the inlet, and ultimately incorporated into the thick organic surface soil, where their impact on inﬁltration rate was minimized.
Conclusion: The upper garden acted as a sediment filter for the lower garden. It seems that the mulch layer also may regulate sediment load. Bottom line: whatever sediment got in didn’t degrade infiltration over time.
From a practical standpoint, the event peak depth (via crest stage gauges) was always lower than the maximum freeboard depth in either rain garden; total inﬂow volume for any event was insufﬁcient to ﬁll either rain garden. This suggests that a smaller proportion of each rain garden was active in inﬁltration and drainage processes. Given the amount of unused surface area (and hence retention capacity) in both rain gardens, future outﬂow events in this network may be better mitigated by increasing the usable area. Some practical approaches that may be generalizable to other rain gardens include: engaging the unused network storage volume via ﬂow-spreaders; facilitate movement of water to the perimeter by re-grading the gardens to create a slight slope toward the outer perimeter of each garden; and limiting the drainage area of underdrains to a close proximity near the inlet, forcing lateral water ﬂow (fully leveraging subsurface storage) once the maximum vertical ﬂow rate is attained.
Conclusion: Their data showed them that only a portion of their rain gardens were “active” in the infiltration and drainage process. Also, they never overflowed their banks. They suggest some practical additions to rain garden design that would allow more of the garden’s area to be active, all directed towards filling the garden up more effectively (flow-spreaders; grading toward the outer perimeter; limiting under drains to closest to the inlets to maximize storage within the gravel underlayer).
Although monitoring of volume reduction ended in fall 2015, ongoing 2016 measurements of soil structural and hydrologic characteristics indicate that soils were overall less compact, and had maintained or increased hydraulic conductivity. Given no other changes in the network, these measurements indicated that retention capacity and the overall operational dynamic of this rain garden network is stable. Retention of half of total inﬂow volume across four years of contrasting rainfall patterns is encouraging news for wastewater management with inﬁltration-type stormwater control measures.
They work! and they’re stable!
Rain gardens can be beautiful - install more of them people!