A Pilot-Scale Evaluation of Greenroof Runoff Retention, Detention, and Quality
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- Buccola, N. & Spolek, G. Water Air Soil Pollut (2011) 216: 83. doi:10.1007/s11270-010-0516-8
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Vegetated roofs are becoming more commonly deployed as a means of mitigating stormflow in urban areas. A greenroof performance comparison of stormwater runoff has yet to be conducted with controlled rain events and quantifiable antecedent soil moisture. This study aimed to investigate the rainwater management provided by various greenroof design schemes. Runoff retention, peak flow lagtime, conductivity, and pH of ten different small-scale greenroof schemes were observed and analyzed under repeatable rain simulations in a pilot-scale study. Sedum rupestre Angelina, Sedum hispanicum, Trifolium repens (white clover), Trifolium pratense (red clover), Vinca major (Big-Leaf Periwinkle), and Lolium multiflorum (ryegrass) were grown in the same type of soil media but separate 2′ × 2′ trays at depths of 5 cm and 14 cm to observe how soil depth and root zone development affects stormwater flow through for each plant type. Results showed that increased green roof soil depth improved water retention and runoff lagtime; the effect of plant type was mixed and inconclusive. Runoff conductivity test results depended primarily on soil depth and the existence or absence of vegetation. Testing results show that pH normalization provided by a greenroof does not depend significantly with the substrate depth.
KeywordsBest management practicesLow impact developmentStormwater managementRunoffNonpoint source pollutionGreenroofsEcoroofs
Increasingly, building owners and city planners are turning to vegetated roofing systems, commonly known as greenroofs, as a means to mitigate stormflow in urban areas. As stormwater retention increases for a given management scheme, the amount of stormwater load on municipal sewer systems decreases. With many older cities that have combined sewers, a stormwater surge during an intense rain event could lead to an overflow of untreated sewage into receiving water bodies. While conventional roof runoff may contain heavy metals (e.g., lead, copper, or zinc) or toxics (such as polycyclic aromatic hydrocarbons) that may be detrimental to water quality, widespread usage of retention devices such as greenroofs can reduce the quantity of water unleashed during a storm, thus reducing the mass loading of any contaminants contained in the stormwater surge. At the same time, runoff detention structures such as subterranean large-diameter pipes or above ground ponds can hold large amounts of water temporarily to reduce the wave of runoff that occurs immediately after a storm event. Greenroofs also detain stormwater by delaying the runoff peak from a storm. By using greenroofs to help lower the peak runoff flowrate for a given drainage area, sewer network design and construction may be more efficient with respect to materials (smaller diameter pipes needed), maintenance (less burden on wastewater treatment plants), and cost.
Many researchers have monitored the rainfall–runoff behavior of greenroofs (DeNardo et al. 2003; Liptan and Strecker 2003; Moran et al. 2004; Whitlow 2006; Carter and Rasmussen 2006). Although the large variation in local climatic conditions, testing period/conditions, growing medium depth/composition, and vegetation coverage/composition make it difficult to compare all of these results side by side, the variation in water retention from these independent greenroof runoff studies mentioned can be compiled and averaged to yield an annual retention rate over wet and dry seasons of roughly 54%. Furthermore, there is a general agreement from the greenroof monitoring literature that greenroofs have a greater ability to retain stormwater during drier conditions and/or lower intensity storm events. Studies conducted by Kohler et al. (2002) and Hutchinson et al. (2003) have also shown that increased substrate depth will improve greenroof runoff retention rates. Teemusk and Mander (2007) also presented evidence that vegetation density can affect the ability of a greenroof to retain rainwater thereby reducing stormwater runoff. Antecedent soil moisture appears to greatly affect the ability of a soil layer to absorb precipitation and delay or reduce storm runoff. Although soil moisture has been monitored by some greenroof monitoring studies (Kohler 2004; Van Seters et al. 2007), it has not been measured explicitly in repeatable experimental hydrologic greenroof monitoring as it has in this current work.
Greenroofs also appear to impact the quality of runoff. Many researchers have found that the phosphorous loadings in greenroof runoff are higher than normal flat roof runoff loadings (Forster and Knoche 1999; Clark et al. 2001; Hutchinson et al. 2003; Van Metre and Mahler 2003; Moran et al. 2004; Hunt et al. 2006; Van Seters et al. 2007). While phosphorus is in an indication of solid particles in solution, electrical conductivity (EC) measures the amount of free ions (salts) in solution and is easily measured in the field. A study by Berghage et al. (2007) showed that greenroof runoff resulted in higher EC than did nongreened roof runoff. High runoff conductivity can limit the amount of habitat-forming plant species in a water body and may indicate that higher concentrations of more toxic trace metals may be present. High free ion concentration in water can also be a nuisance to household plumbing and lead to calcification of pipes in boilers. In areas with acidic rainwater (pH between 4 and 6), greenroofs have been shown to normalize and raise the pH of runoff (pH between 6 and 8; Berghage et al. 2007; Teemusk and Mander 2007). The buffering of roof runoff in urban environments could be an important improvement as many aquatic species are intolerant of a pH of 5.7 or less. With an increasing number of municipalities looking to reuse water, opportunities for on-site water pretreatment such as greenroofs are becoming more popular by necessity.
One of the primary reasons that many designers, architects, engineers, and planners build greenroofs is to decrease storm flow, thereby improving water quality in receiving waters. To achieve these design goals, the optimum greenroof soil depth and plant type(s) should maximize retention without sacrificing water quality. Thus, the need to better understand the behavior of greenroof runoff quantity and water quality as a function of plant type and media depth is the premise of this body of work.
2 Materials and Methods
A range of greenroof plant types, soil depths, and storm conditions were tested and analyzed in order to investigate their potential to reduce (retention) or delay (detention) stormwater flows and nutrient/contaminant loadings from urban areas. This study focused on varying plant type, growing media depth, and storm intensity. Antecedent soil moisture, air temperature, relative humidity, rain water temperature, rainstorm flowrate, runoff temperature, and runoff flowrate were monitored for each plant type.
The greenroof wind tunnel was designed to accommodate two, 0.6 m × 0.6 m square trays side by side at a 0.02° slope (1 cm in 55.4 cm) to facilitate drainage. The trays were constructed of steel sheet metal with 9.2 cm-high sides, expandable to 18.5 cm high during deep-soil experiments. Tray expansions were attached to the bottom tray using fasteners, silicone sealant, and a layer of rubber to limit leakage around the extension interface.
A drainage layer was placed in the bottom of each plant tray (Henry DB-50 greenroof drainage layer material) followed by the growing media (Pro-Gro Mixes’ Intensive Greenroof Blend consisting of pumice, sand, and two types of compost). The first phase of experiments employed a growing media depth of 5 cm while the second phase was conducted using a soil depth of 14 cm. A particle size analysis was conducted to reveal that the growing medium contained on average 11% gravel, 78% sand, and 11% silt and clay by weight.
The plants chosen for the experiments were as follows: Sedum rupestre Angelina, Sedum hispanicum, Lolium multiflorum (perennial ryegrass), Trifolium repens (White Clover), Vinca major (Big-Leaf Periwinkle), and a proprietary wildflower seed mix (Ed Hume brand: “Wildflower Meadow Mix”) that consisted primarily of Trifolium pratense (Red Clover) at the time of testing. All plants were grown and maintained in a commercial greenhouse on campus, and temporarily moved to the test lab for no longer than 1 week at a time. T. repens, L. multiflorum, and T. pratense were planted from seed while Catharanthus roseus, S. rupestre Angelina, and S. hispanicum were planted as starts from a local nursery. Upon the completion of the 5 cm soil depth tests, V. major, L. multiflorum, and S. hispanicum were transplanted and laid atop an additional 9 cm of growing media for a total soil thickness of roughly 14 cm per tray. T. pratense was reseeded at a 14 cm depth while S. rupestre Angelina remained at the original 5 cm depth due to its lack of development and vegetative coverage.
Experimental storm sizes
Heavy (n = 36)
Medium (n = 15)
Average flow (L/s)
Standard deviation (L/s)
Total volume (L)
Effective area (m2)
Prior to testing, each tray of plants was weighed in order to determine its relative soil moisture in relation to the residual soil moisture. It was assumed that the weight of the dry plant material (woody matter) was negligible compared to the water held in them. This follows a general assumption used in gardening literature that plants contain roughly 75–90% water (Tenenbaum et al. 2001; Champagne et al. 2003). A digital scale was modified to interface with an Agilent 34970A data logger. The weight of the water in the collection vessel could then later be post-processed by taking the difference of each set of adjacent weight recordings to yield a mass flowrate for each time increment.
Water conductivity was measured with an Orion conductivity/salinity meter Model 140. Water pH was measured with an Orion Model EA 920 m and Orion 9157BN probe which was calibrated before each day of testing. For both conductivity and pH, tap water was measured as the control specimen on each testing day and compared to greenroof stormwater runoff.
3 Results and Discussion
A total of 51 runoff quantity tests were completed (27 at 5 cm soil depth and 24 at 14 cm soil depth) yielding measurable water retention characteristics for each greenroof. For the majority of these water quantity tests, runoff conductivity and pH were also measured immediately afterward. As measured by the soil supplier, the porosity of the soil used in all experiments was 0.73, the dry bulk density was 0.74 g/cm3, the saturated density was 1.28 g/cm3, and the density at field capacity was 1.22 g/cm3.
3.1 Runoff Detention/Retention
Overall average runoff retention and lagtime values
Overall average retention
Overall average lagtime (min)
5 cm depth
14 cm depth
5 cm depth
14 cm depth
Heavy (34 cm/h)
Medium (3.0 cm/h)
Unexpectedly, the test-averaged runoff data from the heavy storms displayed a clear peak in flowrate before the end of the applied rain for both 5 and 14 cm depth tests. This phenomena is not fully understood, but may be explained as follows: As the experimental rain covered approximately 40% of the greenroof tray area, the runoff flowrate typically increased gradually as the soil under the shower became saturated first. Upon the saturation of this area, there may have been lateral water movement into unsaturated areas may have resulted in lower runoff flowrates before the end of the experimental storm.
Correlation matrix for measured variables
Extrapolation of retention and detention results presented for this pilot-scale study to a full-scale greenroof requires examination. Retention measured at the pilot-scale, as given in Table 2, was comparable to what would be expected for an actual greenroof because the soil depth tested is typical of most greenroof installations. Retention was normalized by the rain flux, in volume per unit area, so it is reasonable to expect a 5-cm soil depth to show 20–50% retention and a 14-cm soil depth to show 55–65% retention. Detention, on the other hand, as indicated by the lag time between the rainfall and the subsequent discharge of that water influx, depends both on soil depth through which the water drains and upon the transverse dimension of the greenroof, the distance that the discharge must travel to the drain. In pilot-scale tests reported here that transverse characteristic length was about 0.5 m substantially less than the 10–20 m for a full-scale greenroof. Published field studies of full-scale greenroofs have shown lag time to be on the order of an hour or more (Spolek 2008) rather than on the order of minutes as given in Table 2. The detention results reported here, therefore, indicate the relative change rather than the absolute value.
Average retention for different plant types
L. multiflorum (ryegrass)
S. rupestre Angelina
3.2 Water Quality
Multiple linear regression with conductivity as the dependent variable
What might be hoped for, a single combination of soil depth and plant type that yields the greatest runoff retention and detention while simultaneously maximally reducing conductivity and increasing pH, was not discovered. As with most engineering designs, a compromise solution must be reached. Since plant type does not seem to have a significant effect on either discharge quantity or quality, plant selection can be based on other factors, such as energy transfer, plant hardiness, and aesthetics. For many of the plant types tested, summertime irrigation would be necessary for sustenance, which adds to both first and operating costs for the greenroof.
Soil depth is a design decision that will affect overall greenroof performance. While additional soil depth will yield greater retention, it will also yield a higher conductivity (a potential indication of suspended solids) in solution. This is detrimental to water quality. As plants become mature and roots take hold of the soil, the amount of solids washed away may be lower with older plants. As different plants age, their root structures may age differently. Due to their continued growth every year, it is reasonable to assume that the roots of perennials would provide more sediment containment than annuals.
These pilot-scale greenroof tests have shed light on the potential water retention and runoff water quality differences between plant type in both 5 and 14 cm depth vegetated roofs. Plant types tested in this study displayed positive correlations with substrate depth, rainwater detention, and rainwater retention. This agrees with previous research: an increase in greenroof growing media depth leads to higher runoff retention rates and extended runoff lagtimes. A deeper soil depth resulted in a greater increase in greenroof runoff lagtimes during medium storms (30 mm/h) than during heavy test storms (340 mm/h). T. pratense tests resulted in the highest average runoff lagtime (detention) when compared with other plant types under similar conditions. The key water quality findings from these experiments have led to important conclusions: Measured pH increases in vegetated roof water runoff had no dependence on plant type or soil depth. Further, a deeper vegetated roof growing media depth displayed higher runoff conductivity. Finally, these results confirm that a vegetated roof will have lower runoff conductivity when compared to an unvegetated roof of bare soil.