HYDROLOGIC IMPACTS OF RETENTION LAYERS WITHIN EXTENSIVE VEGETATED ROOF ASSEMBLIES

G. Frizzi (University of Toronto) & J. Drake (Carleton University)

ABSTRACT

CONTEXT

Natural environments that aid in the cycling of water are being depleted due to urbanization, and without the infiltration of precipitation, cities have been subjected to excessive flooding (Montalto, et. al., 2007). With limited horizontal space, city planners are looking vertically to implement green infrastructure (GI) practices to regain control of the stormwater. In high-density urban environments, green roofs (GR) imitate predevelopment hydrology by allowing for evapotranspiration and infiltration. Extensive green roofs consist of shallow growing media (<6 inches deep) and vegetation and are lighter weight compared to deeper intensive GR (CVC & TRCA, 2010). Traditional extensive system designs only include vegetation, substrate-based planting media and drainage layers (Hill et al., 2017; Carson et. al., 2013; Hakimdavar et. al., 2014). The vegetation cools the surrounding areas via evapotranspiration and provides habitats for wildlife (Berndtsson, 2010; Vacek, Struhala & Matejka, 2017). By capturing the stormwater in its pores, the planting media alleviates urban flooding by delaying its entry into the sewer systems and promoting vegetation uptake and evapotranspiration (Carson et al., 2013; Montalto et. al., 2007; Hakimdavar et. al., 2014). Despite the simple design of traditional GR systems, the planting media layer can be too heavy for roofs that were not originally designed for GI and may not provide enough water holding capacity to satisfy local stormwater codes. In response, vegetated roof assemblies (VRA) are replacing the planting media layer with manufactured lightweight soilless materials that aim to increase the water holding capacity, retention, and detention of green roof systems.

PURPOSE

Despite the hydrological purpose of the new manufactured retention layer alternatives, much of the current research focuses on overall GR system functions like retention (Carson et. al. 2013; Arkar et al 2019), peak flow reduction and delay (Hakimdavar et. al. 2014), runoff coefficients (Abualfaraj et. al. 2018), and thermal performance (Vacek, Struhala & Matejka 2017; Kostadinovic et. al. 2022; Arkar et. al. 2019), and does not assess the direct impact of individual system components like detention layers (Rowe & Getter, 2022). The purpose of this study is to assess the hydrological performance of various generic, ultra-lightweight and soilless retention and detention materials used in GR designs for the Toronto, Ontario, climate.

METHODS

The study area is housed in the Green Roof Innovation Testing Laboratory #1 (GRITLab1), located at the University of Toronto (UofT), in Ontario, Canada. The laboratory consists of 0.8 m raised 2.3 m by 1.2 m testbeds with 2% slopes. Ten testbeds were utilized for the treatments, including a stone roof control, a traditional green roof control (GRC), and duplicates of VRAs with fleece, mineral wool without growing media (MWwoGM), mineral wool with growing media (MWwGM), and a combined reservoir-detention system (CRD). Sedum vegetation was chosen because it can withstand extreme weather conditions and high winds found at roof level, is drought resistant and has high coverage percentages (Li & Babcock, 2014). The growing media utilized is an extensive mix that follows the Green Roof Guidelines developed by FLL (2018). The fleece consists of fabric made of recycled mixed fibers that are needle punched and nonwoven. The mineral wool is a compact felt composed of long rock mineral wool fibers. The CRD system utilizes a honeycomb reservoir that is made up of vertically oriented fused 10 mm diameter polypropylene tubes, and a polyester fabric made of vertically orientated polyester thread between two knit layers of tightly woven polyester fabric.

A tipping bucket rain gauge device with a 6.28 mL tip threshold (Model TB6, HyQuest Solutions) is installed under each testbed’s drainage point to track the amount of water discharged. The tip frequency data is recorded via Onset HOBOware data loggers. There is a weather station located at GRITLab1, which is used to gauge the relative climatic conditions (temperature, ^oC and precipitation, mm). The United States Environmental Protection Agency’s Stormwater Best Management Practices Monitoring Manual recommends quality assurance and quality control (QA/QC) techniques to validate the rainfall data by collecting measurements from two other rain gauges (US EPA, 2002). The two weather stations selected are located at UofT GRITLab2, roughly 260 meters northwest of GRITLab1, and the Environment and Climate Change Canada (ECCC) ‘Toronto City’ station, located roughly 950m north of GL1 (Government of Canada, 2022).

Data analysis entailed weekly compiling of weather and tipping bucket raw data and hydrologic performance calculations on a rain event basis. Rain events were defined when more than one tip (>0.2 mm) was recorded, and discharge from the grey roof was also observed. Rain events lasted until there was >1 hour between tips, and subsequent events were combined if the discharge from any of the testbeds spanned across the precipitation events. Resulting testbed discharge events started from the first tips following the rain event and ended when >1 hour was seen between tips. Event-based data includes rainfall depth (mm), peak rainfall (mm/min), total discharge (L), and peak discharge flow rate (L/min). The hydrologic performance parameters calculated for the VRAs include discharge delay from the onset of precipitation (min), reduction of rain peak flow rate (%) and retention (%) calculated by comparing the total discharge of the testbed to the amount of rain the testbed experienced. Significance testing across treatment types was conducted using the Tukey HSD test provided by R-coding software (R Core Team 2021).

FINDINGS

Over the course of the monitoring period (July to November 2022) a total of 17 events resulted in a cumulative rainfall amount of 220 mm. Of these events, five were classified as small (0.2-4.8 mm) with an average size of 3.2 mm, nine medium (5-20 mm) averaged of 10.4 mm, and three large (>20 mm) averaged of 36.6 mm, all of which fell under the 2-year storm line on local Toronto IDF curve. Discharge happened from all treatment types during all the large events and two of the medium events that were not the largest storms in the medium category but occurred less than 6 days after the previous event, and therefore were impacted by previously saturated conditions. The fleece VRA produced discharge most frequently (44% of the time), followed by MWwGM (41%), GRC (35%), CRD (35%) beds and MWwoGM (26%).

The stone ballast roof was found to be an effective system for stormwater control by retaining 105 mm, or 48%, of the received rainfall. The VRAs significantly (p < 0.05) improved retention, retaining an additional 47% for the MWwoGM, MWwGM, and CRD, and less significantly (p < 0.1) for the fleece (44%) and GRC (46%). On an event size basis, the VRAs completely retained all of the small and the majority (96%) of the medium events. Comparing the large events to the medium, the GRC and fleece beds retention capacity was reduced by 28% and the MWwoGM, MWwGM and CRD systems’ retention declined by an average of 13%, 9%, and 7%, respectively.

Across all storm events, the grey roof significantly reduced the rainfall peak flows by 71% (0.28 versus 1.78 L/min), and the VRAs provided additional flow reduction of 23% - 27%, with all but the fleece and GRC being statistically significant reductions. All VRAs reduced the rainfall peak flows by nearly 100% for the small and medium-sized events, For the large events, rainfall peak flow reduction declined by 25% (GRC), 19% (fleece), 11% (MWwoGM), 7% (MWwGM) and 7% (CRD). Discharge from the VRAs was also delayed by an average of 2.4, 5.7, 8.7, 6.5 and 9.6 hours from the onset of rainfall for the GRC, fleece, MWwoGM, MWwGM and CRD systems, respectively.

SIGNIFICANCE

Development of urban areas has gone too long without respecting the natural environment and has led to harmful impacts in these areas – such as extreme flooding, urban heat island effect, water and air pollution, and habitat disturbance. To reinstate the relationship between natural cycles and urban communities, buildings are being retrofitted with vegetated roof assemblies. Without the regulation of legislation codes and research to provide building and climate-specific insight on which systems would be most suitable, various VRA layering profiles have been implemented. After one growing season of comparing the hydrological performance of multiple VRAs, this study concluded that while being the lightest of all the systems, solely lightweight, manufactured retention layer VRAs (fleece and MWwoGM) provided nearly equivalent stormwater benefits compared to roof assemblies with growing media (GRC, MWwGM and CRD). Due to the additional reservoir detention layer, the CRD system hydrologically performed the best with one of the greatest retention levels and the greatest discharge delay and peak flow reduction. As this is an ongoing study, data will be continuously collected over the dormant season and an additional growing season to discover how the performance results change with bed age and climate.

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