Throughout 2020, Geosynthetica has been celebrating Pan-American contributions to geosynthetics, both in terms of practitioners and companies located in the Americans and events held in the Americas. No event series is more influential in this regard than the GeoAmericas series, a quadrennial, Pan-American regional event from the International Geosynthetics Society.
In this GeoAmericas series installment, we publish a green roofs paper from the GeoAmericas 2016 proceedings, which were published by Geosynthetica. The authors profile the impact of different geosynthetics used as drainage layers in vegetated covers (nonwoven geotextile, geomat, and geospacer) and the resulting water retention performance.
1. INTRODUCTION TO GREEN ROOFS & GEOSYNTHETICS
Green roofs are defined as any kind of roof that there is on its top coat, intentionally and in a planned way, a living vegetable layer. The use of substrate and vegetation on the roof of a building is an old technique and some authors mention the Hanging Garden of Babylon as the first application of this technique, with aesthetic purpose and a source of food. According to Minke (2004), the green roofs are known for centuries, both in the cold climates of Scandinavia, USA, and Canada, and in hot climates in Tanzania. In cold climates, they can be considered hot because they store the internal heat of the building. In turn, in hot climates, they can be considered cold, for keeping the inside of the building with lower temperatures. The green roofs are being used with different motivations, such as aesthetic, vernacular (from Vernacular Architecture), recreational, ecological and finally sustainable. Regarding this last reason, people nowadays are used to deploying larger amounts of green roofs in cities, using as an energy efficiency mechanism, for thermal and acoustic comfort and a potential reducing of flow drained rainwater.
In the 1960’s in Europe, the Germany government started to invest in research aimed to benefit and develop new green roofing construction techniques. Among the studies, it has approved public policies for deployment of this sustainable practice, which brought further expression to the use of green roofs. Following the example of Germany, other countries in all continents have been studying and motivating the use of green roofs (e.g. Japan, USA, Canada, England and Switzerland). In Menlo Park, California, a building with approximately 40,000 m² of green roof is being built to host the new headquarter of a company, as shown in Figure 1.
In Brazil, the green roofs are still poorly known, lacking of skilled labor and further information on building techniques, types of substrate and vegetation and its benefits.
1.1 Classification and composition of Green Roofs
According to Dunnett et al. (2007), green roofs comprise with two basic configurations: extensive and intensive roofs. The extensive roofs are made with a thinner substrate layer (up to 20 cm), support low-growing communities of plants and mosses selected for stress-tolerance qualities (e.g. Sedum spp., Sempervivum spp.) and practically do not require maintenance. On the other hand, the intensive one have a thicker substrate layer, over 20 cm deep, support larger plant species and require more frequent maintenance operations.
A green roof can have several types of compositions, from very simple with only a base waterproofing layer, and on it a substrate with good drainage capacity, to more complex systems made of several layers, comprising, besides the base waterproofing layer, layers for mechanical protection against roots, drainage, filtering, substrate stabilization, thermal insulation, the substrate and vegetation (Minke, 2004). The system to be adopted will depend on factors such as roof slope, region climate, cost, available materials and structural load capacity. Figure 2 shows a typical scheme used on green roofs.
1.2 Importance of Geosynthetics in Development of Green Roofs
The geosynthetics, generally developed with a focus on geotechnical works, are important to the increase of the green roofs technology. There is a wide range of geosynthetics materials with different characteristics and functions.
In green roofs, they can work as waterproofing, protection, drainage, filter and stability of the substrate. The use of these materials in exchange of natural materials brings many benefits, including the ease of handling, better quality control ensuring physical and functional characteristic, possibility of a cleaner and faster work, among others.
The advent of geosynthetic has introduced new materials to compose the impermeable barriers as the geomembranes, which consist of flexible polymeric webs that have permeability extremely low (in order of 10-12 cm/sec) used as barriers to liquids and vapors (Vertematti, 2004). There are some types of geomembranes available on the market as those made of high density polyethylene (HDPE), polyvinyl chloride (PVC) and ethylene propylene diene mixed rubber (EPDM). The Green Roofs Systems Manual (Kirby, 2007) recommends that, regardless of the membrane type, it must present the following properties: low water absorption, low vapor transmission, puncture resistance, chemical resistance (for example, fertilizer) and the ability to resist the loads imposed by project, through the tensile or elongation strength. It also states that it can be used the overlap two or more layers.
According to Minke (2004) the protection of waterproofing must be taken care of, especially when the waterproofing is made with bitumen materials. Some microorganisms that live in the roots tip can dissolve bituminous materials thereby providing the penetration of roots. Therefore the base’s waterproofing and the protection against the roots can be obtained using suitable geomembrane models. The protection against mechanical damage of the sealing can also be obtained by a geosynthetic material, the geotextile, which can be placed between the waterproofing layer and the roots protection layer and/or below the waterproofing layer. For drainage layer, instead the granular materials such as gravel, crushed stones, and expanded clay, geosynthetic materials can be used, such as nonwoven geotextiles, geonet, geomat and geospacer. Both geomat and geospacer must be covered by a filter layer with the function of preventing the displacement of particles from the substrate into the drainage layer. This function is usually performed by geotextiles. In the case the roof provides a steep slope and there is the risk of slippage of the substrate, geomats, geogrids and geocells are usually prescribed.
Therefore, it is observed that the geosynthetics can be used to waterproofing, protection, drainage, filter layer, and even in retaining the substrate, obtaining a good performance.
1.3 Water Retention Capacity of Green Roofs
The population growth in urban areas is cause of serious problems. One of these problems is the waterproofing layers of the soil surface provided by streets pavement, sidewalks and buildings, which reduces the capacity of water infiltration in the soil. In turn, the rain amount remains equal over the years. With a lower rate of water infiltration, there is a greater portion of surface runoff. It means that much of the water that infiltrate and seep into the soil before, start to flows over the surface, coming sooner to the rainwater drainage system and/or to a water vessel receptor. It may happen that the flow of the drainage system or the receiving vessel (e.g. a river or a stream) has a flow rate (discharge) under the flow reaching it (input), causing catastrophic flooding situations. In the figure 3, a hydrogram compares the expected behavior for a hydrographic basin (most of the soil sealed) with another before the urbanization (higher infiltration capacity). It is noted that, before urbanization, there is a larger base flow (underground) due to the rainwater that has infiltrated into the soil and percolates slowly. The surface flow (runoff) before urbanization has a lower, slower peak, with a gradual recession. On the contrary, the flow peak in an urbanized area is higher and happens faster.
As shown by Castro (2011), two urban drainage systems are used to control or minimize the floods effects: Classic Systems based on the hygienists principles, it means, the drainage of rainwater and wastewater must have rapid evacuation to downstream through conduits; and the Compensatory Systems which objective is not to drain water as soon as possible downstream, but to reduce the flow velocity and cause infiltration of the water, to offset the effects of urbanization on hydrological processes. The green roofs fit perfectly in the Compensatory Systems. The capability to retain water of a green cover is linked mainly to the thickness and composition of the substrate, but may vary according to the type of vegetation, the materials used for the drainage layer, the roof slope, the local climate and the number of days without rain. In North Carolina, USA, two green roofs were built and monitored. Each green roof retained a significant portion of the precipitation, as shown in Table 1.
In Goldsboro, the depth of the substrate was approximately 75 mm and in Raleigh, it was 100 mm. The substrate and the vegetation were the same in both roofs. Variations in the amount of retained water depended on, as already mentioned, how rainfall events occurred. For example, a month that the rains were spaced several days had a higher retention rate than a month with many rain events in a short time. Greater intensity storms also result in reduced retention rates. The total percentage retained on the roof in Raleigh is much smaller than the Goldsboro’s one, even though a substrate thicker. It is possibly the result of several reasons: (1) in Raleigh, data were collected during only 3 months, (2) Raleigh has received heavy rains during the hurricane season in 2004, (3) the roof in Raleigh is substantially steeper than in Goldsboro (Moran et al, 2005).
According to Mentes et al. (2006), even in the modest scenarios, only 10% of Brussels roofs are green roofs, and they easily reduce the runoff by 2.7%, due to its capacity to store water. The reduction consists on: (1) delaying the initial flow time due to the absorption of water in the green roof system; (2) reduce the overall flow retaining part of the precipitation; (3) the distribution of the flow over a longer period, by releasing excess of water stored temporarily in the pores of the substrate. Figure 4 shows the behavior of the flow of a green roof compared to a conventional roof. It is noted that in a total of about 15 mm of rainfall occurred in one day, the green cover has retained a large part of the total (less than 5 mm has flowed), while the conventional roof has retained about 2 mm.
This paper has as main objective to emphasize the importance of geosynthetic materials on the development of the green roofs technology, by purposing a study that will evaluate the behavior of different configurations of green roofs with geosynthetics. In addition, it specifically aims to evaluate the benefits of this technique regarding the green roof water retention capability, thus contributing to reducie peak runoff. It will also evaluate the behavior of geosynthetic materials related to constructive and functional aspects, including the development of vegetation, the stability of the substrate, particles entrainment from the substrate to lower layers and the behavior of the waterproofing geomembrane.
2. MATERIALS AND METHODS
Four modules were built in this research. Among them, one will serve as a reference and will be built without vegetation cover (only waterproofing layer). Each vegetated cover will comprise a different geosynthetic configuration, varying the drainage layer (nonwoven geotextile, geomat and geospacer). Using a rain simulator calibrated to two rain intensities, the water retention capacity and the performance of different green roof configurations will be measured. The modules will be built in the Technology Center at the Federal University of Rio Grande do Norte.
2.1 Modules Description
The modules were built with wood Pinus Elliottii, connected by screws as illustrated in Figure 5. Their plant view is 1.1 m wide and 1.3 m long. A 100 mm high wooden wall surrounds all bases’ perimeter. The base, which simulates a slab, is made with 1 inch thick, placed side by side. The inclination of the module base can be changed because there are two hinges in the front part, enabling simulations of either flat or inclined roofs.
2.1.1 Reference Module
The reference module comprises only the waterproofing layer over the base. It will be provided by a EPDM geomembrane. The purpose of this module is to represent a common roof. It will be submitted to the same rainfall simulation processes of the vegetated modules in order to get comparable results.
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2.1.2 Vegetated Modules
The vegetated modules differ from each other in the drainage layer. All three vegetated modules substrates will have the same constitution and depths. The same plant specimens (Zoysia japonica, the Poaceae family) and EPDM geomembrane waterproof layer will be used. The vegetation should be grown exposed to the sun, with semi-annual fertilizer and regular watering. This specie is not suitable for places with intense traffic, or for shaded areas. The vegetated module (1) will be more compact, with only the geotextile responsible for drainage. In vegetated module (2), a geomat will be added, which will be responsible for drainage, sandwiched by two nonwoven geotextiles, for filtering and mechanical protection. Figure 6 shows the layers of vegetated module (1) and vegetated module (2). The vegetated module (3) has the same configuration as the module (2), yet with geospacers as drainage layer. The geospacer will be placed in the way it can retain a certain amount of water.
2.2 Rainfall Simulator
As rain become scarce at certain seasons in Rio Grande do Norte State, the rainfall simulator is essential to the development of this research. The simulator will allow the time and the intensity of precipitation to be defined. The rainfall simulator was built based on the study performed by Furegatti (2012), which developed a simulator for erosion analysis. Furegatti (2012) stated that the variables involved in setting the amount of rainfall generated by a simulation are: the model of the nozzle disperser, the nozzle’s height in relation to the ground, the pressure and flow of the pump, among others. Figure 7 shows details of the rainfall simulator. Logging on to the pump outlet serves as a flow regulator in the sprinklers, because the volume of water passing through it returns to the reservoir. It means it is possible to define the rain intensity by turning the valve.
2.3 Parameters Analyzed
The intensity and duration of the rainfall simulation will be measured by rain gauges placed on the roofs. The water from the runoff in the roofs will be conducted back to the reservoir through a pipe. Information relating to the volume of water collected in the reservoir will be measured at defined time intervals. It will also be observed the performance of the filter layers and waterproofing as well as the adaptation of the vegetation to each configuration.
3. FINAL CONSIDERATIONS
At the present moment the modules where green roofs are assembled and the rainfall simulator are ready and awaiting the arrival of geosynthetic materials to start the simulations. The first simulations should be made early in August, 2015.
This paper presented the geosynthetic materials as important elements in the construction of green roofs, mainly due to ease of handling and reliability of its performance and its features. It is expected to confirm the capability of green roofs to retain a great part of rainwater, reducing and delaying the flow to the rain network. Therefore, it will contribute to flooding reduction. The green roofs alone cannot solve all the problems due to urban development; however, they provide many benefits that can be part of the solution, if combined with other sustainable building techniques.
The authors thank the National Council for Scientific and Technological Development – CNPq by the financial support for this research project.
ABOUT THE AUTHORS
T.S. Louzada, T.A.A. Oliveira, F.A.N. França, and A.C. Scudellari were with the Department of Civil Engineering, Federal University of Rio Grande do Norte, Natal, Brazil at the time of original publication.
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