Tuesday, 08 July 2008
A Study of Green Roofs
Ben Leusink & Chad Brittner
Major Technical Report
Green roofs can be a great asset in storm water management. Green roofs are not only roof gardens, but are there to serve an important purpose. They can help with rain water prevention through absorption. The amount of water a green roof can absorb depends on the soil type, vegetation, and overall design. Green roofs have many other positive aspects as well. These include: reducing the heat island effect, creating natural animal habitats, insulating, and improving air quality and the urban ecology.
The idea of green roofs has been around for many years although they were not always used for managing storm water runoff. They have been dated as far back as 1000 years ago; places such as Iceland used sod on their roofs to act as insulation, especially where building materials were scarce. In North America, settlers used sod as insulation in the walls of their homes and wild grass to cover the roofs. These were not living green roofs, but they helped to build the foundation of the green roofs we know today. Many old countries have outdated sewer systems; the storm water management systems can often be part of the sewer system causing problems during heavy rainfalls. Germany was the country credited for engineering a living green roofing system to help solve the growing problems with aging and over burdened sewer systems during storm events. (Colwell, 2007)
In many countries today storm water systems can be overburdened from the over development of urban areas. In densely populated areas, there is nowhere for the rainwater to be absorbed because the roofs dump water onto the ground which is mainly covered with asphalt and cement. During heavy rainfalls treatment facilities are unable to keep up with the demand and are forced to release unprocessed waste water into the environment. Under designed storm water systems can also lead to street and basement flooding which results in major property damages, as well as breeding ground for insects and diseases. This research project will analyze how effective a green roofing system can be on storm water retention. With Lethbridge being a newer city the infrastructure has been designed with separate sewer and storm water systems. The project will outline the pros and cons of what a green roof can offer in the climate of the Lethbridge area.
A green roofing system absorbs water through the soil and the plants growing in the soil. Water that would normally be dumped straight into the storm water collection system will be slowed down through absorption of the soil. This will lower the peak runoff as well as spreading it out over an extended period of time, helping to lower the flow rate during heavy rains. Not only will a green roof slow the flow rate, but it can decrease the runoff coefficient or even stop the flow from a building all together with a collection system.
There are many different ways to construct a green roof. The designer must keep in mind the extra weight from the soil and plants, and take the possibility of water leaks into consideration. With green roofs growing in popularity, companies are now specializing in the field of green roofing systems and are therefore developing new products with increased technology. Some standard construction techniques used today include: waterproofing layer, drainage layer, filtration layer, soil layer, vegetation and irrigation. These will be discussed in more detail.
- Waterproofing layers need to be very durable and able to withstand root penetration. Some common materials used are PVC which is welded together at the seams. Asphalt roofing systems may be used as well but must be covered with a material that can withstand root penetration like high density polyethylene. The most common problems with green roofs come from a faulty water proofing layer. In the past the materials used would break down over time and the roots would penetrate the material causing water leaks. With the use of new, more durable products such as PVC, green roofs can out last most conventional roofing systems. (Scholz-Barth, 2001)
- The drainage layer will direct the moisture that seeps through the soil to a concentrated point, which may be a holding tank or a storm sewer system. Many materials can be used for a drainage layer, such as gravel or manufactured layers made of a corrugated PVC Material. (Scholz-Barth, 2001)
- The filtration layer must stop fine soil particles from reaching the drainage layer allowing water to soak through it. If the soil particles plug up the drainage system, water may pool on top of your roof which will increase the chance of a leak on your roof. (Scholz-Barth, 2001)
- Some properties of soil which must be taken into consideration include:
- Weight of the soil
- Depth the soil
- Moisture holding capacity of the soil
- Vegetation that will grow in the soil
- Erosion resistance of the soil
- Considerations to think about are drought resistance for an Albertan climate, low maintenance, and cosmetics. A good way to choose vegetation that will be hearty and low maintenance is to find vegetation native to the area. (Scholz-Barth, 2001)
- Given Alberta’s climate, retaining the water for future irrigation will be something to take into consideration.
- Grey water systems could be developed to water green roofs in times of drought.
Figure 1 Structural Components of a Green Roof (Horstman, 2004)
In large cities a problem starting to reveal itself is the increase of temperature due heavily developed areas using concrete and asphalt. The temperature increase varies with different climates, but temperatures can increase as much as 5.6 degrees Celsius. Downtown areas are known to be affected the most due the materials that absorb and maintain the heat. Large buildings can reduce air flow, which in turn can trap heat in the surrounding areas. The heat island effect can also increase city’s energy consumption with the excessive use of air conditioners and fridges. As a result, a family’s energy bill will increase. “The Heat Island Group estimates that the heat island effect costs Los Angeles about $100 million per year in energy”(Wikipedia, 2008). In the United States an average of one thousand people die every year from heat stroke; the heat island effect only increases this problem. One of the best ways to combat this temperature problem is through natural vegetation. Vegetation cools the air through shading and a natural cooling effect called evapotranspiration (evaporation of water through soils and leaves). The best way to introduce more vegetation is through parks, or where this is not possible, a green roof. If green roofs were installed on a large scale they could have a huge impact on energy savings and help to lower temperatures in heavily developed areas.
Large buildings, with lots of roof area, can take away from already dwindling natural habitats. The development of land can involve the destruction of forests and grasslands being replaced with concrete and asphalt. In severe cases, this can lead to animal endangerment or even extinction. There are limited options available to help mend this crisis. It is often hazardous for large animals to live among people; however it is possible to help wildlife such as birds and insects in maintaining their way of life. Green roofs can be made to simulate the natural habit and try to replace the land that is lost. With the flexibility of different types of vegetation a green roof can support, it is possible to target specific endangered wildlife.
A roof can play a critical role in its ability to maintain constant temperatures. Since heat rises, the most vulnerable place to lose heat is through the roof. In conjunction, large surface areas exposed to the sun are more susceptible to absorb heat. This heat absorption can greatly increase the energy needed to cool top levels in all types of buildings. Green roofs have shown to be an excellent source of insulation. Cleveland (2007) states that:
Green roofs decrease air conditioning and heating costs, depending on the size of the building, climate and type of green roof. Environment Canada estimates that a typical one story building with a grass roof and 10cm (3.9 inches) of growing medium would result in a 25% reduction in summer cooling needs. Actual field experiments in Canada suggest that a 6-inch extensive green roof reduced heat gains by 95% and heat losses by 26% compared to a referenced roof.
Green roofs can also offer sound insulation. This is especially valuable in buildings close to airports or areas of high noise pollution.
Sound waves that are produced by machinery, traffic, or airplanes can be absorbed, reflected or deflected. The substrate tends to block lower sound frequencies and the plants block higher frequencies. A green roof with a 12cm (4.7inches) substrate layer can reduce sound by 40 decibels; 20cm (7.9inches) substrate layer can reduce sound by 46-50 decibels. (Cleveland, 2007)
Our main goal is to determine what benefits a green roof can offer in a Southern Alberta climate. The focus of our research is to determine what effects a green roof will have on rain water entering a storm water system during rainfall events. To do this a scaled down model of a green roofing system was built to simulate how this system will perform in an Albertan climate
The design for the green roof was modeled after current designs being used in the green roofing industry. There are a lot of specialized products available to consumers that will ensure the quality and durability of a green roof. Since our project was only required for temporary use, materials were used that could produce an accurate test result to provide a variety of information. The main focus of the design was to collect water that passes through the soil substrate during a simulated rainfall event. This data could help to calculate how much water was absorbed and how much time the peak runoff was delayed.
The green roof was constructed by cutting a one meter by one meter square out of half inch plywood. The size of the green roof was limited by the work space available at the Lethbridge College Greenhouse. The sides of the model were eight inches high so we could obtain a seven inch soil depth. The box was screwed together using 2x2 studs as reinforcement. One corner was notched out to allow a concentration point for the water to flow to. Once the structural element was built to carry the weight of the green roof, it needed to be waterproofed by using a membrane that would resist moisture and root penetration. The inside of the box was lined with heavy duty plastic with a hole cut through the corner that was previously notched out. On top of the plastic membrane a one inch thick layer of washed rock was placed to produce a drainage layer. All of the washed rock was sieved through a 10mm screen to eliminate larger rocks. A layer of landscape fabric was laid over the rocks to prevent soil and grass roots from clogging the drainage layer. Landscape fabric was a good choice because of its ability to prevent root penetration. A silty-loam soil was then placed on the green roof. The Environment Department at the Lethbridge Community College suggested a silty-loam would be a good choice because this soil can support most native drought resistant vegetation and it is able to retain a fair amount of water. For vegetation, a native grass was chosen that would be low maintenance and drought resistant.
Figure 2 Green Roof Model (2007)
To accurately perform our tests we would need to be able to simulate a rainfall event as close as possible. Since the weather is near impossible to recreate, we gathered data on a variety of different circumstances to understand how a green roof would perform, and what benefits it could offer to a city. We designed a gravity fed system where we could accurately control the intensity and amount of the water being applied. The system was built by hanging a five gallon pail in the air connected to a flexible hose run over our green roof in a grid pattern. Small holes were made in the hose so that water would be applied evenly to the entire roof. A valve was used to regulate the intensity of flow we desired.
Our first test was to simulate a slow rainfall. We decided to apply 40,000 ml over a period of 3.5 hours. This is equivalent to a 4 cm rainfall at a rate of 1.14 cm/hr, which shows a good representation of an average rainfall. The plan was to discover how much of the water would be retained and at what rate it will be discharged. During the test the green roof retained all of the applied water until the 117 minute mark. The discharge steadily increased its flow rate and at 180 minutes a total of 1850 ml was collected. The flow rate continued to increase and at 210 minutes the total increased to 5000 ml. After 3.5 hours we had applied all the 40,000 ml. Once the simulated rainfall stopped, the discharge from the green roof was at a peak and slowly began to decline. At the 240 minute mark another 450 ml was collected, at the 270 minute mark an additional 400 ml was collected. Finally after five and a half hours a remaining 300 ml was collected. With 40,000 ml being applied, a total of 6150 ml was discharged from the green roof. This suggests the green roof was able to retain 84.7% of the water applied at a rate of 1.14 cm/hr.
Figure 3 Test #1 Data (2008)
During the test we observed that the soil was able to hold all the water back until it became saturated. Once saturation was reached and water started to pool, the flow was at its highest rate. Even at its highest discharge rate, the green roof was able to slowly reduce the intensity at which the water was being applied. Analyzing the intensity at half hour increments we were applying 5714 ml. The highest output the green roof reached was 3150 ml in half an hour.
Our second test was to simulate a rainfall with a shorter duration and a higher intensity. We applied the water at an intensity of 4 cm/hr for one hour. This also totaled to 40,000 ml. During this test our first drop appeared at the 18 minute mark. The flow steadily increased, and at the time of 30 minutes 470 ml were collected. The flow continued to increase rapidly so we measured the outflow in 10 minute increments. At the time of 40 minutes 750 ml were collected, at 50 minutes 1750 ml we collected. After 60 minutes the total 40,000 ml had been applied and 4200 ml was collected. After 70 minutes the peak flow was reached and 4450 ml were collected. The discharge began to decrease and after 80 minutes 1700 ml was collected. The discharge continued to decrease and after 100 minutes 500 ml was collected, after 120 minutes 100 ml was collected and finally after 2.5 hours another 100 ml was collected. With 40,000 ml being applied, a total of 14,020 ml was discharged. This means the green roof was able to retain 65% of the water applied at a rate of 4 cm/hr. During the highest outflow the green roof discharged 4450 ml in ten minutes. The applied rate was 6667 ml every ten minutes which shows that even at a rate of 4 cm/hr a green roof can slow the output intensity of a rainfall.
Figure 4 Test #2 Data (2008)
It is difficult to test how a green roof will perform. Weather is very unpredictable which makes it hard to simulate. We performed two tests representing average situations on a Lethbridge climate. The average runoff coefficient for a conventional asphalt roof is 0.95. This means that 95 percent of rain that lands on the roof will run off and only 5 percent will be absorbed or held back. Our green roof model showed great capabilities of rain water retention. The first test with an intensity of 1.14 cm/hr produced a runoff coefficient of 0.15. The second test with an intensity of 4 cm/hr came up with a runoff coefficient of 0.35. The average runoff coefficient between the two tests works out to be 0.25. This means that 25 percent of the rain that lands on our green roof will runoff and 75 percent will be absorbed or held back within the roofing system.
We were able to receive permission from city hall to use an aerial photograph of the city of Lethbridge Alberta. We download this picture into ArcMap and imputed a scale so we could get actual measurements. The picture has a resolution of 12.5 cm so we were able to trace the rooftops fairly accurately. We chose an eight block area of downtown Lethbridge as our target.
Figure 5 3rd - 5th Ave and 4th - 8th St Lethbridge AB (2007)
In the picture above we traced out the rooftops, the asphalt roads, grassed areas, concrete sidewalks and concrete parking lots. Using runoff coefficients of 0.875 for conventional rooftops, 0.825 for asphalt, 0.25 for grass and 0.925 for concrete we came up with an average runoff coefficient of 0.838. To prove how much water a green roof can help to save we replace the conventional roofing systems with green roofs on 90 percent of the area, keeping 10 percent of the roof areas intact because there will always be areas where green roofs will be impossible to install. Using the coefficient from our tests of 0.25 the new runoff coefficient for this area becomes 0.618. With the new runoff value, if a 4 cm rainfall were to take place like in our testing, a total of 1372.93 m³ of water would be stopped from entering the storm systems. It is estimated that in the Lethbridge area, between the months of April and September, an average of 25.9 cm of rain will fall. Again using runoff values from our tests, we can calculate that 8879.41 m³ of water can be held back from entering the storm system.
The test results indicate that green roofs are very effective in retaining and slowing rain water runoff in a Southern Alberta climate. The data suggests that in an eight block area, of Downtown Lethbridge, Alberta, green roofs would reduce the total amount of rain water runoff by 22%. The purpose of this research project was to find out how effective a green roof would be in a Southern Alberta climate. Since the research data indicates that green roofs could save 22% of rainwater from entering the storm water systems, future research needs to be done. Further research needs to be done in order to answer the question “Are green roofs feasible, on a large scale, in Southern Alberta”. Although this research project indicated that green roofs can have a big impact on the amount of runoff water in an area, further research could tell us if 22% less water entering a collection system would be enough to reduce the size of the system in place. Further research could also reveal whether other construction practices be implemented to virtually eliminate the need for storm water collection systems.
Green roofs add a lot of other benefits to an area other than just collecting storm water. They are aesthetically pleasing, offer natural animal habitat, reduce the heat island effect and have excellent insulating properties. With the benefits of green roofs come some downfalls as well. They are more expensive to construct, and it may be impossible to construct on some buildings. Also, in periods of drought, they would have to be irrigated. There may be environmentally friendly solutions to these problems such as irrigation by grey water, which would then help reduce flows into sanitary sewer lines reducing the amount of water the sewage plant has to treat.
There is a lot of research that needs to be done in order to make an informed decision of whether installing green roofs in Southern Alberta would be a feasible idea. This research has indicated that green roofs could hold back enough water during rainfall events to make a significant difference on the amount of water a collection system would have to handle.
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