guest feature article
The eThekwini Municipality Green Roof Pilot Project
By Clive Greenstone
Graphics Courtesy and Photos By Clive Greenstone
During this millennium Durban, South Africa, like other cities around the world, will be challenged to provide sufficient food, shelter, basic services and jobs for all its urban residents. This challenge is compounded by the necessarily to fulfill these needs whilst simultaneously building more sustainable cities that mitigate, rather than add to, climate change. There is growing consensus that the future is in cities and the drive to change these urban forms into more sustainable ecological spaces is a necessity.
With strategic and creative planning solutions from both municipal governments and citizens, the urban environmental question offers a vision of a healthier and more sustainable urban space. To do this city spaces need to be re-conceptualized and restructured, we need to examine how existing empty spaces can become resourceful spaces. We believe that one way of enacting this transformation is through the use of empty rooftops. Large, flat and empty rooftops are abundant throughout Durban on institutional, private, residential, industrial, municipal and commercial buildings. These underutilized spaces are ideal locations to create rooftop gardens. Buildings with suitable roof structures could be developed into a network of green spaces, which may provide a mixture of urban food security (through both container gardening and/or extensive rooftop gardens) and city greening as well as providing opportunities to mitigate and adapt to climate change.
eThekwini Municipality’s Municipal Climate Protection Programme
The green roof pilot project is part of eThekwini Municipality’s Municipal Climate Protection Programme. This programme was initiated in 2004 and focused initially on understanding the vulnerability of the city to the impacts of climate change. A strong emphasis has been placed on identifying climate change adaption projects that will improve the resilience of the city to future developmental, social and environmental challenges. Basically the green roof pilot project is a response to the higher temperatures and increase in the frequency and severity of floods and droughts that are expected as the result of climate change. From an urban environment perspective these changes will exacerbate the already high temperatures experienced as a result of the Urban Heat Island effect and the high levels of surface run-off and flooding that result from the hardening of permeable surfaces. Green roofs are currently being used around the world to reduce temperatures and storm water run-off. Green roofs also offer an opportunity to promote inner-city biodiversity on underutilized, empty roofs and to address food security issues through the production of food.
The green roof pilot project is located one on of the buildings in the City Engineer's complex, on two flat roofs on adjacent sides of an arched roof. The entire area of green roof is 550 square metres (5,920 sf), broken down as: 5 x modular systems at 55 square metres (592 sf) each; 3 x direct applications at 55 square metres each; and 2 x control areas at 55 square metres each. The choice of location was informed by the following facts:
• The roof is flat and has an easy access;
• It is in a secure location where scientific analysis can be undertaken;
• The roof was assessed by a structural engineer and found to be suitable in terms of its loading capacity;
• The roof is visible to municipal staff and public visiting the city engineer’s buildings.
Left: Before installation in Durban, South Africa;
Right: Direct application in foreground, modular in background.
Green Roof Pilot Project Emphasis
The pilot study is testing the following:
• Both the direct and modular methods of green roof construction;
• Different growing media across a range of nutrient levels, textures, composition, depths, weights and drainage characteristics;
• A range of different plant species;
• Some containers are being used as ponds, to see which submerged aquatic and wetland plants will survive under rooftop conditions;
• Different watering rates.
• To assess the temperature reduction that the green roof affords both the ambient atmosphere and the building itself (resulting in the reduction of the “heat island effect” and the reduction of air conditioner use, respectively);
Temperature Variances and Data
• The quantity and quality of the surface run-off from the roof;
• The increase in biodiversity of the roof.
Before application of the Green Roof Design to the roof, the structural load capacity was estimated at 100kg/m2. The existing waterproofing was checked and repaired where deemed necessary. Both the Modular and Direct Green Roof applications were installed. Five areas were planted using modules/ containers and three areas using direct application with varied depths.
Left: Mike Hickman and Clive Greenstone, applying 1000mic plastic as a root penetration layer;
Right: Various layers of application for direct green roof.
Different Applications: Modules/Containers
Before installing the Modules, the following was installed:
• A layer of Geotextile Bidum;
• Then a layer of 1000 micron plastic was applied to protect the existing roof from wondering roots. (Roots are attracted to water and if not planned for accordingly could easily penetrate existing waterproofing and over a few decades cause potential damage to structures).
Modular area preparation.
The containers are patented custom modules that are made of recyclable HDPE (High Density Polyethylene) and are UV resistant. They have built-in drainage systems and water reservoirs that aid in both storing water for plant usage as well as slowing down rainfall run off into existing infrastructure. They are made in varying depths of 75mm, 110mm and 200mm thus allowing for greater use of plant species adaptability. All the module weights were recorded in the Soil Aggregate Lab. Fully saturated planted modules weigh between 34 kg – 92 kg/m2 depending on the depth of tray and plants used. This allows for more diversified application to the varied roof types in the city. The containers are raised 20mm of the roof to allow for free movement of both air which aids in cooling down the rooftop and building directly underneath as well as allowing for the unhindered movement of excess runoff water. The modules are movable thus allowing for easy maintenance of the existing roof as well as changing of landscape design. The team is also experimenting with some containers being used as “ponds” to see which submerged aquatic and wetland edge plants will survive in damp seeps on exposed areas.
Modular applications after 6 months.
Different Applications: Direct Application
Before installing the direct application the following was installed:
• A layer of geotextile fabric;
• 1000 micron plastic;
• Then a drainage layer;
• An additional layer of geotextile was applied to create a layer to accept the soil and plants as well as to protect the existing roof from wondering roots. (Roots are attracted to water and if not planned for accordingly could easily penetrate existing waterproofing and over a few decades cause potential damage to structures).
Direct Application after 6 months.
Varying Depths of Growing Media
The direct application was installed using varying depths of growing medium:
• 10- 50mm
• 10- 100mm
Showing the actual depth of green roof against standard business card.
A specialised soil layer is carefully placed on top of the drainage layer. (The soil layer is a light weight, well drained, high moisture retention layer compiled of Vermiculite/Perlite/and well composted growing medium.)
Left: Instrumentation used for recording rainfall velocity of the green roofs and blank roofs;
Right: Temperature probes and loggers recording temperature of green roof, under modules, under direct green roof, under blank roof, air temperature, etc.
The reason both the modular and direct application were installed using varying depths of growing media was in order to measure the following:
• The difference in growing medium weight and fertility for plant cultivation.
• The ability of both techniques to minimise storm water flow and rainfall retention capacity. This will prove beneficial to both future policies around storm water management fees as well as mitigating against infrastructure damage with regards to storm water velocity and impervious surfaces in the built environment.
• Temperature probes and data loggers are installed in both applications to measure air temperature, rooftop temperature both under modules and direct applications. This is paramount to curbing the heat island phenomenon, as well as insulation correspondence regarding energy and buildings. (Results to date show differences of 25 degrees Centigrade between measurements were the Green Roofs have been installed).
• Biodiversity will be monitored as to what fauna will be attracted to Green Roofs, with +/- 100 plant species being used, already the results are showing which species are more suitable for Green Roof applications.
• The plants with the widest amplitude of survivability on our coastal belt are what the project team is determining with this experiment. Much of the inspiration is being found in the wild by observing which plant species are able to grow on extremely hot and dry granite and sandstone rock outcrops and cliffs (cremnophytes – cliff dwellers) that have shallow humus rich “soils.” Some shady cliffs can be seepages and will support water loving plants.
Left: Initial watering: Right: Six months later!.
Test Plant List
Examples of some of the plants are listed below:
Cissus fragilis Cissus quadrangularis
Crinum macowanii Delosperma rogersi Justicia flava
The eThekwini Municipality Green Roof Pilot Project has used only indigenous plants (except for the food plants that will be tested) in support of the Municipality's policy of protecting the city's internationally significant biodiversity and came within a 50km radius of the building.
Roof-top Garden Insect Survey Aim
To determine if insects are being attracted to the Environment Department’s roof-top garden.
Methods and Materials
Eight yellow pan traps were placed at regular intervals on the roof-top garden. They were half filled with water containing a small amount of dishwashing liquid to break the surface tension of the water. The traps were left out for between 3 and 7 days at a time. The water was drained through a strainer to collect the insects which were then put into a collection bottle containing 70% ethanol to preserve them. The insects were then identified to morphospecies (insects with different morphologies are considered to be different species). The trapping was conducted from the 9th of April 2009 to the 14th of May 2009. Half way through the trapping, two of the pan traps were moved to the Durban Natural Science Museum Research Centre parking area roof to act as controls.
A total of 1,448 insects were caught during the survey period. This total was made up of 66 different morphospecies from six different orders of insects (see figure 1).
Figure 1: Total number of insects collected per order.
The order with the most species was Diptera (flies) which had 23 different species, followed by 17 species of Hemiptera (true bugs), 14 Hymenoptera (wasps, bees, ants), eight Lepidoptera (butterflies & moths), three Coleoptera (beetles) and one Thysanoptera (thrips). Each order was dominated by certain morphospecies which contributed most to the total number of insects collected per order (see figure 2).
Figure 2: The dominant morphospecies contributing to the total insects per order.
The most insects were collected on the last trapping date, May 15, 2009, while the least were collected on May 11, 2009 (see figure 3).
Figure 3: Total number of insects caught per trapping date.
The control traps caught a total of 55 insects while the treatment traps caught 612 insects in the same time (see figure 4).
Figure 4: Treatment versus controls.
Discussion and Conclusions
A large number of insects were collected during the trapping period of just over a month. Flies were present in large numbers with 23 different species being recorded. Although flies are generally seen as being pests, they play an important role in the ecosystem. Apart from their role as decomposers (maggots use decomposing material as a food source), up to 17% of all pollinators are flies. Female flies require protein to develop their eggs while male flies only require nectar for energy and hence they are attracted to flowering plants. The majority of the flies were very small with the exception of the sarcophagids (flesh flies) which are forensically important.
Hemiptera (true bugs) are insects that either use plants as a food source or as a hunting ground. Many Hemipterans, such as leaf hoppers, are responsible for damaging leaves and stems of plants as they pierce the plant tissues to access the liquid food. Other Hemipterans such as assassin bugs, are predators and lay in wait for other insects to ambush them. Thus the presence of 17 different species represents a solidly developing ecosystem.
The 209 Hymenopterans (wasps, bees and ants) that were caught were dominated by 10 different species of wasp. Bees and wasps are most commonly known for their role as pollinators but wasps also kill other insects by paralyzing them and feed these to their young. The presence of these insects is positive in a garden as they are pollinators and they are finding a suitable food source.
Lepidoptera (butterflies and moths) are one of the most well known groups of insects. Adult insects feed on the nectar from flowers and use the plants as suitable habitat for laying their eggs. The larvae (caterpillars) can be a pest on plants as they can defoliate trees and potentially kill smaller plants by eating all the leaves. Their presence however again indicates a healthy ecosystem.
Although there were Coleopterans (beetles) present, there were only three different species. Many beetles feed on plant material while some are predatory. As the garden develops and grows, one would expect more species to be attracted to the garden.
Thysanoptera (thrips) are a group of very small insects that are very slender and inconspicuous. They feed on plants and sap which makes them both pests and pollinators. They can reproduce parthenogenically which means large numbers can be produced without the need for a mate.
The traps were left out for different lengths of time due to unpredictable weather. If the weather was very hot for consecutive days, the water in the traps evaporated more rapidly and so had to be replaced. On other days we had cool weather which wasn’t conducive to insect activity or it rained and with too much rain the traps were in danger of overflowing and the insects being washed away. It would be ideal for the traps to be left out for equal periods of time to facilitate statistical analysis.
Seasonality will also affect the suit of insects that are present at a particular time of year. The time of year that this survey took place was in autumn when there are colder nights, but still pleasantly warm days. As insects are directly affected by the ambient temperature, one could expect to find a greater number and diversity if this study was repeated in summer.
There was a significant difference in the number of insects between the control traps and the treatment traps (fig 4). The controls were only in place for the second half of the trapping period and so have only been compared to the treatment traps for the corresponding period. The significant difference in numbers of insects suggests that the insects are being attracted to the roof because of the garden.
Future Research Interests for the Project Team
The eThekwini Municipality Green Roof Pilot Project in Durban, South Africa.
There is an increasing need to promote urban agriculture and rooftops are seen as unused space. Thus future research avenues could be to circulate grey water and rainfall run-off, back on to the existing Green Roof to monitor the viability of vegetable cultivation that requires greater amounts of water for survival. Furthermore, LECA will be tested as a growing medium in order to determine results, as the organic base is resulting in a high percentage of decomposition.
Publisher's Note: See the EThekwini Municipality Green Roof Pilot Project profile in The Greenroof Projects Database here.
Clive Greenstone is qualified as a Water Reticulation Engineer and received his Diploma Nature Conservation at TECH S.A in 2000. He holds a Bachelor of Social Science and an Honours Degree in Political Science & Development Studies focusing on Environmental History and Policy. The Honours research was based on the Point development and appropriate urban agriculture). In 2009 he received his Masters in Town and Regional Planning (UKZN), and his thesis was on rooftop agriculture/ green roofs. Clive has been involved with testing different containers for growing food and plants for +/- 4 years. More recently he has been involved in growing food crops for students on top of the university buildings. Currently, Clive is part of the Green Roof Team for the City of Durban and is principal of Green Roof Designs.
Contact Clive at: +27 83 3986902 or email@example.com
Additional resources can be found on Michael Hickman's website - Mike is principal of Ecosystems Management and a consultant with Green Roof Designs. He has many years experience as a turf management horticulturist, in horticultural development and maintenance, landscape planning and design, arboriculture, re-naturalization of eco-systems, and is part owner in a wholesale and retail nursery. In addition to being part of the installation team of the EThekwini Municipality Green Roof Pilot Project in 2009, Mike has been involved in a German greenroof installation and monitoring it for 19 years. Download the Green Roofs Pilot Project pamphlet here, and also see his Green Roof Project Launch page here, where you can read two articles about the project, and his research page here.
Contact Mike at: +27 82 0612593 or info@ecoman.
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