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Session 2

EVALUATION OF CONCAVE GREEN ROOFFROM WATER – ENERGY – FOOD NEXUS PERSPECTIVE

Mooyoung Han, Eunsoo Lee, Kihong Park

Professor, Seoul National University/ President, Nowon Urban Farmers Network, Seoul/ CEO, Skytree Co., Seoul Korea

Abstract

As a new paradigm of multiple functions of water-energy-food nexus, a full-scale (840 m²) concave green roof has been in operation since 2012 at Building No.35 in SNU, which consists of flower garden, vegetable garden, and a fish pond. The concave green roof is a roof which can retain rainwater at the bottom by putting a 5cm retention board and at top of the soil by simply making the surrounding wall 10 cm higher than the soil level. At an extreme rainfall event, with the record rainfall of 239 mm/day, the concave green roof reduced the peak of runoff by 56% and delayed the runoff for 3 hours, compared to the ordinary flat concrete roof, thus reduced the flooding risk. During the hot weather, the temperature of top soil of the green roof is 24 °C lower than the ordinary roof, which can reduce the heat island effect and the cooling energy of the top floor of the building. This roof garden became not only a food production site, but also a good gathering place for communication. Motivated by this successful multi-purpose concave green roof showcase, several cities in Korea, such as Seoul and Daejeon, made regulations to build a green roof for their climate change adaptation strategy and subsidize the construction cost of green roof.

Keywords: Concave green roof; Energy; Food; Seoul National University; Water

INTRODUCTION

Green roofs are gaining much interest for its social benefits such as creating an aesthetic and green environment as well as climate change adaptation capability including flood mitigation, heat island reduction and reducing building energy consumption by cooling energy consumption during summer (2, 6, 7). Therefore, many modern cities in the world are promoting green roofs as a part of their climate change adaptation measures (5). However, a green roof can cause other problems such as increased demand for water, labor and cost (1). Most of the green

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roof is designed and maintained for a single purpose of landscaping for aesthetic improvement. Although some researches are available which investigated the hydrologic and thermal behavior in a small lab scale experiment, any systematic quantitative verification of engineering aspects at a full scale in actual climate condition is not well reported. In this study, a full-scale (840m²) concave green roof is introduced at Building No.35 in Seoul National University(SNU), which consists of flower garden, vegetable garden and a fish pond. The concave green roof is a roof which can retain rainwater at the bottom by putting a 5cm retention board and at top of the soil by simply making the surrounding wall 10 cm higher than the soil level. The effects of flood mitigation, water conservation, as well as heat island reduction are monitored and analyzed systematically. This newly formed, once abandoned, roof space is revitalized by various activities by the community people sharing vegetables and food.

MATERIAL AND METHODS

The full scale concave green roof has been in operation at the rooftop of building #35, Seoul National University (SNU), Seoul, Korea since December 2012 (Fig. 1). The flower and vegetable garden of 840 m² was constructed at the 2000 m² roof. The green roof consists of 6 parts of 140 m² each. Sensors for measuring runoff and temperature were installed in section A, B, C for assessing their effects on flood control and temperature reductions. At section D, E and F, vegetable gardens are operated by students, faculty and local residents. Later, it was found that sedum planted on top of original 5cm soil depth in Section D could not survive the long drought period in Korea. So, Section D was changed to a pond (80m2 surface area and 20cm water depth) by putting water holding structure made by plastic sheet on top of the existing 5 cm depth soil layer. The water to maintain the minimum height of the fish pond is supplied by the rainwater collected from the nearby roof. Gold fishes inhabit the pond and proliferate, even under the icy environment during the winter. The water quality in the pond is maintained by a self-purifying system consisted of contact biofilm zone made of gravel, sand and charcoal. The water is circulated by pumping water from one end to the front, after passing through the contact biofilm zone.

Figure 1. System outline of concave green roof at building #35 Seoul National University, Seoul, Korea

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This water-energy-food (WEF) nexus became possible by slightly changing the section of the garden. There are three types of the roof: Ordinary concrete roof, ordinary green roof and concave green roof (Fig. 2.). In the ordinary concrete roof, most of the rain is drained quickly in the ordinary roof and dries quickly and is heated easily causing heat island effect. Furthermore, solar heat is transferred to the top floor of the building easily and it requires high cooling energy consumption. In the ordinary green roof, water management has not been considered seriously. Most of the rainwater is designed to flow down to the drain to avoid any harmful effect on the plants. And later, if needed, water is used for irrigation purposes, which requires extra cost for water and labor for maintenance. However, in the concave green roof, extra rainwater is retained in 5cm-high drain board at the bottom which can contain rainwater 20 L/m² at the space made between the inner walls of the drain board. During heavy rainfall, more than 100 mm rainfall can be retained at the space made above the top soil and surrounded by 10 cm high walls.

Figure 2. Cross section of three types of roof

In this research, the 100 m² area of the normal roof of a rooftop room and a 140 m² area of concave green roof (Section A) was selected and compared as shown in Fig. 1. The area of the normal roof has concrete surface with grade and drains through drain pipe which has a flow meter (Model: Signet 2507 mini Flow Rotor Sensor). The cross section of concave green roof consists of retention board (5 cm), then fabric, and artificial soil (10~15 cm) made of cocopeat, perlite and peatmoss with the weight of 100~150 kg/ m2. The top layer is for planting 20 cm height of flowers with the density of approximately 160 units/m2. The discharge and rainfall were checked by installing flow meters at each drain outlet and a rain gauge. (Model: HD 2013 TIPPING BUCKET RAIN GAUGE, DELTA OHM.)

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Figure 3. Cross section of normal roof and concave green roof system

RESULTS AND DISCUSSIONS

Flooding mitigation and water conservation

While the ordinary roof and ordinary green roof is designed to drain rainwater as fast as possible, the concave green roof system can perform multiple roles in water management such as flooding mitigation and water conservation. At an extreme rainfall event on July 12, 2013, where total precipitation amount was recorded as 239 mm with warning of heavy rain, the concave green roof showed a big potential for flood mitigation (Fig. 4). The result showed peak flow quantity reduction of 56% and peak flow time delay of 3 hours. Furthermore, after the rain, the roof retained 40m³ of rainwater/140 m² section (equivalent to 286L/m²), which can later be used for evapo-transpirative cooling and irrigation water.

Figure 4. Runoff from Normal roof and Concave green roof on July 12th, 2013

Thermal insulation and heat island reduction

In Energy management, the concave green roof system can reduce the temperature increase by plant cover with extra cooling effect due to evapotranspiration by stored rainwater. In a field experiment, on August 30, 2014, it showed maximum surface temperature was 47.9 °C on the normal roof and 23.9 °C on the concave green roof. So, the maximum temperature difference between the normal roof and the concave green roof was 24 °C. This temperature difference shows similar trend throughout the summer. Even at a long period of antecedent rainfall up to 20 days, the insulation effect can be maintained due to the retained rainwater. At an extremely long dry period, tap water can be supplied to the retention board to ensure evaporative cooling. Through green roof’s cooling effect, it can reduce the urban heat island phenomenon and improve building insulation to reduce cooling energy.

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And during winter time, the soil or ice on the green roof can become an insulator and indoor temperature of top floor is 1~2 °C warmer at green roof than concrete roof.

Figure 5. Surface temperature at Ordinary concrete roof and Concave green roof on August 30th, 2014

Food management

Through the preservation of the rainwater at concave green roof system, the growth of vegetables and fruits would require less maintenance for watering. The vegetable garden is managed by students, faculty members and local residents, which consist more than 30 households (Fig. 6.) Through urban agriculture on a green roof, they are self-sufficient in food products such as vegetable, root crops and fruits.

Additional benefits

There are additional social benefits which cannot be quantified. This vegetable garden became a place for communication between school and local residents. Every month, participants of urban farming on #35-dong green roof gather, share their harvested crops. Three big events at the green roofs became regular: Potato harvesting and sharing at the end of June, Full moon festival with traditional Korean rice cake making at Aug 15, in lunar calendar, and Kimchi making and sharing with foreign students at the end of November. Students, professors, local neighbors, and foreign students participate and enjoy the culture and sharing. Additionally, Korea Beekeeping Association set 8 hives on the green roof and extract 40 kg of honey a year. Thus, this green roof exists as a learning place for students and urban farmers. This project already became a showcase in Korea as well as in the world and received the National Energy Global Award 2014, and Showcase award at the World Water Forum in 2015. More than 4,000 people from Korea and other countries visited this green roof each year since 2014. Mass media or social media are making news and a special program from this roof garden.

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Figure 6. Various events with local residents on green roof

CONCLUSIONS

Concave green roof systems are suggested as a sustainable solution to urban flooding and urban heat island phenomena. Concave green roofs can also have social benefits such as producing food and being community space for people. Not only the building users but also the local residents can benefit from this concave green roof system. By applying green roof system on the buildings, without destroying existing urban infrastructure, there will be environmental & economic benefits. This concave green roof is already reported in media more than 50 times and awarded at the 2014 Energy Globe Awards(3), 2015 7th World Water Forum Showcase Awards(4) as an urban management model for the water-energy-food nexus. After all, this project implies that a simple change from the ordinary concrete roof to concave green roof system is a sustainable solution for water, energy and food issues of the future, especially as a measure for climate change. A new sense of neighborhood can be created and tradition of sharing in Korea can be revitalized through this multi-purpose green roof movement.

ACKNOWLEDGEMENT

This research was supported by “Development of Nano-Micro Bubble Dual System for Restoration of Self-purification and Sustainable Management in Lake” project and “Waste to Energy • Recycling Human Resource Development” project, both funded by the Republic of Korea Ministry of Environment. Furthermore, this research was supported by the Institute of Construction and Environmental Engineering at Seoul National University. The authors wish to express their gratitude for the support.

REFERENCES

1.Berghage, R., Jarrett, A., Beattie, D., Kelley, K., Hussain, S., Rezai, F., Long, B., Negassi, A., Cameron, F., Hunt, W. (2007). Quantifying Evaporation and Transpirational Water Losses from Green Roofs and Green Roof Media Capacity for Neutralizing Acid Rain, http://www.ndwrcdp.org/research_project_04-DEC-10SG.asp (Nov, 2010).

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2.Castleton, H. F., Stovin, V., Beck, S. B. M., Davison, J. B. (2010). Green roofs; building energy savings and the potential for retrofit. Energy and Buildings, 42, 1582–1591.3.Han, M.Y. (2014). “Concave Green Roof as Water-Food-Energy Nexus.” National ENERGY GLOBE Award Republic of Korea, http://www.energyglobe.info/southkorea2014?cl (July. 9, 2014)4.Han, M.Y. (2015). “Concave Green Roof as Water-Food-Energy Nexus.” 7th World Water Forum, Water Showcase World Final, http://www.worldwaterforum7.org/outcome/overview.asp (Apr. 15, 2015)5.Kumar, R., Kaushik, S. (2005). Performance evaluation of green roof and shading for thermal protection of buildings. Building and Environment. 40, 1505–1511.6.Vijayaraghavan, K., Joshi, U.M., Balasubramanian, R., (2012). A field study to evaluate runoff quality from green roofs. Water Res, 46, 1337-1345.7. Virginia, S., Gianni, V., Hartini, K. (2010). The hydrological performance of a green roof test bed under UK climatic conditions, Journal of Hydrology, 414-415, 148-161.

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