Wetlands are an effective green roof system

doi:10.1016/j.buildenv.2013.04.024

Building and Environment

Volume 66, August 2013, Pages 141-147

https://doi.org/10.1016/j.buildenv.2013.04.024 Get rights and content

Highlights

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We tested the performance of wetland plant species and constructed a wetland for use as a green rooftop.
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An installed rooftop wetland had great thermal benefits and a low irrigation requirement.
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An installed rooftop wetland demonstrated flood-control and can serve as a carbon sink.
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Constructed wetlands are a reasonable system for green rooftops.

Abstract

Green roofs recently have garnered much attention as a means to reduce both the absorption of solar energy in summer and heat loss in winter, especially in urban areas with limited space for gardening. Constructed wetland roofs maintain more stable temperature profiles than terrestrial systems because of their slow heat transfer and high heat storage capacity. We found that wetland roofs were particularly efficient at decreasing the temperature of green roof systems on hot days. Wetland plants have high evaporation rates that are associated with their ability to cool buildings. Constructed wetland had excellent water holding ability, requiring less than 400 l water/m² of irrigation over the entire growing season, which was less than 20% of the expected irrigation requirement for terrestrial systems on green rooftops. Wetland macrophyte species demonstrated high tolerance to flooding and drought and showed great potential for regeneration by rhizomes, suggesting easy maintenance. Plants grown in the constructed wetland accumulated high biomass that can serve as a carbon sink. Wetlands on rooftops would not exceed the weight-bearing capacity of rooftops if water depths are designed and kept under 30 cm. Constructed-wetland roofs offer thermal benefits, a low amount of required irrigation, high tolerance of drought and flood, and flood-control capacities. They also can act as a carbon sink, are easy to manage, and provide other ecological services. Therefore, constructed wetlands are a reasonable choice for green rooftop systems.

Introduction

Urban development alters the environment in many ways, including raising temperatures in cities above those of surrounding areas. This “heat island effect” results from landscape modifications that replace green surfaces with buildings and pavements. These structures are made of materials that retain heat more than natural surfaces. Further, buildings discharge anthropogenic heat and gaseous pollutants that restrict normal patterns of airflow. Green spaces that are necessary to buffer these effects simultaneously disappear. Commensurate with its rapid development into the world's third largest megacity, Seoul, Korea, has experienced one of the highest rates of temperature increase over the last few decades [1].

The development of green areas may mitigate, to some extent, the heat island effect. However, in most cases, available space for greening is limited to rooftops and the outside walls of buildings [2]. Rooftop gardening can be used to create green, living roofs which are expected to reduce absorption of excessive solar energy, thus resulting in a significant savings in the energy used for air-conditioning in summer [3]. Green roofs also serve as insulation in cool weather [4]. Also, green roofs have other social costs benefits such as carbon reduction, air quality improvements, provision of recreational space and Habitat creation [5]. Green-roof methods have advanced with studies of appropriate species and growing media [3], [6], [7], [8], system design [9], [10], thermal and energy properties [11], [12], [13], and economic and environmental impacts [14], [15]. However, the possible use of wetland plants or systems for green roofs has been largely overlooked.

Species of drought-adapted succulents (genus Sedum) currently are favorites for populating green roofs [16]. In comparison, the aqueous barrier provided by a wetland roof system should have better evaporation and insulation performance properties than the substrate used for terrestrial green roof systems. The higher water holding capacity of wetland plants, because of minimal leaching associated with them, increases their effectiveness. Therefore, using wetlands for green roofs offers distinct advantages in comparison to those provided by traditional green roof systems. Additional benefits of creating wetlands in urban areas include improvement in air quality, microclimate regulation, noise reduction, and added recreation-cultural value [17]. To evaluate these advantages, we studied the tolerance and performance of several wetland species under rooftop conditions. We then constructed a pilot-scale wetland to test its actual performance and features under rooftop conditions.

Section snippets

Research site

Field measurements were carried out on the rooftop of a six-story building (the College of Natural Sciences building; 37°27′ 31.01″ N, 126°56′ 53.67″ E; 100 m above sea level) at Seoul National University, Seoul, Korea. The concrete rooftop was recently covered with urethane elastic water proofing coatings (top layer is covered with a mixture of trimethylopropane-neopentyl glycol-1,6-hexanediol-phthalic anhydride-adipic acid copolymer, toluene, methyl methcrylate, butyl acrylate-2-hydrooxyethyl

Temperature, humidity and precipitation

The mean rooftop and wetland temperature and precipitation on the rooftop are shown in Fig. 2. The mean rooftop temperature during the experiment was 21.7 °C and the mean relative humidity was 46.3%. The average external temperature during the experiment was 21.1 °C. The highest rooftop temperature recorded was 38.0 °C on August 5, and the highest wetland temperature was 33.1 °C on the same day. The highest one-day mean rooftop temperature was 32.3 °C on August 3, and the lowest mean one-day

Discussion

Rooftop temperatures (ranging from 14.4 °C in October to 26.6 °C in August) fluctuated considerably more than those in the wetland (ranging from 22.1 °C in September to 25.8 °C in August). Although the wetland temperature was not measured in October, the average rooftop temperature in September was 19.8 °C, much lower than that of the wetland. The highest recorded rooftop temperature, at 38.0 °C, was five degrees warmer than that of the wetland, 33.1 °C. As water is a slow transfer medium for

Conclusions

The temperature of a constructed wetland on a rooftop was more stable than that of the ambient air of the rooftop, due to the high heat release and insulation ability of water. The highest rooftop temperature recorded during the study 38.0 °C, but was only 33.1 °C in the wetlands, clearly demonstrating he thermal benefits of the wetland system. The total evaporation of macrophyte plants during the study period was about 1700 l water/m², indicating their ability for cooling. The constructed

Acknowledgments

This study was funded by the Rural Development Administration of Korea (Project No.3344-20120010).

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¹: Contributed equally.

View full text

Wetlands are an effective green roof system

Highlights

Abstract

Introduction

Section snippets

Research site

Temperature, humidity and precipitation

Discussion

Conclusions

Acknowledgments

Build Environ

Build Environ

Ecol Eng

Energy

Energy Build

Build Environ

Build Environ

Landscape Urban Plan

Ecol Econ

Resour Conserv Recy

Sol Energy

Build Environ

J Hydrol

Aquat Bot

Energy Build

Responses of two invasive plants under various microclimate conditions in the Seoul metropolitan region

Environ Manage

Evaluation of growth and green coverage of ten ornamental species for planting as urban rooftop greening

J Jpn Soc Hortic Sci

Evaluation of cold resistance of ornamental species for planting as urban rooftop greening

For Study China