Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield
Living Roofs as Stormwater Management Tool - in Sheffield? The Interrelationship of Plants – “A Designed Element” and Climate-Dependent Factors on Stormwater Mitigation Daniel Roehr1 and Kevin Kong2 1
Daniel Roehr, Associate Professor, greenskinslab, The University of British Columbia, School of Architecture and Landscape Architecture, Vancouver, BC, Canada, droehr@sala.ubc.ca 2
Kevin Kong, Core Researcher, greenskinslab, The University of British Columbia, School of Architecture and Landscape Architecture, Vancouver, BC, Canada, kevin@greenskinslab.sala.ubc.ca Introduction During the last 10 years, research world-wide has shown that living roofs have multiple and significant positive effects on managing stormwater runoff. However, the implementation of living roofs and consequent runoff mitigation has produced varied results, as a consequence of both regional and seasonal contexts. These results have shown that the total annual runoff reduction achieved by extensive living roofs can vary significantly, from 29% in Vancouver, BC, to 47% in Augustenborg, Sweden, and 74% in Washington, DC. Existing research has shown that runoff generated from and mitigated by living roofs can be influenced by both the design of living roofs as well as contextual climatic factors that lie outside the control of designers. Selection of roof pitch, along with that of type and depth of growing medium and plants, are decisions that can be made by designers. These design decisions must be made in response to carefully researched climatic conditions such as amount, frequency, intensity and duration of precipitation, as well as evapotranspiration, which is related to extraterrestrial radiation, temperature and humidity. In order to analyze living roofs in a holistic way and to find out which elements have the greatest influence on runoff, this presentation shows results from existing living roof research stations in the Northern Temperate Zone that measure stormwater runoff, and analyzes each element to determine its impact on the water cycle of living roofs.
Overview of Methodology After climatic data from Sheffield is collected, it can be applied to designing a living roof that will function successfully as a stormwater mitigation tool. That is, there must be a balance among the growing performance of plants, growing medium, stormwater management results, maintenance and irrigation use. Such balance is strongly related to local climatic condition (such as rainfall pattern throughout a year, air temperature, humidity and wind speed); properties and depth of growing medium; and plant selection and coverage. High
Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield water-use plants can potentially reduce more runoff as they transfer more water from growing medium to the air via evapotranspiration. However, if there is too little rainwater in summer when plants need water the most, plants will wilt. Consequently, irrigation and plant replacement is required in this case. Drought tolerant plants survive better during drought periods, but they transfer less water via evapotranspiration. High water retention substrate could potentially reduce more runoff during a rainfall event. However, substrate with high water retention is usually poorly aerated. Plants would grow relatively slow but survive better during drought.
Key Findings Findings of this presentation can be useful to city planners, engineers and landscape architects. With data based on local climatic conditions, an appropriate estimate can be made of the potential for living roofs to help reduce stormwater runoff on a city-wide scale.
Further Reading Bengtsson, Lars, Lennart Grahn and Jonas Olsson. 2005. Hydrological function of a thin extensive green roof in southern sweden. Nordic Hydrology 36(3): 259-268. Berndtsson, JustynaCzemiel, Tobias Emilsson and Lars Bengtsson. 2006. The influence of extensive vegetated roofs on runoff water quality. Science of the Total Environment 355 (1-3): 48-63. Burton, G. Allen and Robert Pitt, 2002, Stormwater Effects Handbook: A Toolbox for Watershed Managers, Scientists, and Engineers, Boca Raton, Florida: Lewis Publishers. Connelly, Maureen, Karen Liu, and John Schaub. 2006. BCIT green roof research program, phase 1 summary of data analysis, observation period, Jan. 1, 2005 to Dec. 31, 2005, report to Canada Mortgage and Housing Corporation. Ontario: Canada Mortgage and Housing Corporation. Fioretti, R., Anna Palla, Luca G. Lanza and P. Principi, 2010. Green roof energy and water related performance in the Mediterranean climate. Building and Environment 45(8): 1890-1904. Glass, Charles C. and Peter A. Johnson. 2008. Monitoring of a new green roof for water quality and quantity. In Proceedings of Sixth Annual Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show 2008, Baltimore, MD. Toronto, ON: Green Roofs for Healthy Cities. Hilten, Roger Norris, Thomas Mark Lawrence and Earnest William Tollner. 2008. Modeling stormwater runoff from green roofs with hydrus-1d. Journal of Hydrology 358: 288-293. Köhler, Manfred. 2008. Long-term vegetation research on two extensive green roofs in Berlin. Urban Habitats 4 (1): 3-26. Liu, Karen KY. 2004. Engineering performance on rooftop gardens through field evaluation. Journal of Roof Consultants Institute 22(2): 4-12. Snyder, Richard L. and Simon Eching. 2007.Penman-Monteith daily (24-hour) Reference evapotranspiration equations for estimating ETo, ETr and HS ETo with daily data, (PMDay.xls
Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield spreadsheet). Regents of the University ofCalifornia, Davis, CA. http://biomet.ucdavis.edu/evapotranspiration/PMdayXLS/PMday.htm (accessed in May, 2007). VanWoert, Nicholaus D., D. Bradley Rowe, Jeffrey A. Andresen, Clayton L. Rugh, R. Thomas Fernandez and Lan Xiao. 2005. Green roof stormwater retention: effects of roof surface, slope, and media depth. Journal of Environmental Quality 34: 1036–1044.