Hydrologic Restoration in the Urban Environment Using Green Roofs

Anna Palla, Ilaria Gnecco  and Luca G. Lanza (Department of Civil, Environmental and Architectural Engineering, University of Genova) have published a paper on the use of green roofs to help restore the hydrologic environment in urban areas. Loss of natural soil and vegetation within the urban environment can significantly affect the hydrologic cycle by increasing storm water runoff rates and volumes. In order to mitigate these modifications in urban areas engineered systems are developed, such as green roofs, to mimic and replace functions (evapo-transpiration, infiltration, percolation) which have been altered due to the impact of human development. Green roofs, also known as vegetated roof covers, eco-roofs or nature roofs, are composite complex layered structures with specific environmental benefits. They are increasingly being used as a source control measure for urban storm water management. Indeed, they are able to re-establish the natural water cycle processes and to operate hydrologic control over storm water runoff with a derived peak flow attenuation, runoff volume reduction and increase of the time of concentration.

Furthermore green roofs exhibit the capacity to reduce storm water pollution; they generally act as a storage device, consequently pollutants are accumulated in the substrate layer and released when intensive rainwater washes them out. In order to investigate the hydrologic response of a green roof, the University of Genova recently developed a joint laboratory and full-scale monitoring programme by installing a “controlled” laboratory test-bed with known rainfall input and a companion green roof experimental site (40 cm depth) in the town of Genoa. In the paper, data collected during the monitoring programme are presented and compared with literature data.

The introduction to this good paper reads:

As development progresses in a urban environment the impervious areas increase with the direct consequence of higher runoff rates and volumes and shorter times of concentration. Indeed, the impervious surfaces rapidly contribute runoff to the receiving water bodies while the pervious areas store and deliver subsurface flow over periods of hours, days or weeks. The construction of impervious surfaces modifies the surrounding soils through engineered compaction and eliminates superficial soil and its role as a significant pervious storage interface between the subsurface and the atmosphere.

Traditional storm water management in urban areas aims at removing runoff as quickly as possible, eventually gathering excess runoff in detention basins for peak reduction. The conventional approach to storm water management involves the efficient capture, conveyance and sometimes treatment of runoff generated from the impervious surfaces. Sustainable Urban Drainage Systems (SUDS), Low Impact Development (LID) or Water Sensitive Urban Design (WSUD) principles and applications are a relatively new approach to storm water management that represent a conceptual improvement from the conventional sewer system design methodology. LIDs (SUDS or WSUDs) are basically a source reduction approach. These sustainable strategies focus on evaporating, transpiring and infiltrating storm water on-site through natural and/or engineered soil, vegetation and bioengineering applications to reduce and treat the overland flow that is limited for most rainfall events in a natural environment.

LIDs are a way of managing storm water and planning the land development, at the parcel and catchment scale, that emphasize conservation and use of on-site natural features integrated with engineered source controls to more closely mimic the pre-development hydrologic functions. LID solutions include storm water infiltration systems, rain gardens, storm water wetlands, green roofs, etc. to be properly distributed throughout the urban area. The positive aspects of these alternative approaches are that, in addition to reducing surface runoff, it may also be possible to increase recharge of local ground water resources and streams, reduce stream erosion, favor the development of biodiversity and improve stream water quality.

Site planning and storm water management have to be integrated at the initial design phases of a urban project to maintain a more hydrologically functional landscape. Hydrology and natural site features that influence water movement have to guide the layout of roads, structures and infrastructures. Natural soil and vegetation protections, that are strategically distributed throughout the landscape to slow, store and infiltrate storm water flows, are designed into the new projects as amenities, as well as hydrologic controls.

Several studies have been conducted to analyze the effectiveness of various LID (or SUDS or WSUD) practices based on hydrologic and pollutant removal capabilities. Bioretention areas, grass swales, permeable pavements and green roofs are the most common practices investigated in the literature. These techniques are able to reduce the amount of Effective Impervious Area (EIA) in a watershed. EIA is the impervious area directly connected to the storm drainage system and contributes to increased watershed volumes and runoff rates. There are documented case studies that conclusively link urbanization and increased watershed imperviousness to hydrologic impacts on streams.

Kleidorfer et al. Analyzed, e.g., the impact of urbanization, expressed by the increase of impervious area, in conjunction with possible global warming scenarios on urban hydrology and in particular on the efficiency of a combined sewer system. It is shown that a 20% increase of a climate change factor expressed in terms of rain intensity distribution has the same effect, on average, as a 40% increase of the impervious area. Such an increase of rainfall intensities could be compensated, on average, by infiltration measures which lead to a 30% reduction of the impervious area. They concluded that the impact of increased urbanization is sometimes significantly higher than the one expected from global warming.

LID could become a viable approach to reduce Combined Sewer Overflows (CSO), however LID has not been widely implemented in highly urbanized areas due to a perception that insufficient land is available. LID systems could become most effective when applied on private areas, e.g. green roofs on private rooftops, which occupy a large fraction of the land surface in the urban environment.

The goal of the present paper is to account for relevant research on green roof systems as hydrological restoration solutions, and to synthesize the results so that the current status and future research needs can be assessed. In Section 2 the basic green roof technology is recalled and a review of experimental evidences about the related hydrologic performance is presented based on literature results. Section 3 is devoted to the description of a joint laboratory and full-scale monitoring programme undertaken at the University of Genova (Italy) to investigate hydrologic green roof performance in a Mediterranean environment. Finally conclusions are drawn in Section 4.

Read the full paper here in PDF format.

Citation: Palla A.,Gnecco I.,Lanza L.G. Hydrologic Restoration in the Urban Environment Using Green Roofs. Water. 2010; 2(2):140-154.

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