Extensive green roof (Source: Katrin Scholz-Barth Consulting)
Vegetated roofs, also known as green roofs, eco-roofs, or nature roofs, are structural components that help to mitigate the effects of urbanization on water quality by filtering, absorbing, or detaining rainfall.
Modern vegetated roofs can be categorized as "intensive" or "extensive" systems depending on the plant material and planned usage for the roof area. Intensive vegetated roofs utilize a wide variety of plant species that may include trees and shrubs, require deeper substrate layers (usually > 10 cm (4 in)), are generally limited to flat roofs, require 'intense' maintenance, and are often park-like areas accessible to the general public. In contrast, extensive roofs are limited to herbs, grasses, mosses, and drought tolerant succulents such as Sedum, and can be sustained in a shallow substrate layer (< 10 cm (4 in)), require minimal maintenance, and are generally not accessible to the public. An additional variation is the possibility of designing vegetated roofs as urban gardens capable of providing a local food supply. Soil depth and plant suitability should be considered when exploring this option.
Stormwater Management Objectives
Through a variety of physical, biological, and chemical treatment processes that filter pollutants and reduce the volume of runoff, vegetated roofs reduce the amount of pollution delivered to the local drainage system and, ultimately, to receiving waters. In addition, vegetated roofs have a longer life span than standard roofs because they protect the roof structure from ultraviolet radiation and the extreme fluctuations in temperature that cause roof membranes to deteriorate. Furthermore, the construction and maintenance of vegetated roofs provide business opportunities for nurseries, landscape contractors, irrigation specialists, and other green industry members while addressing the issues of environmental stewardship.
A major benefit of vegetated roofs is their ability to absorb stormwater and release it slowly over a period of several hours. Vegetated roof systems have been shown to retain 60-100% of the stormwater they receive. They can provide substantial stormwater retention in urban areas especially when the soil matrix has sufficient opportunity to dewater between discrete rain events. Generally, vegetated roofs treat only the rainfall that falls directly on that particular surface area.
Peak flow reductions of as much as 80% have been observed in the U.S. from extensively vegetated roofs. Water retention rates are known to be higher in the summer than in the winter due to higher evapotranspiration rates.
The selection of the soil material will impact the effluent quality. While materials such as compost will provide excellent volume reduction, the concentrations of nutrients in vegetated roof effluent may increase because of nutrients present in the soil. Typically, non-organic, high-mineral content substrates are recommended for the soil matrix. From a CSO perspective, however, green roofs will provide water quality improvements in receiving waters by reducing the volume and peak rate of stormwater entering the sewer system.
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