Green Roof Stormwater Retention

Abstract

Urban areas generate considerably more stormwater runoff than natural areas of the same size due to a greater percentage of impervious surfaces that impede water infiltration. Roof surfaces account for a large portion of this impervious cover. Establishing vegetation on rooftops, known as green roofs, is one method of recovering lost green space that can aid in mitigating stormwater runoff. Two studies were performed using several roof platforms to quantify the effects of various treatments on stormwater retention. The first study used three different roof surface treatments to quantify differences in stormwater retention of a standard commercial roof with gravel ballast, an extensive green roof system without vegetation, and a typical extensive green roof with vegetation. Overall, mean percent rainfall retention ranged from 48.7% (gravel) to 82.8% (vegetated). The second study tested the influence of roof slope (2 and 6.5%) and green roof media depth (2.5, 4.0, and 6.0 cm) on stormwater retention. For all combined rain events, platforms at 2% slope with a 4-cm media depth had the greatest mean retention, 87%, although the difference from the other treatments was minimal. The combination of reduced slope and deeper media clearly reduced the total quantity of runoff. For both studies, vegetated green roof systems not only reduced the amount of stormwater runoff, they also extended its duration over a period of time beyond the actual rain event.

A Global Geospatial Ecosystem Services Estimate of Urban Agriculture

“…Nevertheless, previous studies have demonstrated rainfall retention characteristics of green roofs (Mentens et al, 2006;Simmons et al, 2008;VanWoert et al, 2005). For example, VanWoert et al (2005) showed that for a 10 cm thick substrate and a heavy rainfall event, defined as exceeding 6 mm rainfall per day, 54% of precipitation is retained by the growth medium. To compute the change in runoff, we used the Natural Resource Conservation Service (NRCS) method for urban hydrology (NRCS, 1986).…”

Hydrological performance of a full-scale extensive green roof located in a temperate climate

“…Furthermore, green roofs can be retrofitted onto existing 189 buildings as well as incorporated into new developments (Castleton et al, 2010). This is 190 particularly beneficial in urban areas where roofs can account for a high proportion of 191 the total impervious land area (Carson et al, 2013;VanWoert et al, 2005). 192…”

The Influence of Hydrologic Parameters on the Hydraulic Efficiency of an Extensive Green Roof in Mediterranean Area

“…From an energy prospective, green roofs also promote evaporative cooling, thereby offsetting the energy requirements associated with air conditioning systems and reducing urban heat island effects [6,7]. From a hydraulic perspective, several studies have shown that green roofs have significant effects on retaining rainfall volumes [8][9][10][11], delaying the peak flow rate [12,13] and reducing the runoff volume discharged into the combined sewer systems (CSSs) [14][15][16]. Many literature studies have, indeed, investigated the hydraulic efficiency of green roofs, but only few have focused on the performance of such systems under Mediterranean climate condition.…”