Overview for green roofs
Water quantity treatment
A portion of rain that falls on green roofs is stored in the green roof media and eventually lost to evapotranspiration. Green roofs therefore provide qater quantity treatment.
Green roof hydrology
Water retention by green roofs
Studies show that, compared to traditional hard roofs, green roofs:
- decrease runoff peak discharge (eg. Berghage et al 2010, Carter and Rasmussen 2006);
- delay peak runoff (eg. Carter and Rasmussen 2006, Van Woert et al 2005, Berghage et al 2010); and
- reduce runoff volume (eg. Carter and Rasmussen 2006, Teemusk and Mander 2007, Van Woert et al 2005)
Green roof stormwater performance is affected by regional climatic conditions, storm size, rain intensity, frequency, and duration, antecedent moisture in the soil, transmissivity of drainage layer, vegetation species and diversity, length of flow path, roof size, growing medium composition and depth, and roof age.
For small rainfall events (typically less than ½ inch) little or no runoff will occur (e.g. Rowe et al 2003, Miller 1998, Simmons et al 2008, Moran et al 2005). Lower intensity storms also result in greater stormwater retention than high intensity storms (Villarreal and Bengtsson 2005).For storms of greater intensity and duration, a vegetated roof can significantly delay and reduce the runoff peak flow that would otherwise occur with a traditional roof.
Annual runoff volume reduction in northern temperate regions is regularly measured to be 50 to 70 percent when the media thickness is 3 to 6 inches (e.g. Berghage et al 2010, Carter and Rasmussen 2006, Van Woert et al 2005, Moran et al 2005, Van Seters et al 2007, Berghage et al 2009, literature review in Magnuson Klemenic Associates. 2007). While no green roofs have been monitored for annual stormwater retention in Minnesota, green roofs in Minnesota’s climate (with shorter storms, and generally enough time to allow evapotranspiration to renew much of the soil water holding capacity between rain events) are expected to retain about 70% of annual runoff. Berghage et al’s results published in 2010 of an extensive green roof with 3 inches of growing medium in Chicago IL, a climate similar to Minnesota’s climate, found 74 percent annual retention over a 2 year study period that included 106 precipitation events. Higher reductions are attainable by maximizing design for stormwater retention and evapotranspiration (e.g. Compton and Whitlow 2006 – see case studies).
- decrease runoff peak discharge (eg. Berghage et al 2010, Carter and Rasmussen 2006);
- delay peak runoff (eg. Carter and Rasmussen 2006, Van Woert et al 2005, Berghage et al 2010); and
- reduce runoff volume (eg. Carter and Rasmussen 2006, Teemusk and Mander 2007, Van Woert et al 2005)
Green roof stormwater performance is affected by regional climatic conditions, storm size, rain intensity, frequency, and duration, antecedent moisture in the soil, transmissivity of drainage layer, vegetation species and diversity, length of flow path, roof size, growing medium composition and depth, and roof age.
For small rainfall events (typically less than ½ inch) little or no runoff will occur (e.g. Rowe et al 2003, Miller 1998, Simmons et al 2008, Moran et al 2005). Lower intensity storms also result in greater stormwater retention than high intensity storms (Villarreal and Bengtsson 2005).For storms of greater intensity and duration, a vegetated roof can significantly delay and reduce the runoff peak flow that would otherwise occur with a traditional roof.
Annual runoff volume reduction in northern temperate regions is regularly measured to be 50 to 70 percent when the media thickness is 3 to 6 inches (e.g. Berghage et al 2010, Carter and Rasmussen 2006, Van Woert et al 2005, Moran et al 2005, Van Seters et al 2007, Berghage et al 2009, literature review in Magnuson Klemenic Associates. 2007). While no green roofs have been monitored for annual stormwater retention in Minnesota, green roofs in Minnesota’s climate (with shorter storms, and generally enough time to allow evapotranspiration to renew much of the soil water holding capacity between rain events) are expected to retain about 70% of annual runoff. Berghage et al’s results published in 2010 of an extensive green roof with 3 inches of growing medium in Chicago IL, a climate similar to Minnesota’s climate, found 74 percent annual retention over a 2 year study period that included 106 precipitation events. Higher reductions are attainable by maximizing design for stormwater retention and evapotranspiration (e.g. Compton and Whitlow 2006 – see case studies).