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Measuring Effectiveness of Green Roofs, Porous Pavements in Slowing Runoff

Apr 11, 2019

Installation of green roofs and porous pavements to slow rain runoff has risen while climate change has made extreme downpours known as rain bombs more common. That raises the question: How well do low impact development (LID) practices such as green roofs and porous pavement handle extreme rainfall?

Clemson University researchers Nigel Kaye and Will Martin designed laboratory tests mimicking rainfall to answer those questions. One goal was to improve the design of modular green roof systems to make them more effective at reducing peak discharge. Their tests showed modular green roof systems as most commonly designed fail to detain significant amounts of rainfall for long periods of time.

Two female students observe water falling onto plants in a laboratory.

Graduate students at Clemson University monitor a lab simulation of a green roof, complete with artificial rainfall. Photo: Glenn Department of Civil Engineering, Clemson University.

Then they tweaked existing green roof modules by raising them and putting empty storage modules underneath. The design alteration reduced peak runoff discharge by 88% compared to a standard impervious roof.

A second component was to assess the impact of LID technologies when a traditional stormwater design is enhanced with the addition of modular green roof systems and porous pavements. The data indicated the use of the newly designed green roof system reduced the peak discharge but provided no significant reduction in total site discharge, since most of the water eventually passes through the green roof system. Porous pavement systems, quite the contrary, did not make a major difference to the peak site discharge but did reduce total site runoff.

A third component focused on the potential benefits to municipalities of land developers using LID technologies, thus indicating whether there are benefits to requiring LID technologies in their stormwater regulations. The study found LID technologies could allow a municipal storm sewer system to handle a 20% increase in design rainfall without exceeding the current peak discharge, and a 10% increase in design rainfall without exceeding the current total discharge.