Rooftop stormwater management technologies for climate change adaptations and resilience

Blue-green roofs

Performance comparison of three rooftop stormwater management solutions.

A blue-green roof consists of a green roof installed on top of a blue roof basin – a reservoir formed by a geo-cellular unit (see Figure 3[b] on page 18). The upper vegetated portion filters and retains the rainwater and provides all the benefits of a green roof. Excess water infiltrates through the green roof and ponds in the blue roof underneath, which is detained and slowly released through control flow drains.

This lower portion enables the blue-green roof to manage back-to-back rainfall events and large intense storms regardless of the antecedent weather conditions (i.e. even when the upper green roof is fully saturated). Water is released slowly, so the blue roof basin is emptied or “recharged” for the next storm. Blue-green roofs offer reliable controlled release of runoff like traditional detention-based systems.

At first glance, blue-green roofs offer the best of both worlds, combining retention and detention to maximize stormwater management potentials. Unfortunately, as detention is achieved through control flow drains, it also inherits many of the blue roof’s weaknesses such as the dependence on zero-degree slope for water storage efficiency and clogging at the drains can affect the operation and effectiveness. As a result, blue-green roofs work best on dead flat roofs, preferably on large regular shaped roofs to minimize the number of flow restrictors required for best economics.

A friction-detention green roof consists of an enhanced retention green roof on top of a friction detention mat (see Figure 3 [c]). The retention of the upper green roof is enhanced by a mineral wool and an optional reservoir cell. The detention mat consists of thousands of fine vertical fibres sandwiched between two geotextiles.  These fibres create friction to slow down runoff traveling to the drain to achieve detention. Water is backed up, filling up the reservoir layer and saturating the retention layer and growing medium.

The friction-detention mat allows small amounts of runoff to flow through unimpeded but slows down large volumes of runoff. As a result, runoff comes out at a slower rate over a longer time, which prevents overloading the storm sewers. The detention mat enables the system to manage back-to-back rainfall events and large intense storms regardless of antecedent weather conditions (i.e. even when the green roof is fully saturated). The water is released slowly within 24 hours and the system is “recharged” for the next storm.

Performance comparison of three rooftop stormwater management solutions – hydrographs.

It is important to note that because detention happens at the drainage level across the entire green roof, this avoids clogging issues associated with single-flow restrictors such as control flow drains. It also enables the system to be implemented on low-sloped roofs without losing efficiency and sloped roofs effectively. Lastly, the friction detention system is particularly economical on irregular shaped roofs where numerous roofs drains would require flow restrictors.

A successful friction-detention green roof design requires collaboration between several disciplines–architects, landscape architects, civil, and mechanical engineers–to provide project-specific details such as size, location, design storm, maximum allowable outflow rate…etc. Using this input data, a proprietary detention modeling program accurately predicts performance and calculates the appropriate green roof profile to meet detention and retention requirements of the project.

The research behind friction-detention technology 

Figure 4  compares the water retention and detention performance of three distinct green roof systems.  Water fills and exists differently given a “traditional” system using simple drainage cups to expedite fast drainage, an “enhanced retention” system with a high performing water retention layer, and a “friction-detention” green roof with both retention and friction-detention layers.

A short explanatory video can be viewed by scanning this QR code.

Figure 5 illustrates the effectiveness of the friction-detention green roof by comparing hydrographs from the same three systems in an actual rain test.

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