A detention-based green/vegetated roof slows the outflow of water also when the green roof is soaked with water from a previous rainfall. This slowing down of water, or stormwater detention, is analogous to the functionality of a bioretention cell where water is collected and the peak outflow is reduced and delayed in order to prevent flooding damages. For the Purple-Roof concept, the detention layer is the "magic ingredient".
Do All Green Roofs Provide Detention?
Far from all green roofs have detention capacity, though this is often claimed due to a misconception and confusion around two concepts: retention and detention.
As it starts to rain and the green roof is dry or relatively dry and there is a lot of pore space still available in the roof profile. At this point there is no runoff – this is retention filling up and not a detention-based outflow delay.
The profile fills up and capillary forces hold the water in the profile micropores. The problem is the substrate macropores where the capillary forces are not strong enough to hold the water against the gravitational pull. Then, runoff begins due to these gravitational forces pulling on the water through substrate preferential flow paths.
Please note that it is essential to separate real retention from the results of the ASTM E2399, which is the standard used to assess the maximum load of a saturated profile. Using ASTM E2399 as a measure of retention is severely flawed and will lead to an overestimation of the retention capacity of the profile.
But back to detention! The million-dollar question is if the profile can delay the outflow of water when it is filled up like a soaked bath sponge, or does it become a pipeline with essentially no peak outflow reduction & delay? Also, even if there is a peak outflow reduction & delay, is the magnitude of these parameters enough to satisfy a stormwater engineer? A 5min peak outflow delay and 5% peak outflow reduction during a significant storm event such as a 100-year storm is insignificant as a flooding protection measure.
Many years of data collection and hydrologic modeling by Green Roof Diagnostics have revealed that traditional green roofs do not provide significant detention to act as a replacement for, e.g., a bioretention cell or a stormwater tank. Traditional green roofs are great at providing retention and many other tremendous secondary benefits, but they do not provide true detention, no matter what you might hear.
What is Detention, and Why is Friction so Important?
There are several detention-type green roof solutions on the market with various solutions to delay the outflow of water. Some use orifice restrictions, meaning that water is collected on the roof, and the drainage outflow points are set to a specific diameter resulting in a predictable outflow rate. There are also smart roofs on the market that will open or restrict the outflow depending on predicted storm intensities. Most of these solutions are Blue-Green roofs and need a completely flat deck to function, making these solutions a good fit for beautiful roof gardens.
The Purple-Roof concept is suitable for flat and sloped roofs and uses a friction-based layer to delay & reduce peak outflow. This friction-based layer is slightly analogous to the natural friction encountered in a watershed. Water follows the path along the sloped forest floor; some water is absorbed (retention), and some is delayed through the crisscrossing over mosses, sticks, and stones. Traditional green roofs have little to no friction as they are built to expedite water away as fast as possible, making it impossible to saturate the profile thoroughly using the macropore spaces.