How Does Purple-Roof Green Roof Hydrology Work?

Traditional green roofs capture rainwater and thus reduce runoff. Traditional green roofs work when they are mostly dry. And traditional green roofs are most efficient during warm months due to transpiring vegetation, but they are far less efficient during cool, wet weather, when the plants are dormant.

Purple-Roof is different.Purple-Roof is a concept for a vastly improved green roof that does not have the limitations above. The Purple-Roof concept is available as a product through multiple suppliers, including

Scroll down to read more, or check out the video below!

Purple-Roof Hydrology Explained

This video might be the easiest way to understand they hydrology behind the Purple-Roof concept, and green roof hydrology in general. The rest of this page gets technical, but here we try to keep it very clean and straightforward!

Fundamental to understanding the differences between Purple-Roof and traditional green roofs is understanding the differences between two common stormwater phenomena:

  • Retention is capturing stormwater that only leaves the roof through evapotranspiration.
  • Detention is temporarily holding stormwater and allowing it to flow out at a later time.

Traditional green roofs only provide retention. The Purple-Roof concept and the Blue-Green concept provide both Retention and Detention.

Understanding retention and detention is so important that we created an entire video series on the topic! The first video is below. Visit our retention-and-detention page for the full series.

Retention and Detention Video Series

Eight-segment video series about the differences between retention and detention, why they are important, and how this relates to green roofs.

The full video series is on the Retention And Detention page.

Purple-Roof works as follows:

  • Rainfall lands on the green roof surface and percolates downward.
  • When the system is dry, initial rainfall, or possibly most rainfall is retained and later evaporated, just as traditional green roofs do.
  • When the system is wet, and rainfall occurs, such as during an extra-large rainfall, or during second-day events, stormwater percolates downward to a detention layer. A detention layer, or friction layer, replaces a traditional drainage layer, and rather than expediting drainage, has a precisely engineered flow rate. This slows water and allows water to backfill layers above. This stored water drains out at a slow, predictable, and engineered pace.
  • This performance, and the engineering behind it, is very similar to bioretention cells. The Purple-Roof concept includes an engineered storage volume and engineered release rate, which allows it to be utilized in the same way as other stormwater detention infrastructure such as retention cells and tanks.

Digging deeper into Purple-Roof hydrology:

Detention calculations are best done by continuous flow modeling. Purple-Roof modeling assumes the worst-case-scenario: that the green roof is already at field capacity when rainfall begins, and that all micropores (smaller pores that retain water for evapotranspiration) are already full.

If you're new to the concept of field capacity, check out the animation below. Water percolates into soil, and the soil absorbs water before runoff is generated. Once field capacity is reached, water flows through the soil rapidly. Detention should be modeled after field capacity is reached. Notice in the animation how water rises and falls within the larger macropores; that is gravitational water modeled during detention.

Animation of pores within soil (or green roof media) first retaining water (micropores absorb water), then runoff starts (preferential flow paths start, and water flows downward in the profile), then finally detention occurs (large macropores fill with water, which later drains out)

The detention layer is typically a 5mm thick mat of densely spaced vertical polyester fibers. The detention layer’s flow rate can be predicted according to the material’s transmissivity, which can be measured using ASTM D4716. Project-specific flow rates factor in roof geometry, design storms, and material transmissivity (ASTM D4716). This can be modeled using a simple continuous flow model.

The detention layer is the lowest layer within the system, and all runoff from the green roof can only pass through the detention layer.

All layers of the Purple-Roof concept: vegetation, engineered soil media, needled mineral wool, honeycomb detention reservoir, detention layer.

Due to restricted drainage outflow rates, macropores (large pores that fill with gravitational water) fill during rain and drain out later. Water storage is provided via macropores within typical green roof materials such as mineral wool and green roof soil. Often mineral wool and green roof soil do not provide sufficient macropore volume to contain larger storms, such as 100-year storms. In those cases, a detention honeycomb is added. This is a layer of vertical tubes that are at least 93% macropore void space. The tubes fill with water from above or below and can only drain to the detention layer below. There is no horizontal flow within the honeycomb. A honeycomb detention reservoir dramatically increases water storage volume.

The detention layer and honeycomb also work on a 2% slope roof. All other rooftop detention practices require a 0% slope roof. Horizontal flow restriction within these layers allows the filling of macropores and reduced flow rates on slopes of 2% or less (and likely on slopes exceeding 2%, to be tested).

Purple-Roof is for managing design storms. Purple-Roofs are typically designed to detain 2.5cm (1”) to 13cm (5”) of water and manage storm events such as 15cm (6”) Type II 24-hour events with peak rainfall intensities of 20cm (8”) per hour. Pending roof geometry, outflow rates are commonly as low as 0.03 m3/s per hectare (0.5 cfs/ac) and may be as low as 0.01 m3/s per hectare (0.2 cfs/ac) or lower. Typical drain-down or "recharge" timeframes (time to completely drain and be ready for the next event) are 12-18 hours.

Hydrograph of instantaneous rainfall and runoff of a storm of 5.2-inch (132mm) total volume, 24-hour rainfall duration, Type II distribution. Notice that runoff rises at the peak of storm, yet runoff remains approximately 90% lower than rainfall intensity. This particular graph is a model of a profile with 2 inches (50mm) of engineered soil media, 2 inches of mineral wool, and 2 inches of honeycomb.

Cumulative hydrograph of the same storm as above. Notice that all rainfall eventually runs off; this is because modeling assumes zero retention potential (i.e. modeling starts after field capacity is reached). Also notice that runoff stops in this example around hour 39, i.e. the "recharge" timeframe (time to empty) is 39 hours after the 5.2-inch 24-hour Type II rainfall starts, or 15 hours after the rainfall ends.

Instantaneous hydrograph of a Dutch R8 storm, a much smaller and faster storm than the first storm example. Notice that the instantaneous rainfall and runoff response is very similar to the larger storm.

Cumulative hydrograph of the R8 storm above. In this example runoff is at a more constant rate than the first example, which is a function of detention layer design, maximum hydraulic gradient reached, and roof geometry.

Storage volume is sized to prevent overflow during a design storm. This is the same as a bioretention cell or tank. If sized for a 100-year storm, then during a 1000-year storm the green roof safely overflows to roof drains. Again, this is no different from sizing any other green infrastructure.

In the hydrograph examples above, overflow would appear as a spike in runoff. Purple-Roof assemblies should be designed to prevent overflow during project design storms.

Modeling is currently provided by Green Roof Diagnostics. Green Roof Diagnostics is an independent research organization focused on raising the scientific integrity of green roof research.