What is Green Roof Retention?

by Brad Garner on Tuesday, March 19, 2019

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Green roof water retention and stormwater management

Retention is the process of holding water on the green roof, i.e., preventing water from draining off the green roof. Retention has been the focus of the hydrologic design of green roofs over the past few decades. Many people familiar with green roofs but less familiar with other types of stormwater management might not realize that most engineers consider retention the “free gift with purchase” rather than their central stormwater goal.

So, what is this odd thing, retention?

The secret of green roof water retention is green

Retention works like this: rain falls onto a relatively dry green roof, and the green roof absorbs the water, usually in pore spaces. Later, the sun comes out, and the plants use the rainwater to evapotranspire. This means that the water leaves the roof as vapor. Eventually, it rains again, and the process is repeated.

A surprising amount is going on there. Let’s unpack it!

Green roof water retention – getting rid of the runoff

In that description, the green roof absorbs water, and water leaves the green roof not as runoff, but as water vapor.

That’s great on many levels!

Because water does not run off, at least not in this textbook example, there is less stress on drainage infrastructure. Less water flows through pipes and sewers are less likely to overflow. Flooding risks drop as does the potential for erosion at outfalls.

The true devastation of floods can be staggering. Consider for a short moment overflowing, stinking sewers, teeming with dangerous disease and contaminants. Flooded houses. The loss of property. Even loss of life during extreme floods.

I think we can all agree that reducing the volume of runoff is a good thing.

Evapotranspiration – not only a tool for reducing floods

Also, notice that water leaves the roof as vapor.
This is the process of evapotranspiration: the combined water vapor losses of evaporation and transpiration. Evaporation can happen when the sun and wind are drying the soil, and transpiration as plants pull water from the soil and emit excess water through stomata, small pores often located on the underside of their leaves.

Evapotranspiration (ET) equals retention.
If you can measure ET, you can measure retention and vice versa.

Evapotranspiration also provides excellent cooling benefits.

Think back to high-school physics: energy is required to convert liquid water to vapor. In the case of green roofs, most of that energy is solar energy, which would otherwise just heat up the roof.
So, increasing retention also increases cooling potential. Pretty cool, huh?!

Green roof – a sponge

In the description above, notice that the green roof started off relatively dry. Think about pouring water on a dry sponge: the dry sponge absorbs the water until it is saturated. After that, pretty much any other water added simply passes straight through immediately. Green roofs usually work the same.

But like sponges, green roofs absorb water more efficiently when they are already a little bit wet. So, for each raindrop that falls onto a green roof, a portion of the raindrop is absorbed, and a portion runs off. The highest ratio of absorption (highest efficiency) occurs when the green roof is already holding some water, usually around 20%-50% of its maximum retention volume.

Retention – a finite capacity

Retention has a finite capacity, just like a cup has a finite capacity. If a green roof has a maximum capacity of 80 L/m2 (2 gallons per square foot) of retention, that means that when it has absorbed 80L of water, every drop beyond that runs off.

The speed of runoff may be highly variable, but retention is black and white: retained or not.

Thus, retention provides great benefits such as a reduction of runoff volume and increase cooling potential, but retention capacity is finite. Retention-focused green roofs provide almost no benefit when already “full”.

This is where another metric comes in: antecedent moisture condition.

Antecedent moisture condition

Antecedent moisture condition (AMC) is just a measure of the moisture level before an event, say before a rain event. For optimal retention, we want our AMC to be around 20% of maximum. That way before it rains the plants still have a water supply, and the green roof is “primed” like a slightly moist sponge.

Another term that is a more direct measure of what we’re going for is antecedent recharge capacity (ARC). This is a measure of the system’s capacity to recharge and retain more water, before the event. In our example, we want an ARC of about 80% to ensure optimal retention. When the green roof is totally saturated and cannot hold another drop of water, we have 0% ARC.

So how does ARC go from a low number, like 0% or 10%, to a high number, like 70% or 80%?
It does this by allowing evapotranspiration to occur.

Stormwater retention and old ways of thinking

So why is retention often considered the “free gift with purchase” of stormwater management tools? Mostly because of the finite nature of retention noted above.

Think of the green roof as a cup. Once the cup is full, it no longer works as a retention device.

Whereas retention-focused approaches work sometimes and are very effective at lowering runoff averages, stormwater engineers usually need to design solutions that work all the time, or at least most of the time. The solutions must also work when there is no time for evapotranspiration to occur, such as between repeat storms, or when recharge is very slow, such as during wet winters. Also, it must work during extreme weather events such as a 100-year storm.

Stay in touch!

Hopefully, this article clarified the concept of retention in green roofs. We will continue this series by posting articles with new research and new takes on existing research to drill deeper into retention and other topics. We will also explore how green roofs can provide stormwater benefits beyond retention.

If you have any questions, please don’t hesitate to contact any of our experts!

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Reading tip: what is green roof retention?

Stovin, Virginia, et al. "The influence of substrate and vegetation configuration on green roof hydrological performance." Ecological Engineering85 (2015): 159-172.

Audio article

This article is available as an audio version on all major podcast directories such as iTunes, Spotify and many more. Just search for Purple-Roof!