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Green roof stormwater benefits

by Matt Downs

In the latest instalment of his regular column, Dr Tom Young looks at the benefits of green roofs with regards to stormwater management in urban areas.

Dr Tom Young is on the GRO Board and Environment Systems Manager at STRI Group.

One of the most celebrated benefits of green roofs is their ability to store stormwater. This is very useful in urban areas where conventional roofs are designed to drain rapidly, releasing large amounts of water into main drainage networks. With increased development and the impacts of climate change, this rapid drainage of water can lead to localised flooding as drainage systems become overwhelmed. Therefore, the ability of green roofs to hold water and reduce runoff can be used as part of integrated Sustainable Drainage Schemes (SuDs) to reduce flooding risk (Getter 2006).

A huge amount of research has been conducted in this area. This research covers many topics, but for the purposes of this article, we’ll just look at stormwater volume reduction.

Stormwater reduction 
A large number of studies have shown that extensive green roofs – less than 150 mm substrate depth – can reduce annual runoff by between 27-81 %, and intensive – substrate depth above 150 mm – by between 65-85 % (Mentens et al. 2005, Berndtsson 2010). However, this only tells the story when averaged across a whole year. It is known that flood events are much more likely to occur during periods of heavy/intense or prolonged rainfall. Green roofs are less effective when viewed in this context, with less rainfall retained as rainfall event intensity, which is measured as the amount of rainfall falling in a period of time, increases (Berghage et al. 2009, Berndtsson 2010, Stovin 2010).

For example, Carter & Rasmussen 2006 found that extensive green roofs retained 88% of rainfall for small storms (<25.4 mm), 54 % for medium (25.4-76.2 mm) and 48% for large (>76.2 mm). However, despite this, the peak discharge during heavy rain events can still be reduced by up to 50% (VanWoert et al. 2005) or at least delayed between 0.5-2 hours by green roofs (Berndtsson 2005, Berndtsson 2010). Both of these will significantly help reduce urban flood risk if green roofs are deployed on scale (Carter & Jackson 2007).

These figures are general figures for green roofs, but different layers of a green roof also affect stormwater runoff in different ways.

Vegetation
Generally, the more complex, diverse and dense the vegetation layer, the more water that will be initially intercepted and trapped on leaves. This can help to slow down the movement of water to the rest of the green roof (Nagase & Dunnett 2012). Deeper and more complex root systems can also help bind the substrate together, allowing it to hold more water. However, in some studies, vegetation has been shown to have had a minimal impact on water flow through a roof, and so care must be given to attributing too much impact on water flow dynamics by the vegetation in some situations (Berndtsson 2010).

Substrate
The type of substrate used on a green roof can vary significantly. This can cause local differences in water holding capacity. However, in terms of stormwater retention, this has not shown to be a significant factor in green roof stormwater retention performance (Stovin et al. 2013). More significant is the depth of substrate, with an increase in stormwater holding capacity as depth increases (VanWoert et al. 2005). More important is the current level of moisture held within the green roof substrate. If this is currently ‘full’ i.e. after a previous rain event, then water will start flowing through the system much earlier than if the substrate was ‘dry’. It is important to consider how quickly this substrate water holding capacity can be recharged between rain events, with vegetation, climate and slope all important factors (Getter 2006).

Drainage layer and additional water storage layer
The effect of drainage board layers can increase the water holding capacity of a green roof. Generally, the deeper and greater capacity of a drainage board, the greater the effect on slowing stormwater release will be (Vesuviano & Stovin 2013). This drainage layer can be increased to a much greater extent (40 mm +) in order to provide significant improvement in water holding capacity. Known as ‘blue’ roofs, this type of system can hold significant volumes of water which can also be used to irrigate the green roof, or for use as non-potable water for a building (Andenaes et al. 2021). These types of systems can increase annual water retention from 30% (green roof) to 50% (blue green roof), and peak flow events from extreme events from 12% (green roof) to 60% (blue-green roof) (Busker et al. 2022). Additional ‘smart’ systems can be used in parallel to discharge water ahead of forecasted storm events in order to free capacity. Using this type of system can result in annual water retention of up to 90% and peak flow events to 70% (Busker et al. 2022). 

Conclusions
Green roofs are fantastic green infrastructure tools which can be used to improve urban spaces due to the many benefits they can provide. Stormwater retention is one of these tools, but must always be treated with caution. As part of integrated site wide SuDs schemes, green roofs have a huge role to play in stormwater reduction. When built in isolation, green roofs can still reduce the impact of extreme rain events, but probably not as efficiently as other SuDs options. Therefore, care must be taken to ensure all aspects of a site’s drainage scheme are fully integrated and calculated in order to get maximum benefits for the site solutions implemented.

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