Green Roofs and Blue Roofs – contributing to successful SUDS strategies

The increased frequency and severity of flooding in recent years has highlighted the need to supplement conventional drainage systems with additional, sustainable capacity.

Here, Simon Poe, Product Director with Alumasc Roofing, looks at how Green Roof Systems and the recent development of Blue Roofs in the UK, are contributing to successful SUDS strategies.

To date, the inclusion of green roofs within SUDS strategies has been limited, but they offer a number of advantages complicit with the Government’s Water strategy for England (DEFRA 2008). Specifically, green roofs will:

• Occupy otherwise redundant roof space

• Extend the roof waterproofing’s durability

• Reduce energy consumption; and

• Provide additional amenity space

Green roofs also help to reduce the peak rates of runoff (that can cause the sewer network to overflow, leading to localised flooding) by 2 main methods:

1. Retention

Reducing overall and peak runoff volumes that discharge directly into watercourses; and

2. Detention

A delay in peak runoff rates associated with the time it takes for rain to infiltrate the green roof build‐up, discharging at a later time and/or over an elongated period.

Long term hydrological research carried out at the University of Sheffield, indicates that green roofs are effective in the reducing run-off volumes, however, as with any flood mitigation measure, drainage engineers would be particularly interested in the capacity of a green roof to manage the kind of storm events that would typically induce localised flooding.
Whilst all drainage systems have a finite hydrologic capacity, there are added complexities with green roofs (and other vegetation-based solutions); namely, with regards to the extent to which this capacity is available at any given time – this is known as the Available Water Capacity (AWC). This capacity is reduced by residual moisture within the green roof layers and will therefore vary according to:

• Nature of substrate

• Vegetation

• Incidence of rainfall; and

• Atmospheric conditions

Research into the predictive modeling of a green roof’s hydrologic response is at an advanced stage and is progressing towards a means of being able to estimate that response – accounting for configuration and climatic factors – with greater accuracy.

However, by combining a blue roof system – where detention and maximum discharge are calculated using hard engineering principles – with the additional retention capacity of a green roof, it is possible to realise not just the hydraulic performance requirements, but every aspect of SUDS design criteria.

Blue Roofs – Looking to the past for future solutions

Blue Roofs are not a new concept, having been adopted in US cities for approximately 50 years as a means of managing stormwater runoff in urban areas.

Blue roofs are designed to alleviate flood risk by reducing and controlling the peak rate of discharge, facilitating the detention of stormwater up to a prescribed maximum hydraulic head, for subsequent discharge over an elongated period.

By restricting the peak rates of runoff from the roof, rainfall does not instantly discharge into the subterranean drainage or waterways, therefore the probability of sewer overflow and localised flooding is reduced.

A blue roof system comprises 3 primary facets:

1. An engineered restriction device is used in conjunction with a drainage outlet to restrict the maximum permissible discharge of stormwater from the roof. Consequently, during more intense storms, rainfall will exceed the rate of discharge, leading stormwater to temporarily back-up on the roof until such time as the drainage outlet has the capacity to discharge it.

2. Technical guidance pertaining to stormwater management calculations, provision of safety overflows and the adoption of a robust planned and preventative maintenance programme etc.

3. Waterproofing – the specification and detailing of which is integral to the success of a blue roof system – protecting the building against water ingress.

Green and Blue – The Optimum Solution

Blue roofs can include open water surfaces but can also be used in buried applications, such as raised deck surfaces or green roofs. Indeed, by partnering a green roof with a blue roof system, the retention of stormwater in the green roof’s layers, complements the detention of stormwater by the blue roof to provide valuable additional drainage capacity.

The Government’s Water Strategy for England acknowledged that below-ground pipe systems cannot cope with extreme rainfall events, identifying the importance of SUDS and green roofs for water management and climate change mitigation. The combination of green and blue roof systems represents the ultimate sustainable roof drainage solution and the need for inclusion of these systems in our every-day architectural designs is already a reality.

To learn more about Blackdown Green Roof systems click here or for information on BluRoofs please click here.  Alternatively please call 03335 771 500.

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