Do Green Roofs Provide Storm Water Attenuation?
If you were at the World Green Roof Congress in London last week, you may well have heard James Berryman speak on behalf of Micro Drainage about their research on green roofs.
The general consensus from the conference was that the momentum behind the implementation of green roofs is shifting from aesthetic to the wider benefits. These include (but are not limited to) amenity space, urban heat sink, enhanced biodiversity, improved insulation and pollutant removal, air quality improvements and finally (and what seems likely to be most important) storm water management.
Previously green roofs have required a leap of faith by engineers, as they have been unable to quantify the storm water benefits and calculate a cost benefit analysis. The joint research between Micro Drainage Ltd and the University of Sheffield now finally allows engineers to do this. The method has also been recognised in the Environment Agency’s Green Roof Tool Kit
The scale illustration below helps us visualise the benefits. Consider a 2 ha warehouse roof discharging to a storage tank. If a green roof is used the volume of storage required would reduce from 450m³ to 300m³.
This idea of quantifying the benefits of green roofs is a recurring theme and is by no means limited to storm water management. Several of the conference speakers quantified the benefits of green roofs, including their ability to provide reductions in air conditioning costs and to increase the rentability of buildings. An International example from Germany highlighted how tax rates for buildings with green roofs are reduced. If such a system were applied in the UK, this would be yet another financial incentive for their use.
As the onus on green roofs shifts to meet the more complex benefits, there is a need for correct and more detailed design, specification, installation and maintenance. Failure to adhere to these four criteria could undermine the green roof movement as systems fail due to poor designs and inexperienced implementation. With this in mind, there remain requirements to improve UK standards, regulation and training to sustain a high level of quality.
For more, download the presentation here:
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Green Roofs can also contribute greatly towards stormwater management on a small scale also. Referring to ultra-light weight extensive green roof systems we are working with here in Florida, it is amazing to see the data on absorption and sequential ET for small to medium rainfall events.
With engineered soils targeted to drain – not hold water and plants designed to rapidly grab the rainwater passing by roots we see extensive green roofs capture most of the rainfall on events one half inch or less.
Granted these do not come everyday here in Florida and hence there is time for recovery, however during the rainy season – July-September – the plants grow quicker and tend to acclimate to the additional water, absorbing more.
While 1/4 to 1/2 inch may not be much (12 – 25 mm) it all adds up, and here in Jacksonville on the St. Johns River, keeping first flush contaminants (nitrogen, phosphorous and Oils/greases) out of the stormsewers is important.
Finally, these roofs are very light weight – 15 lbs or so per SF live saturated load.
This means expensive retrofit is not always necessary.
It is exciting to see advances in green roof technologies move forward.
Cheers!
Kevin
kevinsonger.blogspot.com
metroverde.com
greenbmp.blogspot.com
Thanks for the comment Kevin! It’s great to hear about your work in Florida and particularly rewarding to hear of the obvious benefits green roofs are having.
We’ve been in Scotland this week looking at legislative changes on the uptake of BMPs or SuDS that may or may not include green roofs.
As you say, it’s an exciting time for the industry!