This week’s article is a follow up to the previous article on green roofs looking at SUDS. Within the ‘To Green or Not to Green?’ article I mainly asked the question of why are we not using green roofs more often within our urban landscapes, the main focus was on the aesthetics qualities of green roof as well as the benefits in regards to well-being. This time around I will be focusing more on the technical aspects, in particular the Sustainable Urban Drainage Systems, otherwise known as SUDS.
Sustainable drainage systems can be incorporated into our urban landscape in a multiple of ways such as through green roofs, permeable pavements, rainwater harvesting, infiltration trenches, infiltration basins and many more. Green roofs can be extremely useful in reducing the amount of rainwater run-off especially in urban environments where much of the area is often hard surfaced which in consequence creates stormwater management issues within our urban landscapes. One solution to this issue can be through specifying and installing SUDS into our urban systems. If SUDS are is integrated into a green roof the benefits are numerous. Not only can it improve the aesthetics of an area as discussed in my previous article on green roofs, it can also brighten up the grey drab urban landscapes many of us are subjected too. Further benefits can be gained through improving environmental and drainage system management in technical terms as the plant species and substrate held on a green roof will naturally absorb rainfall thus reducing run-off into man made drainage systems considerably.
As there are on occasion, limited or no natural stormwater management systems in urban areas most if not all of the rainwater runoff has to be managed by manmade systems, these systems can become overloaded at times of small and large events (storms or heavy rain). Green roofs can provide a double edged solution to help reduce this issue. As stated in the Green Roof Guidelines (2012) “Once a green roof has established, both peak flow rates and total runoff volume of rainwater from the roof are significantly reduced compared to a conventional roof.”
Green roofs are particularly efficient at reducing rainwater run-off when encountering small events (light rain) but are less effective at times of larger events (heavy rain/storms). According to CIRIA in (2007), green roofs will reduce the annual volume of rainwater run-off by between 50% and 85%! The variation in the percentages is down to the variations which can be found in the construction of the SUDS green roof, which as you can imagine are vast. But if were to take those figures as an average that’s a reduction of 67.5% in rainwater run-off! That’s a significant amount. Even when a SUDS green roof becomes saturated it still has beneficial affects as the run-off is at least slowed down through having to pass through the drainage systems, vegetation and substrate.
To understand exactly how a green roof can reduce rainwater run-off it is important to understand the process of evapotranspiration in SUDS systems. When a green roof is exposed to a small event, much of the rainfall which falls on the roof is captured by the substrate, the drainage layer and also upon the surface of the plants and vegetation. A large amount of the rainfall which is held on the green roof, through the process of evapotranspiration is generally removed. The rainwater that is absorbed and passes through the vegetation and substrate layer and then runs off will have reduced pollutants as much of it will be removed through the natural filtration process as it passes through the layers of the roofs. That nature and passive design at its best doing all the work for you!
So as I talked about earlier a green roof can significantly help to reduce rainwater run-off and thus reduce the pressure on man-made drainage systems which in turn reduces the peak flow rates by reducing the volume of total rainwater run-off. The amount of rainwater which will run-off a SUDS green roof is dependent on the construction type used which as touched on earlier can have many variables such as; the depth of the substrate, the type of vegetation, the specification of the drainage layer and of course the local weather. According to the Green Roof Centre of research (2012), run-off can be prevented from all rainfall events up to 5mm. Further figures from the same source state that, “In summer, green roofs can retain 70–80% of rainfall and in winter they retain 10–35% depending on their build-up”. The variation between the two seasonal periods is due to the higher intensity of winter rainfall and the reduction in evapotranspiration by the vegetation, which in essence is the process when the water retained in the SUDS systems and evaporates as I discussed earlier.
Now believe me many lucky people may not see this factor as an issue, which in many cases it may not, however when you live in a built up urban area and you leave your front door to casually make your way to work, with a well thought out length of time allocated for your morning journey only to realise that with a shock the whole walkway is blocked due to the ‘gentle’ waterway that usually presides 50 metres from you path has now burst its banks and completely flooded your route the issue become a more pressing concern!
Now over the last two articles I’m not out right stating that green roofs are perfect for every case, what I’m eluding to is that under the right circumstances green roofs are a viable if not extremely beneficial specification. Some of the concerns or issues which should be understood when fully assessing the viability of green roofs and SUDS include the on-going maintenance which a green roof will require. For extensive green roofs it would be minimal, annual maintenance would suffice after the first few years bedding in period has passed (which many companies generally provide as part of the installation costs). However intensive green roofs do require some more regular maintenance, such as planting bedding plants and maintaining the removal of weeds etc. so additional costs have to be accounted for over the whole life of the roof. If the right vegetation is selected the maintenance levels can be reduced as many types such as wildflowers will naturally look after themselves.
Looking at the whole picture there are multiple benefits to green roofs and SUDS such as reducing the rainwater run-off, improving the energy performance of buildings and improving the biodiversity of our habitats. Yes there may be additional costs through increased set up and maintenance costs but much of that can be offset if you look at the whole lifecycle cost of the roof. Aiming for a reduction in pollution and an improved low impact and sustainable environment, in spite of the recent recession should still be a topic that we as responsible inhabitants of our landscape should continue to drive towards. My hope is that with the obviously needed cutbacks and austerity measures that this ethos isn’t cast aside for cheap, bottom line construction.