Google+

Archives for May 2013

Sustainable Drainage Systems for Green Roofs – SUDS

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.

www.gardenvisit.com green roof suds

www.gardenvisit.com

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!

Sourced form- Google Images

Sourced form- Google Images

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!

www.susdrain.org.jpg SUDS green roof

www.susdrain.org.jpg

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. 

(3251)

To Green or Not to Green? Article on Green Roofs.

 Modern beautiful architecture is on the rise within the more vibrant sectors but with the recession still firmly in our lives and conscious new inner city buildings are not exactly popping up at a rapid pace. With this in mind for the most part, what buildings do we all have to enjoy today? How does your city scape look, beautiful and architecturally stunning or grey, drab and dull?
 
www.freeelectricitygenerator.co.uk

www.freeelectricitygenerator.co.uk

We may not be able to knock every grey Brutalist building down within some of the concrete jungles we live in, and some may even appreciate the grey backdrop to their workplace or residence but there are some retrofits that could help to make the environment we live in a greener and more pleasant one. I’m not out right knocking concrete, yes it is a useful versatile material but after the post-war construction drive to repopulate many of our cities what we are now left with 50-60 years on is a multitude of grey. One possible way to work with the stock we have is to retrofit the buildings with green roofs. This may be something which in new builds is often at least considered if not implemented but why not in the case of retrofits, the benefits are multiple.

For retrofits then intensive and simple intensive green roofs will pose a much greater problem in regards to structural stability of existing structures, whereas extensive design on the other hand presents a far greater opportunity in that the loadings are pretty much comparable to a standard flat roof construction. Extensive green roofs generally consist of a thin layer of soil, generally 100mm or less with relatively simple vegetation such as Sedum or Moss’s. Intensive green roofs have an increased level of depth, and hence more consideration is required in regards to structural support.
 green roof

www.worldchanging.com green roofs

 

Firstly let’s consider the technical and environmental impact of green roofs. Green roofs can be seen as one way to help with the global issue of climate change, however small an impact overall they may have, for global low impact buildings to become a reality every effort is needed no matter how insignificant it may appear in the wider context. One of the ways in which a green roof can help in this aspect is through increasing the thermal performance of a building. This is achieved through the green roof substrate and vegetation layer acting as a natural insulation barrier keeping unwanted heat during summer months whilst at the same time reducing heat loss during colder times.

As stated by CIRIA, Building Greener (2007), “Green roofs have a substantial thermal mass, a moderate insulation value and some cooling effect through evapotranspiration. These combined properties significantly reduce daily range of temperatures at the boundary between green roof and building structure” Making for a greater level of stability and comfort in regards to the internal environment.
goats on green roof

www.ecogeek.org goatsonroof

The vegetation that is provided through having a green roof can help increase the biodiversity of the surrounding context. By using a mix of vegetation, not only can the overall maintenance of the roof be reduced but the biodiversity can be increased. Now…I’m not suggesting that we should be farming goats on our urban green roofs as per the image to the left but you get the idea. To improve the biodiversity the simple step is to move away from the standard moss or sedum blanket and move closer towards the use wild-flowers which can be regional and hardy.  Wild-flowers will help to reduce the impact of higher winds whilst also enabling the roof to cohesively integrate with the natural wildlife which currently strives in the local habitats. 

 

Green roofs which include SUDS management can have further beneficial effects in terms of Carbon Sequestration and Storage (CSS). Looking at studies carried out by Getter and Rowe (2009), who carried out a study over the space of 2 years on 32 extensive green roofs in three US cities. The average CSS rates which they calculated was 0.375gC m-2. Through this research they calculated that if the city of Detroit were to install green roofs onto the 15,000ha of their rooftops then, “55 252 tons of carbon could be sequestered in the plants and substrates alone (not including avoided emissions). This is similar to removing more than 10 000 mid-sized SUV or trucks off the road for a year.”
 

Danny McGough green roof detail pdf

Danny McGough

As stated by Dr Charlesworth in (2010) a University lecturer specialising in SUDS, “Whilst there are many studies of the (CSS) abilities of certain SUDS devices, such as constructed wetlands, these would not necessarily be installed into urban centres. Green roofs, on the other hand, offer great potential both for new build and retrofit.” I will cover SUDS in greater detail within a later article as the details and benefits of specifying SUDS drainage are vast. So with all these benefits why don’t we paint the town in green roofs?
Retrofitting existing roof structures will obviously have a cost impact however if the roof needs repairing or upgrading regardless than obviously the impact of the costs is reduced. In a new development project  however the cost difference between specifying a green roof or not is far less or even non-existent in some cases. If we assess the green roof over the whole life instead of just the initial outlay then significant economic benefits can be seen.  As stated in Green Roofs – Benefits and cost implications (2004) “As whole life costing for new  development is emerging as an important tool for sustainability,  the fiscal benefits of a green roof during the life of a building after construction are likely to become more relevant.” Green roofs can also extend the life of a roof and reduce the service and maintenance that is normally required. This is achieved by the substrate and vegetation layer acting as a protection layer over the top of the roofing membranes, protecting them from UV light and frosts. Savings can additionally be made through the reduction of fuel costs through increasing the thermal performance of a building. This is achieved through the green roof acting as a natural insulation barrier reducing the variations in temperature within the building, keeping the building cool in the summer months and insulated further in the winter months. By reducing the heating and cooling load through the installation of this additional insulation layer, the demand on the energy required to heat and cool the building is thus reduced. With the targets for hitting carbon reductions looking tougher and tougher every small step that can be made is important to the overall grand scheme of things.

Moving beyond the technical jargon of the benefits of green roofs I also think it’s important to consider the social impact that a green roof can have on the people who inhabit the surrounding environment. With urban regeneration the focus can sometimes
be on creating more robust and versatile spaces rather than creating more visual and natural spaces.
 

www.greenroofstoday.co.uk  green rooftop

www.greenroofstoday.co.uk green-rooftop

A local space that I work near in Coventry has gone from this (1930’s):
www.bbc.co.uk  coventry broadgate 1930

www.bbc.co.uk

 


To this(1970’s): 

Sourced from- Google Images Coventry broadgate

Sourced from- Google Images

Sourced from- Google Images Coventry broadgate

Sourced from- Google Images

To This (2012);

www.newsrt.co.uk Coventry broadgate  new

www.newsrt.co.uk


Now, granted the vehicle traffic has been removed which  improves the robustness, versatility and openness to footfall fair enough, but could the planners not of kept some of the greenery? With us losing or have already lost plenty of green spaces  in our urban areas could green roofs be the answer to creating a more enjoyable and visually appealing urban environment, which in most cases is an urban grey cloud?

Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog. 

(9178)

By continuing to use the site, you agree to the use of cookies. more information

The cookie settings on this website are set to "allow cookies" to give you the best browsing experience possible. If you continue to use this website without changing your cookie settings or you click "Accept" below then you are consenting to this.

Close