Is torrential rain a valuable resource?5 May 2021
Can we protect our cities, rivers and flood plains from damaging climate-driven rainstorms? Yes, through a new approach to urban landscaping with multi-layered solutions that slow, store, filter and turn storm-water into a resource for irrigation, new habitats and community spaces. Green roofs are an excellent starting point.
As we come out of the pandemic, housebuilding and large commercial units are set to see a new delivery boom. But the traditional way of delivering these important assets needs a radical rethink, and possibly legislation to make key elements like green roofs mandatory.
By solving many complex interconnected environmental issues successfully, green roofs and rain gardens are rapidly becoming storm water management’s first line of defence on high-density urban developments.
If that seems surprising, it is worth noting that companies such as United Utilities in Manchester have been offering various storm-water bill reductions for many SUDS projects (including green roof solutions) on non-commercial properties where runoff and peak flows are reduced.
More green roofs over our heads
Add the versatility of “green” and “podium” solutions, and green roofs become ideal when creating intensive hard-paved communal spaces, private gardens, plus bio-diverse roofs which replicate rare habitats that attract solitary bees, invertebrates, reptiles, and even rare birds like the black redstart.
In fact, the potential for huge combined acreages of modern urban green roof space have important implications not only for flooding but alsourban heat island cooling and the future of under-pressure habitats and much-needed community amenity areas.
More on that later, plus different green roof approaches for retaining water, improving air quality, supporting renewable energy and even producing food in the modern urban environment.
Green roofs within SUDS
However, green roofs are just one component of a wider set of Sustainable Urban Drainage System (SUDS) elements that are gaining significant traction as design solutions for growing environmental challenges.
Instead of relying on hard below ground infrastructure, SUDS where possible seek to mimic natural hydrological processes based on the four key principles of water attenuation (volume), quality, amenity, and more recently bio-diversity. How these are applied varies across the UK.
SUDS across the UK
Traditionally, the England and NI focus has been volume (flooding) and quality. In Wales and Scotland priorities are slightly different; Wales has now enshrined SUDS project delivery in law.
With the current emphasis on whole green infrastructure and natural capital systems, SUDS is increasingly being used to not only attenuate and clean water but also increase biodiversity, create amenity and add value through place-making.
To start we need to review the core problem of flooding.
Historically, our response has been to channel storm water away as quickly as possible via hard infrastructure to rivers and on to the sea. By necessity, this is now changing swiftly.
Today, the growing frequency and intensity of erratic storms, large areas of impermeable surfaces, old combined Victorian sewerage systems, plus our growing use of vulnerable flood-plain land to meet development targets, is overloading these old systems.
This is further compounded by rising sea levels and sea surges that are forcing estuaries and rivers to back up more regularly, spilling water with nowhere else to go, but out across low-lying land.
A 180 degree change of direction
Our new response is quite different. Instead of removing storm water rapidly – overloading streams and rivers with huge peak flows – the contemporary idea is to reconnect with natural systems and hold water back in a myriad “small places” as part of “source control”.
By seeking to mimic natural processes, significant reductions in peak flow can be achieved. This in turn increases groundwater and aquifer recharging (maintaining a more consistent baseline) where infiltration is possible, and forms important new habitats.
The integrated nature of many small solutions can call for the combined skills of hydrologists, civil engineers, ecologists, arboriculturalists, landscape architects and planning and permitting teams like Enzygo’s (https://www.enzygo.com/our-services/).
Ideally, SUDS options are considered right at the start of projects during the land acquisition stage. Rather than crowbarring a solution into a fixed design, the right SUDS approach should inform design from the outset, leading to more considered, integrated and cost-effective design that is multi functional delivering on amenity, recreation and biodiversity, as well as volume and quality.
Sustainable water management
I would like to look at modern SUDS approaches here because they help to put green roofs into context as a key component. Our new flood water management approach is reflected in the work of CIRIA’s (Construction Industry Research and Information Association – https://www.ciria.org).
CIRIA believes that combining the current emphasis on volume (flooding) with the other three pillars of SUDS, biodiversity (wildlife and plants), amenity (community infrastructure) and quality (clean water) creates massive opportunities for multifunctional landscapes and innovation.
A more detailed SUDS definition may be helpful here too. It is often seen as a series of integrated management practices, control structures and strategies designed to drain surface water away sustainably while minimising pollution and safeguarding the quality of local water bodies.
SUDS works best where land is available that can be profiled into shallow swales, reed beds, filter trenches, retention ponds and basins that clean and store storm water temporarily until it can infiltrate into the ground, or discharge slowly into the wider catchment.
No artificial energy is needed beyond that used for maintenance. Compared to hard below ground infrastructure, SUDS are intrinsically robust, attractive and environmentally sound.
In high-density urban situations where large conveyance and detention features are less feasible, more use can be made of the hard landscape, with green and blue roofs that store water, attenuation on landscaped podiums, rain water harvesting for irrigation, toilet flushing and wash down use, rain gardens designed to hold roof run-off temporarily, structural root cell systems for urban tree planting and permeable paving.
Another concept being pioneered in the Netherlands is water squares where a piece of hard infrastructure, such as a MUGA (multi-use games area) or parking area, is allowed to flood during peak events.
In short, the goal is to make the urban landscape work harder through layered multi-functional solutions with SUDS and green infrastructure at its heart. Surface water management is, and should be, seen as a valuable resource to be managed for maximum benefit.
Towns and cities
The concept is so powerful that the SUDS approach might have been a better option than the new 25km (16-miles) Thames Tideway Tunnel mega-sewer currently being completed under central London to take extreme weather discharges from 34 combined raw sewage and rainfall overflows to five modernised wastewater treatment works on the lower Thames.
SUDS could potentially have halved the cost, while improving immediate air quality, shrinking the city’s carbon footprint, reducing the urban heat island effect, delivering an attractive green environment and increasing biodiversity. Importantly, both operating costs and customer bills would have been lower. (Thames Blue Green Economy Group)
Millions of small places
Having looked at SUDS and green roofs on a macro-scale, we should consider micro-scale gains – green roofs with a thin growing medium (subject to loading calculations) can be installed over community buildings, supermarkets and factories, but also homes, extensions, garages and garden structures.
The idea is that multiple-provision of individual leaves, grass blades, mosses, bogs, marshes and puddles jointly extends the time water takes to reach sewers, tributaries, streams, rivers and the coast.
Incidental losses – through leakage, storage, evapotranspiration and infiltration – are a key component; where appropriate, “leaky” SUDS solutions slowly recharge local ground water and sub-surface aquifers, leading to a more consistent water table and base line supply for both people and nature in intervening drought periods.
Making systems more “leaky” also means that rather than coping with current models of, say, 1-30 year major flooding events, we can build in resilience to potentially deal with 1-100 year events. That’s a crucial gain!
What trees can teach us
A good example is a tree. It is calculated that the “rain shadow” of trees prevents 35% of all rain fall ever reaching the ground. Roots break up the soil, allowing infiltration of water to the ground and bed rock. Evapotranspiration from a mature tree can also run into the hundreds of litres. In an urban setting this also has the added benefit of cooling the local micro climate and filtering out air pollution
Other options include leaving swamps and high upper catchment, moorland environments to develop and clog up naturally – reintroducing dam-building beaver populations, (a keystone species and nature’s SUDS engineers) which have evolved over millennia, helps to hold back water for critical periods and create new mosaic habitats for wildlife
This is particularly important for cities like Sheffield where only 30-35% of surface runoff is urban. Therefore, improving the resilience of upstream catchments with, for example, foresting and upland wetlands will reduce peak flows, the risk of flooding downstream and improve water quality.
Vegetation-covered roofs work on many scales. Some are being retrofitted into hard urban environments, often to tackle persistent local flooding issues. Others have city-wide targets.
Washington D.C, New York, Los Angeles, Chicago, Atlanta and Portland are among US cities pioneering green roofs and integrated SUDS solutions. In Europe cities like Stuttgart and Copenhagen are leading the way; mandating green roofs on most new construction. In fact, green roofs are now seen around the world from Australia and Canada to India and the Philippines.
London is leading the way on SUDS in the UK with its Urban Greening Factor initiative. Manchester is following suite with its City of Trees project and structural tree pits; with the conurbation’s green infrastructure plans an important part of the ambitious Northern Forest project. Across the rest of the UK, green roofs can now be seen from Southampton to Hull and Aberlour in Scotland.
Bringing the necessary skills together
So, how can we bring these many different solutions together as the environmental crisis seems to be deepening? Enzygo offers several options (https://www.enzygo.com/about/).
As an independent, multi-disciplinary environmental consultancy delivering creative, integrated and cost-effective solutions which maximise the potential of development sites, the company is well-placed to deliver SUDS and green roof projects on any scale.
Our integrated consultancy teams work on many different types of developments, from major infrastructure programmes and residential housing schemes to renewable energy strategies that draw on the joint environmental disciplines of Planning, Hydrology and Drainage, Permitting and Regulation, Landscape, Ecology, Transport, Geo-Environmental and Hydrogeology, Noise and Vibration, and Arboriculture.
In addition we have recently partnered up with http://www.ecogreenroofs.co.uk/ as part of our offer of comprehensive SUDS design solutions to clients.
Case study examples
The best way to illustrate the effectiveness of integrated SUDS in action is perhaps through several recent practical case examples where we were able to turn problems into green solutions.
Blackfirs – residential scheme, Congleton
This residential development required an integrated design approach, balancing the need for surface water attenuation with existing wetland habitats to the south and new housing development in Congleton. The scheme introduced a new central SUDs feature and then utilised the existing wetland area by enhancing its usage with new pond habitats, planting and management techniques.
Planted with wetland tolerant meadow and tree species – like Birch and Common Alder – the new ponds and enhanced wetlands can cope with 1-in-100 year rainfall events filtering run off and slowing the release of storm water. This was complimented by a 15year Landscape and Ecological Management plan (LEMP) Creating a new wildlife corridor around the Northern and eastern boundaries and a significant increase in wetland habitats (Meadow, Long grass, marshland and ponds). This led to a positive Biodiversity net gain and integrated functional landscape providing valuable amenity and character to the development.
Logistics and Distribution Park – Yorkshire.
Enzygo were commissioned to provide full environmental services to support the outline application for a sustainable extension to an existing logistics and distribution park in Yorkshire.
Working with our ecology, hydrology, arboriculture and transport teams, we developed an integrated design solution which responded to the existing landscape constraints. This led to a surface level cascading SUDs solution which enhances and creates new green corridors.
The approach also seeks to improve biodiversity by creating additional habitats, (woodland, scrub, meadow and wetland) and adopting less intensive management practices (such as wildflower verges and triannual cutting) which allow wildlife to flourish.
Other inspiring examples
Grey to Green Sheffield (http://www.greytogreen.org.uk/) also shows what can be achieved in a wet urban environment where exciting public spaces now double up as a rain garden, moderating water flows as part of a SUDS scheme in a part of the city twice damaged severely by floods.
“Sponge Park” at West Gorton in Manchester is a £1.3 million SUDS project designed to prevent flooding by holding excess rainwater from nearby streets while it infiltrates grass and soil instead of rushing quickly over impermeable surfaces into overloaded local drainage systems.
Queen Elizabeth Olympic Park (https://www.queenelizabetholympicpark.co.uk/) is part of London’s newest park created to be sustainable. Its wetland bowl is a natural feature that supports plants native to Britain, including 30 species of rushes, reeds, grasses, wet wildflowers and irises.
The result is a leisure space for visitors, plus a wildlife habitat that is not only home for young fish, amphibians and nesting birds but also helps to remove contaminants from the nearby River Lea while protecting nearby housing and venues from flooding.
Types of green roofs
Green roofs generally are classified as four types, with some cross over. The first is “intensive” used as parks, gardens and for urban agriculture; the second “semi-intensive” for green garden roofs.
The third is “extensive” in the form of natural low maintenance green roofs typically planted with sedums, but also succulents, mosses and wild flowers. The fourth are bio-diverse roofs which are naturally colonised extensive roofs with varying habitats and substrate depths to mimic local or lost brownfield sites. However, there are interesting variations.
Blue roofs, for example, can be a cell system under a roof, or more recent approaches that include a growing medium but no drainage layer. Natural cross flows to a restricted outlet and overflow hold water back with a controlled slow release.
Weight is usually no issue and falls within snow-loading allowances. For either approach major saving can be made compared to the installation, running and maintenance of tanks and pumps used for below ground attenuation on high density sites.
Blue-green roofs can also incorporate technologies that maximise water storage, plus grey water cycling. Bio-solar green roofs do the same for renewable energy. Other designs support food production and reduce food miles! The only limit is imagination.
One area I haven’t mentioned is the legal position. By law, a SUDS option should be included in development planning applications. The revised National Planning Policy Framework (NPPF) requirements also infer an expectation of a net biodiversity gain to be made. On dense urban/industrial sites this can be achieved through offsetting, commuted sums, or integrating SUDS e.g. via the roof-scape.
Far too often, green infrastructure-driven SUDS is omitted on the grounds of adverse site conditions, perceived costs and barriers to adoption. I think it is inevitable that SUDS must and will be become mandatory. ( as is now the case in Wales) The good news is that it offers a broad range of very diverse and extremely successful solutions.
Please contact me if you would like to discuss urban water management further, or would like more information.
Graham Bailey, Associate Director Landscape, Enzygo
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