Changing our approach to flood defences

Materials World magazine
,
1 May 2016

Peter Wilder argues for a flood defence approach based on natural systems integrated into every building and space within our urban environment.

An exciting new project at the Natural History Museum in London will put mankind’s 200,000-year existence into the context of the Earth’s 4.5 billion year history. Apart from being a historical timeline, the project will explore mankind’s relationship with the environment. It will ask some fundamental questions about the measures that will be necessary for us to take if we are to survive in an ever more urban setting, including the issue of resilience and self-sufficiency. A major part of this story is how we live with water in an increasingly unpredictable climate.

The Pitt Review, carried out in the wake of the devastating British floods of 2007, recognised the need for sweeping reforms in the UK’s management of surface water run-off. Many of the recommendations were aimed at a greater collaboration between water authorities and the Environment Agency, and measures to enable communities to obtain funding for flood resilience measures. The report, however, aimed most of its recommendation at improving the lives of people who had been struck by flooding rather than at the root cause. The Flood and Water Management Act subsequently introduced in 2010 brought in a range of measures to manage infrastructure assets but with only rudimentary guidelines on the adoption of sustainable drainage systems (SuDS).

SuDS, which introduce more natural hydrology into urban areas, have been around for several decades. Their performance, cost and resilience to extreme storm events has been proved time and time again, yet the SuDS Manual, a definitive guideline for their implementation, was only released by CIRIA in November 2015. This delay in the widespread adoption of SuDS is less to do with a lack of empirical data on their performance than a lack of willingness to turn away from an infrastructure-led approach to drainage. This is a common problem in the developed world and even more so for countries like the UK that invented and exported their drainage technology to the rest of the world. It is, perhaps, the amount of existing and ageing piped networks that keeps us held in the grip of maintaining and extending existing systems. 

An alternative approach

In other developed countries such as Australia, USA, China, Germany and the Netherlands, a more sympathetic approach to urban drainage known as Water Sensitive Urban Design (WSUD) is gaining momentum. This approach aims to incorporate rainfall into public open spaces in order to provide time and space for storage, infiltration and treatment as well as improving the biodiversity and microclimate of the city. It seems obvious that reducing the pressure of stormwater run-off on our ageing sewers by dealing with rainwater where it falls is better from both a cost and an amenity point of view, yet the UK has struggled to get to grips with it.

While there are already good examples of new build schemes in the UK, such as Cambourne and the East Village wetlands in Stratford, we seem to have accepted that our cities are too difficult to retrofit. This presents a new challenge for architects, engineers and landscape architects to extend the natural drainage approach up and onto our new and existing buildings. There is an abundance of buildings in London with flat roofs that could be used to create green-blue roofs, which collect or store water during storm events and use it for the irrigation of green façades or roof gardens. One example is the façade of the Rubens Hotel, which was implemented as part of London's Victoria Business Improvement District. 

With so many new high-rise buildings under construction in London, it should be mandatory to integrate rainwater harvesting and irrigation systems into the roofs and façades for the reduction of stormwater run-off and improved microclimate. An approach based on natural systems integrated into every building and space within our urban environment would help to take the strain off our already stretched pipes and sewers. A decoupling of stormwater systems from the foul sewer network could begin to introduce rainfall as a resource for our urban deserts. This is a relatively simple task that would have a greater potential benefit than the Thames Tideway Tunnel, set to cost around £4.2 billion. 

But it’s not just tall buildings that can make a difference. A decoupling of residential properties in London would also make a huge difference to the amount of stormwater run-off entering foul sewer networks. While the Flood and Water Management Act introduced measures to prevent the paving over of gardens, there should be a similar pressure to connect existing homes to water butts, attenuation tanks and, space permitting, soakaways. In the American Portland Clean River Rewards Scheme, residents receive up to 100% rebate on their stormwater management charges if they manage it on their own property. This scheme, introduced in 2006, also allows industrial and commercial properties to gain exemption from charges if they implement similar measures. 

Using green spaces

Our parks and open spaces would also be transformed if changes in stormwater management were implemented. Currently underfunded, many parks have opted for ‘pay and play’ elements of the ‘Go Ape’ ilk that turn playgrounds into commercial ventures. This not only eats into the amount of space that is publically accessible but also creates a park elitism that excludes those unable or unwilling to pay. I have long advocated an alternative method for raising revenue for the upkeep of our public open spaces – allow them to contribute to flood storage during storm events by receiving stormwater from surrounding streets and buildings. This would not only make the parks more engaging and biodiverse but would create a revenue stream from their adoption as a vital piece of drainage infrastructure, and the cost could be recovered through water rates. This would be a great starting point for a wider retrofit campaign for our streets and squares throughout the capital. 

The current demand for new housing in the southeast should also be harnessed for the introduction of better drainage systems. The Code for Sustainable Homes, scrapped in 2015 as part of the Housing Standards Review, introduced a number of key criteria for better homes, including water management, environment, health and well-being. While many of the principles of the code have been incorporated into the National Planning Policy Framework, many housebuilders will choose to build homes that just get a roof over people’s heads. That may meet the demand for new homes, but it should not be allowed to impact on already stretched sewers and pumping stations. The positive legacy of the Code for Sustainable Homes lies in the schemes that were constructed in the eight years that it ran its course. Schemes such as Rowan Road in Merton, where 2.4ha of public open space was integrated as part of the stormwater management system, helped deliver better parkland and reduced infrastructure with increased flood resilience for neighbouring properties. 

Demand for new housing is also pushing development ever closer to land at risk of flooding. Our current portfolio is full of schemes that skirt the edges of flood likely or probable zones, which require the creation of additional flood storage and sustainable drainage. While some architects will advocate the development of flood resilient buildings in such locations, development should never occur where there is a possibility that the problem migrates elsewhere. The frequency and severity of flood events in 2015 have shown the alarming regularly at which our flood defences, designed for one in 30-year storm events, are breached. Some of this has been attributed to global warming, with the atmosphere able to absorb greater amounts of moisture and release it in far more violent storm events. Our current floodzone maps are likely to change considerably, putting even more homes in the path of destruction. 

It stands to reason that at some point we will no longer be able to defend against floods and will have to adapt. In the book Cities Beyond Oil, the UN Habitat Sustainable Urban Development Network recognises that cities of the future will not only have to be more resilient to climate change, but will have to achieve it with less energy. To continue to live in the city we will need to live with flooding and with the effects of climate change. That will either mean an acceptance of flooding as a regular occurrence, with buildings that are able to be quickly restored, or the construction of new buildings that are able to adapt during flood events. Ultimately, we may even need to consider retreating from areas that can no longer be defended from long-term flooding. A sustainable approach introduced through natural systems woven into the fabric of our towns and cities may not be as glamorous as a flood barrier or a high tech tunnel, but will cost less, last longer and provide environmental improvements even on a bright summer’s day when there isn’t a cloud in the sky. I know where I would put my money.

Sustainable Drainage Systems (SuDS) – provide better resilience against flooding. 

SuDS are a cost-effective way to reduce surface water flood risk and damage by tackling the problem through many small features, close to where the water falls. Through the creation of ponds, wetlands, swales and basins, which mimic natural drainage by absorbing or attenuating water into permeable and vegetated surfaces, we can better manage the flows of water and avoid creating one large flooding problem in one place at one point in time. 

SuDS are an increasingly important part of our green infrastructure providing not only green networks but also much broader species diversity. 

Water Sensitive Urban Design (WSUD) – an integrated solution to flooding, droughts and water quality. 

WSUD means thinking about water supply, waste water, surface water and flooding from the start of the planning and design process for new places – rather than as an afterthought. This approach improves the quality of water entering the waterways, allows for stormwater and grey water harvesting and reuse, and delivers significant reductions in potable water needs. Black water can also be considered for site recycling. WSUD and SuDS reduce pressure on existing infrastructure, directing floodwaters away from homes and businesses, and reduce the amount of water entering the sewers. 

Reducing the pressure on sewage infrastructure enables development where the existing capacity of the system is overloaded, makes sewage treatment plants more efficient, and lessens the need for investment in sewage infrastructure, or allowing that investment to be directed elsewhere.

Green (and blue) roofs

Green roofs are covered with vegetation and soil and can reduce run-off and subsequently relieve the pressure on drainage systems, particularly at times of high-intensity rainfall. Additionally, the benefits afforded to biodiversity can be significant by providing wildlife habitats, particularly in urban areas. They enhance the thermal performance of buildings and have an important role to play in reducing the urban heat island effect. They also have the potential to contribute to the wider landscape character in a particular location.

Blue roofs are non-vegetated systems that detain stormwater. Weirs at the roof drain inlets and along the roof create temporary ponding (damming on a small scale) and gradual release of stormwater. Blue roofs complement long and flat roofing styles, and have wide gutters with a sturdy watertight liner. This design works especially well in highly urbanised areas. Some stormwater may be temporarily stored on the roof while the discharge can be released to a stormwater harvesting or infiltration system, or a portion can be discharged to the drainage system at a relatively slower flow rate.

Peter Wilder is a landscape architect and urban designer whose work is focused around the integration of water into the built environment. His work on resilient design has led him to places like Bermuda, Kenya, Brazil, Australia and China where he recently completed the Gui’an Innovation Park – a model for 'Sponge City' development. He was the Strategic Landscape Architect for the Hampstead Heath Ponds Project and is an Associate of the BRE. He teaches Landscape Science and Techniques at the University of Greenwich, London.