With towns and cities home to the majority of the UK’s population, demand on drainage systems has increased significantly over the years. While localised hard engineering schemes such as improvements to culverts, highway drainage and sewers have their place, upgrading these systems is both expensive and disruptive. With the number of homes at risk of flooding set to double to around 10 million in only 30 years time, it is critical that we do all that we can now to try and alleviate the demand on traditional drainage infrastructure.
So does natural flood management really have a place when it comes to managing water and flood risk within the urban environment?
When the term natural flood management (NFM) is used in conversation, consideration will nearly always lean towards more rural based works in the upper catchment involving the likes of in-stream structures such as woody dams, woodland creation, land management techniques as well as floodplain restoration and storage options. But does natural flood management fit the urban landscape? And, if so, what does it look like?
The natural flood management definition and the context in which it is used leads to much debate about what actually classes for, or technically qualifies as NFM. In breaking the term down, it appears to be the ‘natural’ element which causes the most confusion. Does it pertain to the use of ‘natural’ materials and features alone, or does it in fact lay more emphasis on water being managed through ‘natural’ processes?
For example, some say that soft engineering is within the remit of natural flood management, because earth bunds or woody dams are using natural materials. Others disagree, because any form of ‘constructed’ engineering is not reflective of the landscape in its natural state, as would be achieved with river and floodplain restoration or the reintroduction of sphagnum moss to the moorlands. Similarly, some consider Sustainable Drainage Systems (SuDS) to be a form of NFM, while others do not. So, does the answer lie more in the destination rather than the journey – the end result? Regardless of whether the objective is achieved through soft engineering or through SuDS, if the water is managed through natural processes, then should it not qualify as natural flood management? After all, it makes allowances for landscape features, whether rural or urban, to be used, restored and more importantly altered to process water in a ‘natural’ way.
In which case, many (but not all) SuDS would technically be natural flood management measures. Not only do they have the same ideologies of controlling and processing water at source, improving infiltration to the land, increasing attenuation, retention and controlling conveyance, but many of them deal with water through natural processes and with natural materials. It is also worth noting that both up-stream, rural based NFM works and urban SuDS also share many other similarities and benefits; in particular biodiversity and conservation, improving water quality, reducing carbon and other pollutants, building resilient ecosystems, improving green spaces and building on community spirit!
For instance, many would not consider a green roof to be a natural flood management tool as it uses some non-natural materials, such as for the insulation layers and waterproof membranes. In reality though, a green roof uses natural materials such as soil, sand, grass, plants and flowers to manage water naturally through the process of infiltration, transpiration and evaporation. The green roof is effectively adapting the urban landscape to restore what would otherwise be the natural foot print of the land if the built environment was not in situ.
So, with this reasoning in mind, how does NFM fit into the urban landscape?
Traditionally, the design of urban landscapes and buildings has resulted in a low adaptability to natural flood management techniques with the majority of examples seen today having been implemented at the project design stage.
In more recent years, changes to planning policy and focus on sustainability have helped to bring sustainable drainage systems (SuDS) to the forefront of building design and urban regeneration. SuDS directly help to manage and reduce the amount of water entering drainage systems through source control, retention, infiltration and conveyance. Innovation in design and build methods are not only helping to create ‘greener’ new-builds and neighbourhoods, they are also leading to the increase of retro-fit sustainable drainage systems.
Green Roof in Brazil. Image by João Alexandre Peschanski.
‘Green infrastructure’ with ‘soft’, permeable surfaces are a key feature of SuDS. Green roofs, green walls, rain gardens and infiltration trenches, all collect and process water where it falls. Worldwide, more and more inner-city developments are incorporating green infrastructure into building design. Swales, infiltration basins, trenches and soakaways all naturally collect and store surface water, and then allow the gradual infiltration of it back into the land. With groundwater accounting for 30% of the UK’s water supply, this natural infiltration continually recharges groundwater reserves and helps to ease demand on not only drainage systems but water supplies too.
Endcliffe Park, Sheffield. Image by Mark Morton.
For communities with prominent flood risk from urban watercourses and rivers, urban infrastructure such as playing fields, parks and inner-city woodlands can also offer a solution to urban flood mitigation. In many cases these areas will often occupy what would be the natural floodplain and allowing water to leave the channel at desired locations, water distribution and retention techniques can help to connect these areas for use as storage and attenuation - in much the same way as NFM online and offline storage options work in the upper catchment. Sheffield city centre has suffered numerous devastating floods in recent decades and after public consultations, planners are considering the use of some of the city’s parks as water storage areas.
Whether you considered SuDS to be the natural flood management tools for the urban environment or not, the principles, objectives, methods and processes are undeniably similar in many ways. Hopefully, one thing everyone can agree on is that utilising a range of natural and sustainable flood alleviation solutions does not only compliment both the urban and rural environment, but the towns and cities of the future will fundamentally rely on its integration in order to help combat and mitigate the flood risk faced by future generations.
To find out more about the different methods of sustainable drainage systems, download our SuDS booklet here.
Sources used: The James Hutton Institute - "What is natural flood management?" and Houses of Parliament Parliamentary Office of Science and Technology Postnote - Natural Flood Management.