Why is flooding an issue for the built environment? 

Flooding is nothing new in the UK and other parts of the world. Storm surges, rivers bursting their banks and heavy rains have been a part of the world’s ecological system for millennia. 

But climate change is increasingly affecting weather patterns in never-before-seen ways. Heavier and more prolonged rainfall is becoming common, and the risk—and reality—of severe flooding across both urban and rural areas is becoming a major headache for planners, housebuilders, the wider construction industry, residents and businesses. 

The pressure to build more new homes has resulted in hundreds of housing schemes in areas of high flood risk being granted planning permission.  

In recent years, the engineering community, working alongside construction companies, has come up with a number of solutions to avoiding the worst excesses of flooding incidents. 

What is a sustainable urban drainage system? 

Increasingly, projects are turning to sustainable urban drainage systems (SuDS). These work by channelling water away from surface areas, without the use of pipes or sewers, to nearby water courses. 

According to the British Geological Survey (BGS), SuDS mimic natural drainage regimes, reducing surface water flooding, improving water quality and enhancing the amenity and biodiversity value of the environment.  

“SuDS achieve this by lowering flow rates, increasing water storage capacity and reducing the transport of pollution to the water environment,” according to BGS. 

There are three different types of SuDs, depending on the location and type of development being proposed:  

At source: Control of water at source sees the flow of water into a drainage system reduced. Runoff water is reduced by being stored and gradually let back into the surrounding soil, or is intercepted and channelled elsewhere where it won’t pose a risk. 

Site and regional control techniques tend to be used on larger projects.  

Site control: Uses filter strips—strips of vegetation which filter out sediments and other contaminant material—and swales, which are shallow, broad vegetated channels used to collect and move water to reduce peak flows in rivers. 

Regional control: Sees management of runoff carried out from several sites. This technique uses regional ponds, wetlands and other water courses. Natural lakes, rivers and the sea can also play a part in drainage management. 

And since hard surfaces encourage water runoff, permeable surfaces are increasingly considered for residential developments, along with “greening”—green or sedum roofs—and rain gardens.  

The four pillars of sustainable urban drainage systems 

Along with mimicking the beneficial aspects of a natural drainage system, SuDS strive to meet four criteria, known as the “Four Pillars,” as set out by the Construction Industry Research and Information Association (CIRIA): 

Water quantity 

• Reduce the volume and rate of urban runoff 
• Recharge groundwater where appropriate 
• Reduce erosion 
• Control the quantity of runoff both to support the management  
  of flood risk and to maintain and protect the natural water cycle 

Water quality 

• Reduce pollution in groundwater and water courses 
• Manage the quality of the runoff to prevent pollution 

Amenity 

• Provide recreation and landscaping for the community, creating and sustaining better places for people 

Biodiversity 

• Improve and enhance wildlife habitats, creating and sustaining better places for nature  

These pillars act as a guide for developers, local authorities and planners when a new construction project is being considered. 

Given the pressure on the environment from development and the increasing risk of flooding, local authorities and central government insist that SuDS be implemented when a new project is being considered for planning. 

For example, Camden Council, in north London, requires “major developments to achieve greenfield runoff rates. Where it is reasonably practical they should constrain runoff volumes to greenfield runoff volumes. This is for the one-in-100 year, six-hour event. Greenfield runoff is the rate that water flows over land, which has not been previously developed.” 

Tips for designing a sustainable urban drainage system 

When it comes to designing a sustainable urban drainage system, there are several points to consider. 

Yorkshire Water, the private water company, sets out what it expects the design of such a system to consider. It requires that designers “shall ensure that the community, environment and local wildlife are considered in the SuDS design.”  

“SuDS design,” the company continues, “should consider requirements for urban design that may be specified by the local planning authority, particularly in relation to landscape, visual impacts, aesthetics, biodiversity and amenity.” 

The company says that a site’s characteristics should guide the selection of the most appropriate SuDS assets. The systems chosen—swales, rills, bioretention systems, ponds, wetlands, basins, tanks, infiltration trenches, filter drains or “soakaways”—should be decided upon with relevant stakeholders.  

System designs for greenfield sites should consider factors such as water runoff rates, estimates for surface water storage and similar predictions for infiltration volumes. 

While housing can be protected, so can other built assets, like schools. Moulsecoomb Primary School, in Brighton, revamped a courtyard garden, installing downpipe disconnections, rain planters, channels and rills, permeable paving and retention ponds. 

The benefits of sustainable urban drainage systems like this one include reduced flood risk, lower maintenance and an environment that children could enjoy and other local schools could see was a way forward in the event they needed to reassess some of their open space. 

Potential benefits for residential and industrial projects 

It’s not just homes and schools that can benefit from SuDS technology. Industrial properties often leak pollutive substances into the environment through water channels, and SuDS can address this. 

A team of researchers studied the potential for SuDS, concluding that the technology helped to “overcome the challenges related to the pollution problems in rivers and respond to the climate emergency by capturing the pollutants present in runoff from the industrial landscape.”  

Greater awareness, a range of different solutions and incentives could help promote SuDS further in the industrial environment, the researchers believed.  

“While there are barriers to retrofitting, installing SuDS and green infrastructure features is technically and economically feasible, leading to improved water quality and alleviation of flood risk … increased biodiversity and amenity values, as well as other ecosystem services,” their report added.