With UK weather seemingly wetter than ever and flooding causing problems across the country, demand for sustainable drainage systems has never been higher. Hannah Hyslop of Cemex discusses the role of permeable paving
Increased urbanisation and construction, such as impermeable roads, driveways, car parks and pedestrian schemes, has resulted in a greater likelihood of surface water flooding.
This urbanisation, combined with the fact that weather data shows there has been a 50% rise in the number of three consecutive day storm occurrences since the 1960s, means continuing growth in the volume of surface water run-off, which we must now handle effectively.
Of the 57,000 homes affected in the 2007 summer floods, Government figures demonstrated that over two thirds were the result of surface water run-off, not swollen rivers. Another severe flooding event in 2015/16 changed the landscape of planning applications with many new schemes being required to include a full, compliant sustainable drainage system (SuDS) as part of the standard detail.
In light of these flooding events, the UK Government introduced a requirement for SuDS on developments in England, which is implemented through the planning system. Its expectation is ‘that Sustainable Drainage Systems will be provided in new developments wherever this is appropriate.’
The measures are applied by local planning authorities on major developments of 10 or more dwellings and equivalent non-residential or mixed sites, however, smaller developments may also incorporate SuDS installations. SuDS are designed to reduce the likelihood of flooding due to surface water run-off.
As part of our work with architects, one of the main questions we deal with is a request to explain the different types of SuDS that are available. There are many different SuDS techniques, including retention ponds, green rooftops, and pervious pavements, however it is pervious pavements that suppliers such as ourselves offer to architects, and construction projects, and which we focus on in our RIBA certified CPD online training.
Pervious pavements provide two functions; to effectively capture the maximum theoretical rainfall and flooding event (known as a design storm event) and discharge it in a controlled manner to the subgrade or drainage system; and to provide sufficient structural resistance to withstand the loadings imposed by vehicles travelling on the surface.
Pervious pavements do this by providing a hardened surface suitable for pedestrian and/or vehicular traffic, while also allowing rainwater to infiltrate through the surface and into underlying structural layers. Cemex’s Permaflow is a pervious concrete mixture that works perfectly for SuDs, and is carefully designed to have a network of interconnecting voids which allow the movement of water, providing the optimal solution for surface and stormwater management.
Once it has reached the underlying structural layers, rainwater can be processed in three different ways. It can be temporarily stored beneath the overlying surface before alternative use, such as flushing toilets. This is usually achieved with the use of a geo-cellular system in conjunction with a rainwater harvesting system. Alternatively, it can infiltrate to the ground, returning to the natural water table, or it can be discharged in a controlled manner into the sewer system.
Looking more closely at pervious pavements, there are two types available; porous pavements, which infiltrate water across their entire surface material e.g. porous concrete and porous asphalt; plus permeable pavements, which have a surface that is itself impervious to water but the materials are laid with void space through the surface to the sub-base e.g. modular permeable paving.
An example of a market in which a specification of this type of surface becoming a standard is Germany, where over 20 million m2 of permeable pavements are installed annually and treated as standard highway construction.
Architects are also especially interested in whether climate change has affected SuDS planning and setup and this is undeniable when considering the increase in the number of three consecutive day storm events mentioned previously; meaning that continuing growth in the volume of surface water run off needs to be dealt with effectively.
As more sustainable development becomes ever more important, we are also seeing architects ask whether SuDS can be incorporated into environmentally conscious building certification initiatives like BREEAM. Fortunately, the answer to this is yes – we can use porous materials to ensure that sites are managing water correctly and with less impact on the natural environments. Additionally, it’s also worth taking into consideration that several studies confirm permeable pavements demonstrate significantly lower total pollution loadings than standard pavements.
Certainly, we have seen SuDS playing a growing role in developments over the past decade or so, as architects and developers try to meet the challenges posed by our changing climate, and this is sure to continue. SuDS are an increasingly vital part of construction, ensuring those who live in our communities and use our infrastructure can do so safely, comfortably and without fear of expensive flood damage.
Hannah Hyslop is specification manager at Cemex UK