Talking sustainable drainage

In recent years, local planning authorities have had the key responsibility for implementing Sustainable Drainage System (SuDS) strategies. The National Planning Policy Framework 2012 went as far as outlining an expectation that all housing developments of 10 houses or more should include a SuDS solution. In addition to this, many local planning authorities are requiring the use of SuDS for any remaining developments as a planning condition.  With so many different solutions available, some developers and contractors have been reluctant to engage with the issue, with cost and complexity often cited as the key problems.

At its simplest, SuDS systems closely imitate the characteristics of a natural environment, and manage surface water attenuation along with enhancing water quality and amenity. This is a marked change from traditionally designed paved areas and driveways, which prioritise removing large volumes of water as quickly as possible by channelling it directly to the drain.

This change in approach can leave paving installers, who have used the traditional building methods, feeling uncomfortable as they try and adapt to what they perceive as an unfamiliar system. However, some technical information and product advice will go a long way to allaying fears that the system is difficult to use.

SuDS solutions can be divided into two distinct categories, “engineered” solutions and “soft” ones. Both of these can either work in isolation, servicing a single property, or as part of a larger development-wide scheme as part of a water management train.

Examples of “soft” SuDS solutions are ponds, swales and reed-beds, which often feature on individual projects. “Engineered” solutions such as underground water storage and drainage channels are much more practical for development-wide systems as these are significantly more effective in environments where space is limited. Regardless of which type of system is used, both will aim to capture as much water as possible whilst also providing functional pavements for pedestrians and vehicular traffic.

Although installation is different to a traditional non-permeable paving solution, installing a SuDS solution is not complicated. However, to be effective in real-world use, the basic specification and installation decisions must be correct. Ensuring that structural performance and hydraulic design are right is essential to the functioning of the system. To achieve this, specifying the correct sub-base material is just as important as choosing the paving itself.

A permeable pavement typically consists of a geotextile laid onto the sub-grade; a sub-base of 4/20mm coarse graded aggregate followed by a laying course of 2/6mm coarse graded aggregate and then the permeable paving blocks. The use of the coarse graded materials without any fines allows the water to easily drain and pass through whilst also offering the structural integrity required. 

The thickness of the sub-base is designed to accommodate both the trafficking loading requirements and designed hydraulically for the volume of water entering it. 

For the hydraulic design, an understanding of the sub-grade is required, determining whether an infiltration or non-infiltration system is appropriate.  The system type will depend on the subgrade conditions (such as soil type, etc.) and this will influence the volume of water that needs to be stored in the sub-base. The volume of water will depend on rainfall data for the area and the ability to cope not only with rainfall directly on to the paved surface, but possibly also from adjacent areas impermeable areas, such as roofs and impermeable paving.  This run-off will need to be considered when calculating the water storage capacity required. 

With this knowledge, the depth of the sub-base required can be calculated to manage the volume of water designed for. The function of the sub-base is to store and eventually discharge the water, so the depth of this layer will vary depending upon all the considerations listed above.

For the construction of the pavement, advice is available to ensure a successful construction is achieved.

Whilst on the face of it SuDS may appear to be complex, it is a topic that will remain constant in the coming years. As flooding becomes increasingly prevalent as a result of climate change disrupting global weather patterns, it will no long be acceptable to avoid the issue, and contractors and developers will have to be open-minded about adopting SuDS. Whilst it is tempting to opt for non-permeable solutions, this will not be feasible in the long term, as flooding and water management become politically unavoidable topics.

Our advice is to consult technical experts and use online specification tools, which are capable of eliminating much of the hard graft involved in correctly specifying and applying SuDS solutions.

As a manufacturer of permeable paving solutions, Brett has a technical team to help with such specifications and has produced PermCalc – a free online tool that not only generates precise design suggestions for the individual site, but provides accurate costs, and cost comparison information against non-permeable paving alternatives.