Mark MacIntosh-Watson, national engineering manager at Brett Landscaping, investigates the possible reasons for a general reluctance, to implement a broader sustainable drainage systems (SuDS) strategy
Since the release of the 2015 Planning Policy, local planning authorities have been ultimately responsible for implementing sustainable drainage systems (SuDS) strategies; the document outlined an eventual goal that a SuDS solution should be included in all housing developments of ten houses or more.
Whilst there is now a broad variety of SuDS solutions available for developers and contractors alike, a reluctance remains amongst some who may harbour concerns based on the perceived complexity and cost of implementing changes.Imitating the natural environment
Sustainable drainage systems, which manage surface water volume and attenuation, are best when they imitate the natural environment as closely as possible. This means that SuDS solutions approach water management in an entirely different manner to driveways and paved areas designed in the traditional manner – systems that prioritise rapidly removing water, channelling it down the drain.(Pictured) Sustainable drainage systems, which manage surface water volume and attenuation, are best when they imitate the natural environment as closely as possible
Such a fundamental change can be daunting, and a bit of expertise will go a long way to alleviating fears of contractors and specifiers who are considering adopting SuDS.
On hard surfaces SuDS solutions work most effectively by draining surface water close to the point at which water falls. From this point, water can either be allowed to slowly filter through into the surrounding ground, or it can be attenuated, before being discharged at a controlled rate. SuDS solutions can either work in isolation for a single property, or implemented on a larger scale, on a development for instance.
Although “soft” SuDS solutions such as swales, ponds and reed beds are possible, developers will clearly favour an “engineered” solution, which will offer better drainage for the space they occupy. Regardless, using permeable paving for driveways and carriageways will achieve the objective of capturing surface water early, where it falls, whilst also providing pavements for pedestrian and vehicular trafficking.
Permeable paving is easy to install as long as the basic specification and installation decisions are made correctly. Structural performance and hydraulic design criteria must be met – to do this, specifying the correct sub-base material is crucial.
An effective permeable paving solution consists of a geotextile which is 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, with the permeable paving blocks on top.
The thickness of the sub-base is deduced from an assessment of anticipated traffic loading as well as the ground’s strength. Correct 4/20mm material specification maintains the structural integrity of the finished site; substituting the specified material for a cheaper one is a surefire way of compromising the entire system.
Various factors, such as soil type, and whether any system may have to avoid a specific feature (such as an aquifer), all help to determine whether an infiltration or non-infiltration sub-grade solution will be necessary.
The function of the sub-base is to store and eventually discharge water. The depth of this layer depends on a number of considerations.
Calculating the water storage capacity requires a number of factors. Rainfall statistics for the local area are considered, and calculations must also take into consideration the need for any system to cope with not only direct rainfall, but also run-off precipitation from adjacent areas such as roofs. It is only after these factors have been considered that a fully informed decision can be taken for the appropriate sub-base depth to manage the volume of water.
Even at this stage there are still issues to contend with. Other activities taking place on site for example can cause problems. An error as innocuous as storing construction materials like sand or soil on top of a finished area of permeable paving can lead to clogged joints and ultimately, restricted water flow.
With so many factors to consider, it is understandable that some subcontractors can see permeable paving as a daunting challenge. Avoiding, rather than embracing, the change is no longer possible, as flooding becomes an increasingly common sight in Britain.
Luckily, much of the complexity of SuDS can be taken care of with the use of online specification tools, or by consulting technical experts. As a manufacturer of permeable paving solutions, Brett has a wealth of knowledge which can be leveraged for an accurate specification.
Furthermore, PermCalc is a free online tool from Brett which can both generate precise design for individual sites, as well as providing accurate costs and cost comparison information with non-permeable paving alternatives. By removing complexity and margins of error from calculations, permeable paving solutions will eventually become ubiquitous in British construction.