Drain strain and picking the right permeable paving

With urban flooding back in the headlines, Mark MacIntosh-Watson, Engineering and Technical Development Manager, Brett Landscaping, outlines how architects need to approach sustainable paving…

The recent flooding in South Yorkshire highlighted how important an issue urban flooding management is for the construction and built environment sector.

Even the humble driveway has been influenced by the change in government building policy over the past decade, coupled with more people wanting sustainable solutions in their daily lives and a growing desire for off-road parking. 

This has had an unplanned impact which has increased the demand on surface water drains, as there is a much larger area catching the rainfall plus increasing rainfall intensities. 

During a high intensity storm a driveway could have up to 450 litres of water running off it every fifteen minutes, it is easy to see how the issue of water management can become more challenging without proper design consideration.

In Greater London alone, we know that in 2005 land on private properties approximating a minimum area of 3,124 hectares – the size of 22 Hyde Parks – had been converted from the original design of a front garden or pathway into driveways. By 2015 the Royal Horticultural Society estimated that 25% of all homes in Britain had paving or gravel instead of a front garden.

To address these issues the Government passed The Flood and Water Management Act (2010) that applies to England and Wales. This Act took forward some of the proposals in previous strategy documents published by the Government and replaced existing drainage regimes with a Sustainable Drainage System (SuDs) regime.

Permeable pavements are seen as offering a cost-effective method of attenuating and filtering surface water and satisfying the SuDs philosophy.

Architects and designers need to account for the fact that water captured and managed off paved areas cannot be directed onto a public highway and, without planning permission, cannot be drained into the existing local drainage network. 

In fact, the granting of planning permission is significantly weighted towards the outright specification of a sustainable system. A planning permission application requesting discharge into local networks can be made, but will likely be flatly refused by planners, leaving the homeowner and builder with no choice but to use a design and products that can dissipate water within the plot of land.

To solve these planning challenges and avoid a catch-22 situation, architects can turn to permeable pavement systems. By using these solutions paved areas that can capture all the rainfall at source, so there isn’t any rainfall runoff, can be installed. 

Permeable pavements work by replicating the drainage patterns of an undeveloped site where up to 95% of rainfall is absorbed into the land, with only 5% becoming surface water runoff. In this way the flow of water is attenuated, removing the early spike in surface water runoff which creates the greatest risk and pressure on downstream water management.

From a whole life cycle perspective, permeable paving helps to improve the quality of the water captured on the surface. Research by Construction Industry Research and Information Association (CIRIA) has shown that permeable pavements can remove between 60% and 95% of suspended solids and up to 90% of hydrocarbons. The result is that the water which infiltrates into the ground or is drained to the next stage of a water management system design, is of a significantly higher quality than if using an impermeable surface, relying on attenuation tanks.

Following specification permeable pavements are straightforward to install. Typically, once the level of the subgrade is chosen, a geotextile is laid before a permeable sub-base (4-20mm Coarse Graded Aggregate) is set across the area. This is covered with a 50mm thick laying course (2-6.3mm Graded Aggregate) – before the permeable paving blocks are installed on top and jointed with the same 2-6.3mm aggregate.  This allows the water to soak into the ground. It may be the water is attenuated instead of simply soaking into the ground.

Occasionally features such as unusual ground conditions, steep gradients arise, but there are rarely installation requirements that are beyond the available products on the market.

It should be emphasised that there are a number of advantages for designers opting for permeable paving beyond the primary purpose of the management of water. There is a greater range of products and finishes available across the market all designed with SuDS legislation and system longevity in mind. For example, Brett Landscaping’s residential permeable paving systems Omega Flow and Alpha Flow, which offer between 15 and 20 years of low maintenance service.

In addition, from a long-term performance objective being achieved at the design stage by the architect, using paving ensures loose materials are not transferred from a driveway to the highway, while providing a more pragmatic finish for mobility access to comply with the Equality Act – a major consideration for many family home projects for an aging population.  Think about the effort required to push a wheelchair across an unbound surface, like gravel or chippings, or using walking aids on the same surfaces.

Architects choosing to comply with the Code for Sustainable Homes, as under the credit system of this guidance a housing development will gain credit for ensuring there is no runoff from the first 5mm of rainfall, credit for improving the water quality and credit if the overall flood risk is reduced. With more homes being designed in areas of increased flood risk, every improvement helps.

Featured in abcd | January 2020 Edition