Permeable paving combines hard standing with a proven Sustainable Drainage System (or SuDS), and works in a very different way to traditional paving. It is designed to allow rainfall to permeate immediately through the surface near to where the raindrop lands – so surface ponding is completely eradicated without the need for an additional channel drainage system.
The water flows into a specially prepared sub-base, where the voids between the stones act as a temporary reservoir. During a rainstorm, the water is collected in the sub-base before it is released slowly, either by natural infiltration into the ground beneath the pavement, into the main sewer at a controlled rate via a flow restrictor, or a combination of both.
However, many are still wary about implementing permeable paving due to some common misconceptions. It is important to eradicate these modern day myths, as flooding is becoming an increasing concern for many, and when planning a construction project which involves hard landscaping it is vital to take drainage into consideration – even if the build isn’t taking place in a flood risk area. Preventing excess water run-off in all areas reduces the worst of the effects for areas which will be affected downstream.
Marshalls’ water expert Chris Griffiths discusses some of the many commonly-held misconceptions surrounding permeable paving, and how developers shouldn’t fear using this product:
1. Cannot be used on clay soils
It is commonly believed that because clay is dense and difficult for water to permeate, stored rainwater will ‘back up’ and fill up the sub base. This situation can be avoided using simple engineering. When the ground isn’t sufficiently porous to allow the stored water to infiltrate into the ground quickly enough, the sub-base can be tanked and outflow pipes installed to release the water to a specified location or watercourse at a controlled rate. The aperture of pipe dictates the rate at which the excess is released. This kind of tanked system (making up approximately 50% of Marshalls systems) can be used regardless of the ground conditions, and still slows the flow, holds back and cleans the water.
2. Damaged by frost/freezing in winter
Any surfacing product which is slightly absorbent can potentially suffer with frost during the winter months, as water collects in voids in the surface and then expands when it freezes – thus ‘blowing apart’ the surface. However, with permeable block paving, water never sits on the surface for long enough to freeze, as it flows quickly down the voids and into the sub base. This is not always the case with permeable macadam or permeable concrete, where the voids in the material are far smaller and therefore likely to collect rainwater.
3. Clog easily and stop working
We have been installing permeable paving for over 15 years and in all that time we have never experienced any issues with clogging. Even if mud or weeds appear to have blocked the voids, it’s important to remember that these aren’t impermeable materials! The gaps between Marshalls Priora blocks have been engineered to be far wider than they need to be, so even when they appear to be clogged, water will still find its way quickly through to the sub-base. Even if the worst were to happen and a local area of a permeable pavement were clogged with some kind of impermeable material (a concrete spillage, perhaps), the water would flow to the nearest area that’s not clogged and infiltrate there.
4. Expensive to buy
As with any project, you have to compare the total cost of a system rather than just the surfacing product. Priced on a per metre basis, permeable blocks may be slightly more expensive than an equivalent area macadam surface, but when other factors such as linear drainage and water storage are factored in, permeable and non-permeable paving projects work out at broadly the same cost. However, when linear drainage gets blocked, the rectification costs are significant because it’s difficult to access and clear the underground pipes. Should permeable paving require any kind of maintenance, a surface brushing is all that’s required, so ongoing maintenance costs of permeable paving are usually lower than impermeable alternatives.
5. Can’t use permeable paving in heavy loaded areas
Work Marshalls has undertaken with Professor John Knapton has proven that permeable paving can certainly be used in heavy loaded areas. Our research (which comprised a combination of desktop studies, field testing process and in-situ performance data) proved that Priora blocks interlock better than standard block paving – reducing the pressure on the sub base by up to 40%. This makes permeable paving an excellent solution for heavy loading areas regardless of its hydraulic advantages. We have numerous case studies where Marshalls’ Priora paving blocks support the heaviest loads on the road.
6. Lack of aesthetic options
Permeable paving now comes in a variety of colours and plan sizes, so there really will be a paving option suitable for every project. Marshalls offers many different types of permeable paving in numerous colourways in both block and flag sizes.
In short, the management of surface water needs to be considered at the start of every build project in order to slow the flow and hold excess water back in the most effective manner. Ignoring the issue altogether or being cautious about implementing new SuDs solutions will only increase flood risk for catchments downstream.
There’s no doubt that using green space and natural ‘soft’ SuDS solutions are the ideal way for water to be managed, but for trafficked urban areas which require hard landscaping, permeable block pavements are a pragmatic, dual purpose alternative which are easy to install and maintain.