Flooding is now a depressingly regular feature in UK news reports. Areas that have previously never been at risk of flooding somehow find themselves under significant volumes of water. Quite apart from the devastating physical damage, the mental strain on flood victims is something that never really goes away.
There’s no doubt that weather patterns are changing - it’s raining more frequently, more intensely and for longer periods of time. Our drainage systems weren’t designed to cope with this volume of water so it’s inevitable that from time to time they’ll overtop and the resulting excess surface runoff will end up flooding homes and infrastructure. We can’t stop the rain – so is the UK destined to continually suffer this relentless cycle of unpredictable flooding?
Maybe not, because there’s another factor at play – one which may be less visible, but something we can all do something about.
Increased rainfall and new developments
As urban areas expand to cope with a fast-growing population, we’re covering over increasing amounts of open green land with impermeable surfacing. Previously, rain that fell onto bare ground would naturally soak into the earth and be absorbed at source. This would make its way safely to aquifers below ground and eventually maintain the water table at a healthy level. However, rainwater is now intercepted by hard surfacing and is prevented from soaking into the ground below. Instead, this water accumulates until it travels across the surface in large volumes and at high speeds into our already overburdened sewer system, which wasn’t designed to cope with these deluges. It is inevitable, then, that as a consequence of hard surfacing, our existing drainage systems will from time to time overtop and create flood risk.
It’s the combination of these two factors – changing weather patterns and increased urban creep – that is the cause of increased flooding. And while we can’t stop the rain, we can certainly change the way we develop to minimise the impact of these events by using Sustainable Drainage Systems (or SuDS).
How do SuDS work?
Sustainable Drainage Systems (or SuDS) are a series of philosophies, systems and products which are all designed to emulate natural drainage processes. The idea is to collect rainwater as close to the source as possible and allow it to soak into the ground at every opportunity. Where this isn’t possible, a well-designed SuD system allows for water to be stored (or attenuated) before being released at a controlled rate into a water course, in the process avoiding the peak flow that would normally be created by a heavy rainfall event.
Opportunities for reducing pollutants should be considered, as should possibilities for the drainage system to create pleasant, attractive natural spaces in which people can relax and enjoy themselves. The four pillars of SuDS which should be balanced evenly are quantity (dealing with the volume of water), quality (cleansing the water that flows through the system), biodiversity (improving habitats for diverse planting and wildlife) and amenity (creating pleasant, lush environments for people to spend time in).
For visual reference, we've included a video at the bottom of the article, demonstrating how SuDs work in the form of an interactive model.
What is the best type of SuDS?
The easiest (and arguably the best) type of SuDS is a “soft” (or green) system. This effectively involves contouring a landscape to collect, control and direct any water that falls onto it or is directed into it – attenuation ponds, rills or reed beds are good examples. This allows water to be stored safely until it can soak slowly down into the ground over time – and if they’re well designed they also look great and provide a wealth of additional benefits.
The only problem with these features is that they’re land intensive. In an ideal world, every developer would choose to include ponds and swales in the schemes they design - there’s no doubt that well designed soft SuDS add real visual appeal (and value) to a development – but the realities of budgetary constraints frequently drive these features off sites to make space for more profitable dwellings or car parking. This is where permeable paving can prove to be a great option.
Can permeable paving help to stop flooding?
Permeable paving, such as Marshalls Priora, looks like standard hard standing, but it’s been carefully engineered and installed to allow water to soak down through the surface where it can be stored in a specially prepared sub-base. Once in the sub-base, the attenuated water will either soak down into the ground, or alternatively it can be channelled away at a controlled rate to a receiving water course. Either way, surface water is controlled and cleansed as it flows through the system, and reduces peak flow in a flood event. What’s more for cash-strapped developers, the hardwearing surface provided by permeable pavers can take the heaviest loads on the road so is ideal for access roads, driveways or paths. Permeable paving is an ideal dual-purpose SuDS method.
However, practical though they are, permeable pavements don’t provide biodiversity benefits. And while the systems can be designed to store large amounts of water, in tight urban spaces they may still need to be supplemented by additional storage crates or tanks.
There isn’t one single solution to the flooding crisis; a catchment-wide, joined up approach to rainwater management is the only way we can feasibly make a difference – but there’s no doubt that permeable pavements are pragmatic, low-maintenance flood mitigation devices which in construction and house building should be considered as the first choice whenever hard standing is required.