Feb 16, 2019 Last Updated 2:53 PM, Feb 5, 2019

How to score top marks with sustainable urban drainage

Published in Education
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The sharp increase in severe weather events such as droughts, storms and flash floods over the past decade has emphasised the need for educational facilities to employ effective, future-proof water management solutions. Matthew Rolph, Managing Director at GRAF UK, looks at rainwater harvesting and stormwater management solutions and how they can be combined to deliver long-term financial and environmental benefits.


With the effects of climate change making headlines almost daily, it’s no surprise water-saving measures are gaining in popularity in the public sector. Rainwater harvesting (RWH), for example, provides a straightforward and sustainable way to collect and repurpose water, which is particularly important in areas with a high risk of shortages. Harvesting rainwater not only lessens the pressure on the mains potable water supply but also saves a valuable natural resource and cuts bills at the same time – so it’s easy to see the environmental and economic appeal.

What’s more, the public sector is also under pressure to lead by example when it comes to complying with ever-tightening flood defence regulations. In fact, all new projects are now legally required to assess their flood risk and incorporate a sustainable urban drainage system (SuDS) to minimise it. SuDS are stormwater management solutions that provide controlled protection against flooding, replicating natural drainage as closely as possible.

Against this backdrop, what are the true benefits of RWH and SuDS for educational buildings? And how can they be combined together to deliver a space-saving water management solution fit for the future?

The benefits of rainwater harvesting

There is a clear environmental case for collecting and reusing rainwater, as opposed to simply letting it drain away. This is especially true nowadays, as premises are very often designed to meet BREEAM criteria, which assesses a building’s environmental, social and economic sustainability performance. To achieve the coveted ‘Excellent’ rating, these must adhere to a number of energy consumption, sustainability and water-saving standards.

In fact, there are up to three BREEAM credits available for RWH, which means it can be the deciding factor between whether a building reaches the highest possible accolade – or not. The rating is based on the percentage of the total hard surface of the site (i.e. both roof and hardstanding) that is designed to allow the harvesting of rainwater for reuse. To fully comply, the rainwater yield should offset the building’s non-potable water demand that would otherwise be supplied using valuable mains water (e.g. cooling systems, toilets, dishwashers etc.).

It’s important to consider that the larger the building and the higher its water usage, the more economic sense an RWH system will make. Most of the water we use day-to-day does not need to be drinking-quality (e.g. flushing toilets), so installing an RWH system can help significantly reduce ongoing bills by minimising reliance on costly water from the mains where it’s not absolutely necessary. In fact, the most efficient systems can reduce mains water usage by between 40 and 50% – relieving significant pressure on the potable supply, saving valuable water and substantially cutting costs all at the same time.

The payback period of installing a large-scale RWH system is usually between three and five years, and with additional tax incentives available to encourage investment in water-saving technologies, the pull is hard to resist.

Next stop: sustainable urban drainage systems

While collecting and storing rainwater does provide a layer of security when it comes to flood prevention, RWH cannot replace SuDS. An efficient, purposed-designed stormwater system will provide long-term security against flooding and help meet minimum run-off requirements set by local water authorities. There are two types of stormwater management:

• Attenuation – where water is retained and then slowly discharged into the surface water drainage system or watercourses

• Infiltration – where water is gradually released back into natural groundwater reserves.

Attenuation is more commonplace in the UK as the clay-heavy soil in many areas of the country is not suitable for infiltration. Nowadays, modular, underground stormwater tanks are proving most popular. They are less obtrusive than above-ground SuDS, can be installed virtually anywhere and can be fully inspectable too. The most modern tanks (such as GRAF UK’s EcoBloc range) feature inlet shafts and in-built inspection channels – ensuring optimum performance for the lifetime of the tank and complete peace of mind.

Joining forces

The benefits for both RWH and SuDS are clear, but one cannot replace the other. That said, they can actually be combined to become a cost-effective two-in-one solution.

Stormwater attenuation can often be incorporated into an RWH system with a one-tank solution, while infiltration generally requires a separate system. For larger-scale projects, such as educational facilities, attenuation tanks can be designed from modular cells wrapped in an LLDPE-welded membrane. The outflow rate is regulated to allow the water above the outlet to be discharged into the sewage system in a controlled manner, whilst the water below the outlet, at the bottom of the tank, is retained for reuse.

In parts of the country with the right type of soil (i.e. not clay- or rock-based), stormwater infiltration can be incorporated into an RWH system. This is done by adding a crate-style soakaway or a tunnel system, which is sized according to the infiltration rate of the soil.

How to navigate the rulebook

If you’re considering RWH, SuDS or a combined system, you should have a good understanding of the rules and regulations driving the sector. For example, the Flood and Water Management Act was introduced in 2010 following the widespread flooding of 2007.

By bringing sustainable drainage of surface water to the top of the construction industry’s agenda, it aims to increase resilience to climate change and ensure a more comprehensive management of flood risk – right from the planning stage, and no matter the type of building.

Additionally, under the National Flood and Coastal Erosion Risk Management Strategy, the full range of measures available to manage the risk of flooding at a site needs to be appropriately appraised and adopted wherever suitable. Such measures include using combinations of water storage methods and SuDS.

A winning double-act

Today, professionals working on projects in the education sector have to contend with the rise in the number of floods in the winter, and the risk of drought in the warmer months. Climate change is rewriting the rules of the game, and putting the spotlight firmly on solutions that can successfully integrate stormwater management systems and methods of collecting rainwater for reuse.

Ultimately, those that are able to keep their finger firmly on the pulse and offer future-proof solutions that are both efficient and compliant are in pole position to reap the benefits.

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