CPD: Sustainable Urban Drainage Systems
Managing surface runoff against demanding criteria was achieved with ACO Stormbrixx at this site in Telford
Gary Morton explains the importance of achieving flexibility, compliance and efficiency when constructing sustainable urban drainage systems.
In war, there is a famous phrase: “No plan survives contact with the enemy.” Away from the battlefield, nowhere is this truer than in the world of construction. Even the best laid plans fall prey to the need for alterations, rescheduling and different skills required on site – all of which can impact on construction’s traditional enemies of time and cost.
Add Sustainable Urban Drainage Systems (SuDS) into the mix – a relatively new and less well-known construction method, which has regulatory backing but no clear construction guidelines – and it is little wonder that some constructors regard sustainable urban drainage as a headache.
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Drainage has a huge capacity to interrupt construction plans, as it is one of the first stages of a build. Get out of sync on timing and the impact on costs and resources can be catastrophic. New regulatory guidelines that are dependent on project type, and even the area in which a build is located, mean any alterations can present difficulties.
Nevertheless, sustainable drainage is now an essential element of UK construction, but there are still barriers to uptake, from a client, constructor and environmental perspective.
The SuDS challenge
This is not a new subject, but the construction industry, understandably, has difficulty interpreting the guidelines. A recent survey conducted by ACO revealed that 68% of construction professionals felt there was a lack of understanding of SuDS among key decision-makers. What’s more, over 50% of SuDS experts recognised that “site constraints” posed a real difficulty in design and construction.
ACO has been working with a SuDS Task Group to deliver sustainable solutions on sites. The outcome is clear guidance on how to deliver “achievable SuDS”, ensuring that they are planned early and potential obstacles are factored in.
By being realistic about what is achievable and adopting the most suitable combination of products and methods, time and cost can be better managed.
This CPD aims to provide the reader with a clear understanding of SuDS as a practical, cost-effective and value-producing approach to surface water management. By understanding the key principles and the alternative approaches and products available, it becomes possible to address the wide variety of challenges that will inevitably arise.
ACO’s Stormbrixx system can be constructed to form large integrated tanks that are easily maintained during and after construction
The secretary of state for communities and local government has required Lead Local Flood Authorities (LLFAs) to provide guidance on the management of surface water to local planning authorities. The local authorities need to satisfy themselves that the proposed minimum standards of operation are appropriate and ensure, through the use of planning conditions or planning obligations, that there are “clear arrangements in place for ongoing maintenance over the lifetime of the development”.
Developers must demonstrate that any surface water drainage scheme:
- is designed to ensure flooding does not occur on any part of the development for a 1 in 30 year rainfall event;
- manages runoff within the development to ensure no flooding within any building or part of a utility infrastructure in a 1 in 100 year-plus climate change event; and
- provides adequate exceedance capability.
Defining the solution
SuDS need to demonstrate three core attributes: economic viability, resilience and sustainability.
- Economic viability The intended implementation of national standards for SuDS was downgraded to non-statutory guidelines in part because of the difficulty in demonstrating economic viability. The ACO approach is to deliberately seek to use the components most suited to a given project, integrating both soft SuDS and proprietary products that will ensure both constructed and long-term viability.
- Resilience This means expecting the unexpected, which for SuDs means that failure is inevitable, because of the nature of storms. What matters is that a scheme “fails gracefully”, not catastrophically, and the system is able to recover.
- Sustainability The CIRIA SuDS Manual (C753) suggests that sustainable drainage rests on four pillars, as specified in the diagram shown below. These are: water quantity; water quality; amenity; biodiversity.
Designing an appropriate system
Adapting or designing systems that can deliver the benefits of these four pillars can prove to be problematic, but is achievable. ACO supplies projects that range in size from a fraction of a hectare up to several hectares – the typical area of focus for SuDS.
It is possible to achieve positive outcomes against the “four pillars” ambition through a combination of soft and engineered solutions. Moreover, it is also important to consider constructability and cost as well as maintenance. These considerations are particularly important in situations where the constructors are having to adapt designs during the build process.
Construction phase: engineered or soft?
The incorporation of engineered products in SuDS schemes is relatively easy from a construction perspective. “Soft” SuDS elements – those that make use of green landscaping – are very different and require careful attention, not least because they need to be protected from damage during the ongoing construction programme. Compaction, pollution, erosion, silt or sediments can all cause significant ongoing problems – or even failure – if they are not properly planned for.
Maintenance is one of the major difficulties in designing and constructing SuDS. Opinions are divided on whether maintenance of SuDS is cheaper overall than conventional systems.
Whether soft or engineered SuDS are used, it is critical that provisions are made for inspection post-construction and performance, along with enforcement, if SuDS features are not maintained as designed.
Modifications to designs will require technical support and guidance, which is accessible from manufacturers that provide engineered solutions designed to interface with soft systems. CIRIA also provides useful information in its freely accessible publications, such as Guidance on the Construction of SuDS (C768).
ACO’s QuadraCeptor filtration system
The maintenance requirements for proprietary products vary. Channel drainage is a good example of how water can be effectively conveyed to infiltration points. And, because it is on the surface, a visual check can be easily made to determine when maintenance is required.
ACO’s oil/petrol separators can take years to fill, but the removal of material is relatively straightforward with the use of a suction device. More advanced filtration systems work where material is vacuumed out, with only the filtrate requiring replacement.
Specification: quantity and quality
Quantity and quality are the key challenges with SuDS. Quantity is the criterion most in the public eye and of most concern to specifiers as it is a crucial element in any planning approval.
The focus is on both rate and volume control and the implications of different return periods (the estimate of the likelihood of a storm). When a design caters for a set storm-profile, the challenge is controlling the calculated volume. In other words, impermeable surfaces create additional runoff volumes that, ideally, must be reduced to greenfield runoff conditions. This is an area where local planning authorities take a strong interest.
Allowable discharge rates are frequently now set to “greenfield” levels. A site that has previously been developed may have a higher rate than greenfield. The rate and maximum volume for an impermeable surface happens more quickly and is far greater than that of the permeable surface. The key is to manage the rate economically and sustainably while still being resilient.
Geocellular storage structures, which are normally modular, accessible and sub-surface, are often used to manage rate. Designed to withstand a variety of loads, the best systems can be constructed to form large integrated tanks that are easily maintained during and post construction. Typically, they are used in combination with flow controllers like ACO’s Q-Brake, which controls water release.
A maintainable SuDS detail featuring an ACO channel system with an ACO Swale inlet
Any greenfield, unless saturated, may not produce runoff at all. This is due to the size and duration of storm events. Where most storms are short and of low intensity between 0-5mm depth, zero runoff can be achieved. Channel drainage has proved to be one of the most effective ways of intercepting surface water, and by a bit of careful planning it is possible to link the process of interception with infiltration by simply conveying water to swales, tree pits or other vegetative areas.
Other methods include the use of tree pits and other vegetative areas.
How to improve runoff quality
Human activities produce pollution – deliberate or accidental, chronic or acute. This is particularly true for surface water. The type of pollution does need to be carefully considered at design stage, as very specific controls are often essential. A typical example would be on a petrol forecourt where very specific requirements are laid out.
The apparently innocuous practice of salt spreading for de-icing is a good example of how pollution can be mobilised. Salt can cause what is known as an ionic exchange which mobilises pollutants such as metals captured in permeable media. This, in turn, enters the drainage network which, if connected to the wider environment can cause widespread damage.
There is nothing worse than being told a product has to be installed but being faced with challenging onsite conditions and time pressures. While guidance can be accessed, on many projects where adaptations are necessary, time is critical. Even the best laid plans can prove inadequate when faced with a rapidly evolving situation on the ground.
My advice to the construction industry is to get advice from manufacturers who spend their time developing SuDS products and, crucially, understanding the regulatory environment. That advice needs to be accessed as early in the project as possible because it is in the early stages that potential problems can be unearthed and solutions put in place. We’re here to help – and, ultimately, to help you save time and cost.
Gary Morton is head of business development at ACO Water Management. For more information visit aco.co.uk/cpd
An ASDA site in Leicester shows how working with the right partners delivers results
What’s driving the flow of SuDS?
To understand the inevitable problems on site, it is important for constructors to appreciate the five principal drivers behind the use of SuDS.
1: Population growth
The growing population is providing a major challenge for UK infrastructure, with projections forecasting an increase of 9.7 million over the next 25 years, resulting in a total UK population of 70 million by mid-2027, according to predictions from
the Office for National Statistics.
The UK has seen both a dramatic increase in impermeable surfaces being created and a reduction in key wetland areas – a combination that creates drainage and water management challenges. The Coordination of Information on the Environment (CORINE) land-use change map details that 2,500 sq km of land has been altered in a six-year period between 2006-2012.
If this land has all been made impermeable, the runoff caused by 10mm of rainfall is roughly equivalent to 2.5 days’ worth of water consumption for the whole population (150 litres/day/person x 60 million).
3: Water inundation
In 2016 about 16,000 houses in England were flooded during the wettest December in a century. The Environment Agency estimates that around three million people are at risk of surface water flooding in England and Wales. In simple terms, flooding is caused when the volume of rainwater fails to drain away through the existing drainage systems or soak into the ground, lying on or flowing over the ground instead.
4: Climate change
Climate change is increasing rainfall, which is predicted to rise by about 20% in the next 40 years, and is also increasing the intensity of storms – which has a direct impact on the amount of flooding.
5: Water supply
We may have more water but we still have a water supply deficit. England, Scotland and Wales are projected to be in deficit by 800 million to 3 billion litres per day by 2050 (5–16% of total demand) and by 1.4 billion to 5 billion litres per day by 2080 (8-29% of the total demand). SuDS are a key part of the solution, infiltrating water into the ground to assist in the replenishment of ground water.
This article is sponsored by ACO Water Management