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Sustainable Urban Drainage (SuDS)

- Construction Management

  Flexibility, Compliance & Efficiency

In this paper, Gary Morton, head of business development at ACO Water Management (aco.co.uk/cpd)
explains the importance of achieving flexibility, compliance and efficiency when constructing sustainable urban drainage systems.
A maintainable SuDS detail featuring an ACO channel system with an ACO Swale inlet

  It's A War Out There!

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 a Dose of SuDS

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.

  The Drag of Drainage

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.

   Achievable SuDS

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.

   Understanding SuDS

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.

   Government requirements

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".

   What Developers MUST do.

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.
 
  1. 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.
  2. 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.
  3. 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 phase

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.

   Greenfield Level Discharges

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

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.

   Interception

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.

   Salt Spreading Pollution

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.

   Conclusion

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.

   A Word of Advice

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.
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.
 
2: Urbanisation
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 DEFICIT!
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.

   Build Off-Site for Manufacture & Assembly

We don't design or build SuDS, but our off-site solutions centred around timber can help you save massive amounts of water during the construction process. Timber is now making strong progress that to become the material of choice for designers, engineers and developers. These challenges further emphasise the role for Off-Site DfMA (Design for Manufacture & Assembly). They also point up the need for greater collaborative 'partnering' between architects, builders and  manufacturers. The greater speed, accuracy and quality that can result from manufacturing components in a dry, controlled factory environment gives builders a much better chance of achieving improved on-site build quality AND controlling costs.

   Resistance Disappearing

While resistance to the change to DfMA has been the norm, the wish of most UK Builders to deliver excellent customer service and top notch homes is now driving an increasing number to choose Off-Site for their developments. Also, as 'OffSite Hub' note, architects and designers are moving toward DfMA, something w have been encouraging for over 20 years. The emergence of LA Developers will only speed up this process.

   Easy Timber Frame

To assist them in doing so our "Easy Timber Frame" now offers standard size modular timber frame elements for them to use as a design base, cutting down on technical design and engineering to produce win-win results.

   Better all-round Savings

Our specialist services and CUSTOM homes division deliver best value and  protect the interests of land owners, house-builder developers, professionals and private individuals seeking to build their dream home. Our off-site construction systems also reduce the amount of material that gets wasted on site, helping to avoid land fill, hazardous waste separation and handling, They also deliver better value for you and your customers.
PLEASE CONTACT US FOR AN INFORMAL DISCUSSION AS TO HOW WE MIGHT WORK TOGETHER
 
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Some of our previous weekly insights:
 
 
 
We'll have more information for you next week. In the meantime, head to Build News for your daily insight into the construction industry.
 
 
 
 
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