How to Ensure Quality in SuDS Construction

Written By: Regional Technical Manager, Hydro International UK Stormwater Divison

Berewood SuDS during construction with show homes in the background

Have we put the cart before the horse when it comes to ensuring quality in the construction of Sustainable Drainage Systems (SuDS)?

Encouraging greater uptake, even compulsory use, of SuDS in new developments in the UK has led policymakers to put a great deal of effort into promoting best practice in design, but much less, so far, into informing the industry on quality installation.

As a consequence, many contractors struggle to know exactly what success with SuDS looks like, and how to master specialist components and techniques. Equally they often face challenges in managing runoff during construction, as well as retrofitting SuDS schemes into the wider drainage and sewerage infrastructure on and off a site.

Those responsible for overseeing the quality and value for money of drainage construction must be confident that the SuDS will be compliant and therefore avoid the risk of a scheme either being left unadopted, or of becoming a flood or pollution risk.

Technical Standards

To help ensure quality, contractors do have design standards to turn to. The English and Welsh Governments both have non-statutory technical design standards for SuDS, although they are significantly different (see Welsh Standards Light The Way For Future SuDS Delivery). For Scotland, the new Water Assessment and Drainage Assessment Guide is an excellent reference and Northern Ireland published its own Sustainable Water Strategy earlier this year.

The SuDS Manual, updated by CIRIA at the end of 2015, is an invaluable repository of information. However, all of these documents are written firstly with design, rather than construction, in mind, although there is useful information on the available ‘toolbox’ of SuDS components.

CIRIA Guidance

Fortunately, CIRIA is currently leading a project to produce much-needed industry guidance for construction of SuDS, and is consulting widely with contractors, designers and regulators. The new guidance document expected to be published early in 2017 will provide practical guidance, information on common challenges encountered and best practice case studies.

As a member of the project steering group, I have been able to provide advice on the correct installation and maintenance of proprietary systems and manufactured devices during construction, particularly from the point of view of our expertise in Hydro-Brake® flow controls and precision-engineered flood storage, together with filtration and treatment devices.

CIRIA has been gathering very useful feedback from contractors about their experiences and the problems they face with constructing SuDS, which will inform the content and style of the new guidance.

The first challenge that many contractors face is dealing with SuDS features and components successfully during the construction phase without breaching discharge consents to nearby watercourses. The additional silts and sediments in the runoff from a construction site can also clog and silt up above-ground features such as ponds or permeable paving.

Equally, problems with soil compaction due, for example, to site traffic can make it difficult for infiltration components such as swales or detention ponds to operate as designed.

The same is true for proprietary devices, but here support from a manufacturer who can attend on site to inspect the sediment build up in a vortex separator, for example, and set out a site-specific maintenance schedule can be invaluable. Furthermore, the ability to provide off-site solutions can simplify on-site processes or circumvent some of the challenges associated with building SuDS components in situ.

No Substitutes

Substitution of cheaper, and inferior, materials and devices is a very common problem that is by no means new or even limited to drainage construction and installation. However, with the specific intentions of SuDS and the additional challenges of a mixed management train, ensuring that the scheme is built so that it achieves the designed performance is even more important than ever.

An attempt to save costs through, for example, switching a storage crate or a flow control device, could have significant consequences. In either case, the temporary storage volume could be dramatically different to that designed for the site, and therefore leave the finished scheme vulnerable to flooding. Any failures could lead to a claim for the developer.

Flow Controls

In the case of flow controls, our experience at Hydro International tells us that both designers and contractors can sometimes fail to understand the consequences of a substitution. If a contractor merely assumes that a flow control that produces the same upstream head is good enough to be substituted, then, to use an extreme example, a vortex flow control could be switched for an orifice plate which could lead to increased flood risk on-site and downstream, premature flooding of the storage facility or unauthorised over discharge from the site. .

Spec switchingc

Figure 1

In Figure 1, the amount of flow passed forwards is plotted against the depth of water (or head) building up in the storage structure behind three different types of flow control. Whilst all three flow controls reach the same “peak discharge rate” at the maximum head (when the storage structure is full), they pass forwards different amounts of water as the structure is filling or emptying. Changing the specified unit for either of the alternatives could therefore have a major impact on the flood risk both for the development itself and for the downstream infrastructure.

Substituting a flow control device that does not achieve the required performance curve can be a false economy, because the cost of building the additional storage, as well as the extra land-take, could be much more than the apparent saving.

Current inconsistencies with quality in SuDS construction and the need for better understanding and knowledge of specification and construction will take some time to resolve. Much as the SuDS Manual helps to ensure consistency and quality of design, the new guidance for the construction of SuDS will help to deliver consistency and quality in how these designs are transferred from the drawing board to ground.

However, controversy over the long-term maintenance and ownership of SuDS will continue, especially where clarity in regulation is still needed.


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