Sustainable Drainage Systems (SuDS) are a useful tool in managing surface water flooding at a local level. However, while guidance on their design has been available for several years, most notably through the SuDS Manual (C753) published by CIRIA in 2015, it was felt that to be effective, practical companion guidelines were needed specifically for those directly involved in building SuDS to reduce the instances of poor construction and to increase confidence in their performance.
The new SuDS Construction Guidance (C768) was therefore launched by CIRIA on 1 November 2017 as a practical companion to the SuDS Manual.
To ensure that the new guidance fulfilled the knowledge gap in the construction industry, a detailed questionnaire was sent to all those known to be involved in SuDS construction, to understand the nature of the problems that were experienced on site, and the context within which they had happened. Responses demonstrated both a high level of consistency in the challenges faced and a great sense of frustration from designers and contractors alike.
The main causes of difficulty reported fell into three categories:
- level changes (from the design drawings),
- cut and fill issues not resolved prior to construction,
- conflicts with services once works started on site.
Problems frequently cited included inadequacies in the design drawings (ie full details of levels throughout the system, capacity, flow controls etc not provided), existing site information not being comprehensively recorded – nor any consequent implications accounted for in the drawings, hard surfaces being added during the contract without addressing their drainage requirements, changes of materials (to those with different functionality), changes on site, or poor construction arising from a lack of knowledge. Many projects are still undertaken by contractors who may never have built SuDS previously, and therefore do not understand either what it is crucial to get right, or how to achieve ‘good construction’.
Overall, the consequence was either poor visual quality, poor physical construction or functionality, or a combination of those factors.
The new Guidance reflects the layout and style of the companion SuDS Manual, with contents that reflect the information in the Manual in terms of the list of SuDS components, how they are constructed, and what needs to be checked to ensure they are built to fulfil their design purpose. However, its approach is entirely different, as it does not deal with design and is not presented in a technical manner.
The Guidance is written so anyone can understand it, regardless of their prior knowledge of SuDS. It deliberately uses simple English, with minimal text, and pictures used wherever possible to illustrate and explain the issues being discussed. Symbols are also used to show whether the images are illustrating good or bad practice.
Industry experience is shared throughout, reviewing problems encountered on site to identify what lessons can be learnt, and how these challenges can be avoided in the future. Technical jargon is avoided and where necessary, terminology is explained in a short ‘jargon buster’ at the end of each chapter.
Within the Guidance, traffic light symbols and ‘Handy Hints’ highlight useful points of industry advice ; ‘Watch’ points anticipate problems so they can be avoided, or offer a simple solution; and ‘Hold’ points identify stages in the construction process where the works must be inspected before moving to the next stage of construction.
Most chapters include ‘mini-case studies’, which explore a specific issue that has occurred on site. The book also includes a section of whole project case studies.
Whilst the four pillars of SuDS should underpin every SuDS project, unless the practical requirements of successfully constructing SuDS are understood, these principles can easily be undermined.
The four pillars of SuDS
SuDS construction requires a different approach to ensure that the soils are maintained in good condition, so their structure will still sustain plant life, will drain effectively, and do not become polluted with silts or sediments, as trying to reinstate damaged soils can be very difficult. Similarly, protecting and maintaining the existing trees and vegetation resource and/or specific habitats must be understood and planned, as they are very difficult to replace.
The guidance has therefore sought to address these challenges by working through the typical construction contract stages, considering what needs to be done differently to enable SuDS to be constructed successfully. With this in mind, it looks at:
- How to prepare properly before starting the site and scheme
- Planning the works to get it right
- Planning to avoid potential site problems
- Managing the site and works during construction
- The typical challenges of specific SuDS components
The typical roles of both the design team and site construction team are analysed to explain their roles in a traditional contract and SuDS contract to explain how they differ.
Four key problems that arise on site can ultimately affect the ability of the site to deliver the designed scheme.
- Managing soils
- Site access
- Managing soil erosion and site silt
- Establishing planting
Site access is known to be a major problem on site, and is seen as a factor as to why pervious paving cannot be used due to the mud and silt that will be deposited on site roads. This section therefore explains how this can be done effectively.
Managing soil erosion and silt is also seen as fundamental to the successful delivery of SuDS due to their potential to block or damage systems if not properly managed.
Many respondents saw establishing planting within SuDS as a challenge, mainly due to a lack of understanding of plant needs in terms of soil condition. Knowing that the right plants are being delivered and used in the right place (for their functionality and their appearance) and that they are supplied to the right specification also presented challenges.
Using the wrong type of soil or soil mix will affect its ability to either drain freely or retain moisture, affecting the ability of plants to thrive within the system and their ability to improve water quality.
The presence of contamination also requires forward planning to avoid adverse impacts on remediation schemes and management of excavated soils that are contaminated.
The guidelines also identify industry standard tolerances that are particularly relevant to SuDS. The standards generally relate to paving falls and levels, materials, such as aggregate sizes, and geomembranes, or the porosity values/infiltration rates for geotextiles or soils. The Guidance says, for example, that many geosynthetics appear to be similar on visual inspection but they can have widely varying performance in practice. It recommends that product substitution should only be done with the agreement of the designer and that some geosynthetics require additional protection (eg geomembrane liners).
It also stipulates that inlets and outlets should be constructed exactly as shown on the design drawings and finished in an aesthetically pleasing manner. The specified size of flow control should also be provided.
Another factor that can make SuDS construction difficult is high groundwater. The Guidance advises adequate forward planning to manage water inflows to excavations.
The Guidance illustrates different types of bioretention system to show the wide variety of application, with disconnected downpipes draining to in-ground systems or raised planters, road drainage systems adjacent to the highway or more simple trenched components within an overall green system. Diagrams explain the key parts of each system and how it connects back into the drainage network. The challenges that typically arise in constructing bioretention planters are also identified with images showing either good practice or what can go wrong, and a mini case study for each type of component.
Each practical chapter ends with a checklist for construction. These are not intended to be exhaustive, but to identify key issues to form the basis for individual checklists for each particular project. All the checklists provided within the guidance are collected as individually downloadable Word files at the end of the document.
In summary, the Guidance seeks to help deliver better quality construction for SuDS. Understanding how to build SuDS correctly can help inform the design of SuDS generally. The team working necessary to build SuDS successfully will help improve communications both within the design and construction team, and encourage them to work together to overcome the problems that will inevitably arise. Ultimately this should ensure that better outcomes are achieved, and that the full design intent is realised on the ground, thereby delivering better water management and helping to reduce surface water flooding.
The SuDS Construction Guidance was supported by the collaborative funding of the Environment Agency, Thames Water, ACO, CAPITA, BPDA (formerly CPSA), Conways, Hydro International, Kier, LoDEG, Morgan Sindall, NASC Foundation, Permavoid, Polypipe, SEL Environmental, Stormwater Management and Wavin. It is available both in print from the CIRIA bookshop and as a free download from the Susdrain website.