Sewer guidance signals shift on SuDS
The new code for sewers adoption, which comes into effect in April 2020, includes certain SuDS assets for the first time. British Precast sustainability & product manager Hafiz Elhag explores the implications along with the issues affecting selection of attenuation solutions.
A historic change to the Design & Construction Guidance (DCG) for the adoption of foul and surface water sewers in England comes into force in April 2020. It introduces for the first time a mechanism for water companies to take ownership and responsibility for sustainable drainage systems (SuDS).
Many conventional sewer systems in the UK and other parts of the world are no longer able to cope with increasing volumes of rainfall and increasingly frequent flooding incidents due to climate change. In 2014 SuDS was introduced as one of the main measures to control and mitigate flooding through the government’s National Planning Policy Framework (NPPF).
SuDS offer a range of alternative techniques to conventional piped sewerage in managing stormwater. These methods are designed to be more sympathetic to the natural landscape and conventionally include using ponds, swales, wetlands and natural waterways to control, manage and store surface water runoff.
The new DCG replaces Sewers for Adoption 7 and will offer clearer rules and requirements on where responsibility lies for the long-term maintenance of new SuDS. It is produced by industry trade association Water UK and was approved by Ofwat on 25 October 2019.
Water UK says water companies have taken the initiative on SuDS responsibility in the absence of a governmental plan. This has involved reinterpreting laws on sewers going back to the Victorian era.
The basic criteria that need to be met for a sewer to be “adoptable” can be found in Water UK’s Sewers for Adoption in England report. They include those assets that convey and returns flows to a sewer, surface water body or groundwater and those that have an effective point of discharge into a water body or onto land.
It is also worth noting that attenuation tanks are included in the new guidance for the first time, but underground tanks holding water without discharge will not qualify. Ultimately, it is for the water and sewerage company to apply these criteria to assets that are being offered for adoption.
Not only do SuDS ease pressure on drainage networks, they also improve the urban water cycle by enhancing the quality of surface water. Additional benefits include enhancement of the landscape, the environment, biodiversity and quality of life in urban communities.
Alongside natural sustainable drainage systems, manufactured proprietary technologies can be employed within a sustainable water management train to manage surface water runoff. Like the more natural techniques, manufactured SuDS can be used to intercept, collect, treat or store stormwater and direct them to nearby waterways, depending on specific site requirements.
In heavily built up urban developments, it may not be possible afford land area for natural SuDS solutions such as ponds. For many projects, stormwater attenuation is carried out using proprietary SuDS, most likely an underground tank system comprising a lightweight or precast concrete tank, box culvert, or piping system.
A SuDS survey of engineers, developers and others by the Institution of Civil Engineers and Wavin last year indicated that attenuation was the preferred SuDS solution of 70%aof those responding in England and 63% in Wales. Given that this is the first code that enables the adoption of SuDS, it is worth considering five issues affecting developers’ choice of attenuation solutions.
Top five issues affecting attenuation choice
As with any other project, the cost of building, running and maintaining a SuDS attenuation solution can be the main factor in determining which system to go for. Residential developments and housing estates in England have a significantly long lifespan.
According to the English Housing Survey (2014-2015), well over 75% of the 23.4 million homes in England are over 40 years old, over 56% are over 55 years old and more than a fifth of occupied houses are over 100 years old. It is therefore reasonable to assume that entire neighbourhoods and housing estates will have a >100 years lifespan.
SuDS will need to meet this life expectancy as they will need to perform effectively over the lifetime of a development. For SuDS not expected to last 100 years or more, the cost of replacement, renovation or major refurbishment will need to be incorporated into the whole-life cost assessment.
Members of BPDA are aware of these expectations and of the need for any underground attenuation solution, such as pipe-based, box culvert based or modular tank systems, to have an expected working life that matches the lifetime of the development - 100 years or more.
Landscape design and site requirements are the main factors determining particular attenuation systems. Ponds are usually preferred by planners, but they take up significant land area which may not be available in heavily built-up areas.
In such cases, underground tank systems may be the only choice and in places where even underground storage space is tight, developers may prefer systems that make the most out of available space underground with the lowest cover depth possible.
Constraints associated with construction can always pose engineering and execution challenges. Unsuitable ground conditions, such as elevated water tables, may make it impossible for some types of lightweight attenuation systems to be used.
The nature of soil can also pose risk for some types of underground attenuation systems, unless they are treated effectively to resist corrosion. The availability of space for lifting and installation machinery can be a challenge, but as all installations require excavators, these same excavators can usually be used in the installation of any engineered storage products requiring mechanical lifting.
Weather can also pose a challenge as some systems cannot be installed with stormwater inside the excavated area due to risk of floatation. Climate change could mean such events occur more frequently. Some engineered systems may also be adversely affected by bursts of bright sunlight and hot weather during installation, due to the high degradation rate of polypropylene components when exposed to UV light.
Manufacturers of precast engineered SuDS have solutions for these challenges that would not affect the effectiveness of the system or lead times.
Developers and adoption authorities need to have confidence in the structural integrity of an engineered attenuation SuDS system. The new DCG code offers detailed guidance on the standards to be used in constructing underground tank systems.
For example, concrete pipelines design will need to comply to BS 9295, British Standards’ guide to the structural design of buried pipelines. Other concrete structures will need to comply to relevant Eurocodes standards.
The adoption of SuDS will face further challenges as new product types, such as geocellular boxes and precast concrete tank systems, are brought under its remit. The absence of an established track record of deployment, along with limited experience of working with certain types of systems, poses a risk that designers may deviate from the established, universally agreed standards.
Where standards like BS 9295 or BS 5911 for concrete pipes and ancillary concrete products or CIRIA’s C737 report - Structural and Geotechnical Design of Modular Geocellular Drainage Systems - are not used, designers need to employ established engineering principles and justify why European and national standards should not apply.
SuDS provide a valuable contribution to sustainable development planning and execution, offering an environmentally sensitive approach to stormwater and surface water management. In this capacity, they should also have a lower impact and environmental footprint over their lifetime. This would include lower operational and capital carbon emissions, lower embodied water, transparency and responsible sourcing throughout the supply chain.
There is still a strong need for the sustainable drainage systems industry to improve transparency and provide better quality data so that the benefits of both natural and engineered SuDS become more visible. The differences between engineered SuDS and natural techniques can also be judged sustainably through a complete whole lifecycle assessment approach.
Published on 11th March 2020