News & Events

May 2022

Surface Water Drainage Design: Are we doing enough?

The terms ‘Global Warming’ and ‘Climate Change’ have seemingly been around forever and constantly in the news with people stating one fact or another to back up their arguments and predictions. Occurrences of large storms, such as Storm Ciara closely followed by Storm Dennis in 2020 which caused widespread damage throughout the UK, are being far more commonplace. With the global average temperature predicted to increase by 3°C[1], those of us in the construction industry need to ask ourselves; are we doing enough to mitigate against this issue or is there more we can do?

The national demand for housing has never been higher, combining this with the stability of construction as an investment platform, the UK finds itself in the midst of a housing boom. Whilst this is great for the housing market and the economy, it’s sometimes not so beneficial for the environment. In any typical housing development scheme between 50% and 70% of the total site area can be covered in impermeable surface. This reduces the time it takes for rainfall to flow to the edge of a development, increasing surface water discharge to existing flows in watercourses (rather than infiltration into the ground) which increases flood risk elsewhere at the same time as impacting the environment. It is therefore our obligation as members of the construction industry to recognise this impact and provide mitigation whilst building the accommodation and infrastructure required to meet demand.

Flood risk and sewer over-spilling issues have long been a hallmark of bad weather. Almost everyone will have experienced flash flooding and inundation in one form or another with the unlikely few losing everything they own to extreme weather. These events are unfortunately predicted to increase, with more people likely to be severely impacted. As Designers, our main source of flood mitigation is to control the rate of water discharging from proposed development. The idea of greenfield discharge rates and the brownfield rates has been around for over 40 years and all drainage systems use the simple concept of not increasing post development flow rates when compared to the pre-development case. This is included in the National Planning Policy Framework (NPPF) and agreeing a discharge rate for a site is a fundamental requirement of a planning application for a scheme designated as a ‘major development’. In recent times this has been extended to smaller developments due to network capacity issues. Changes over a number of years to the NPPF with regards to drainage discharge have seen a progressive reduction on allowable discharge rates when considering a brownfield site. It is now a minimum requirement to achieve a 50% betterment on existing rates[2] however, it is common for the Lead Local Flood Authority (LLFA) to require all sites to discharge to greenfield rates regardless of the previous use.

Water quality has now become more prevalent in the planning process and with the latest guidance on Sustainable Drainage Systems [3](SuDS) there are a number of options available to suit varying site constraints and meet planning expectations for water treatment to manage the quality of discharged surface water. Use of SuDS provides an alternative to directly channelling development water to the nearest watercourse and can include but is not limited to inclusion of; Ponds, Swales, Tanks, Filter strips and proprietary systems which can also be coupled with more traditional interceptors and vortex separators for managing pollution control. A properly designed system in accordance with the CIRIA SuDS Manual and government guidance can not only provide the required storage in a network but also offer improvements in water quality/pollution control, biodiversity enhancements and local amenity areas for the community.

SuDS have traditionally been thought of as a risk by developers and contractors. However there has been a marked step change in mindset where ponds and swales are considered. This is a positive step forward though more still needs to be done. Maintenance of these assets has always been a limitation, with very few housing developers wanting to take responsibility in perpetuity for a pond/swale. Understandably, solutions which do not include ponds or swales have to date been preferred by some developers. It is encouraging that more recently, adopting and maintaining authorities, such as water companies and local authorities, are beginning to thaw on their adoption stance when it comes to SuDS features. Cost, complexity of construction and space requirements are also reasons cited for removing SuDS features from a scheme, being ignored for a more traditional network of concrete pipes and manholes to store flows and limit discharge to agreed rates.

We as designers understand that the land taken for large attenuation features is land that could otherwise be assigned as developable area, therefore we feel strongly about working with developers at an early stage to promote and where viable, include the possibility for use of SuDS as part of drainage design at the beginning of the masterplanning process to incorporate efficient systems that can provide multiple benefits. Drainage systems should be designed to provide a positive improvement to any development and no longer thought of as something that is just required to obtain planning approval and which occupies valuable space. With sufficient early-stage input, SuDS networks can be designed to be both hydraulically efficient and an effective part of the landscaping masterplan, improving biodiversity and public amenity. Implementing this process, our clients are kept fully informed as the design evolves, are fully informed of the reasons/requirements for SuDS features and understand the benefits that SuDS can bring.

So, to answer the question: The construction industry is already doing a lot to combat the impacts of developments on the environment in terms of water control. However, more can and should be done on the water quality aspect. SuDS systems can provide an end-to-end system that can not only tick all planning considerations but also improve the site in more ways than one whilst providing a functional asset. It is up to us as designers and developers to lead the way in building sustainable infrastructure systems and provide the best possible future for generations to come.

[1] Agreed in the Paris Climate Agreement

[2] CCTV survey of existing network, proof of connection and modelling required to confirm existing rates. Some water companies require water rates to have been paid within the last 12 months before they consider a site a brownfield.

[3] CIRIA The SuDS Manual 2015