Sustainable Drainage Systems (SuDS) manage surface water run-off as close to its source as possible. This reduces the risk of surface water flooding, as well as fluvial and tidal flooding, by lowering the peak flows that enter rivers, the sea and the sewers.

Whether you are a property developer, architect, landowner or planning consultant, this complete guide tells you everything you need to know about SuDS, SuDS design and SuDS reports — and how they affect your proposed development.

In England, Government policy has required developers to consider SuDS in planning applications for major developments since April 2015. Lead Local Flood Authorities (LLFAs) also frequently request a SuDS report for minor developments at the planning application stage.

Unfortunately, much of the information in this area is full of jargon or feels disconnected. Let’s fix that. Below, we set out the key facts about SuDS in a clear and simple way.

In this guide, we will cover:

• How do sustainable drainage systems work?
• Benefits of SuDS
• What is the purpose of a SuDS report?
• Are SuDS and SuDS reports compulsory?
• What is included in a SuDS report?
• Will I need to carry out any other drainage reports?
• Which properties require a SuDS assessment?
• Nimbus’ approach to SuDS reports
• Conclusion

How Do Sustainable Drainage Systems (SuDS) Work?

As far as practicable, you should manage surface water from a developed site in a way that mimics the natural flows from the site before development. This reduces flood risk to the site and elsewhere, and accounts for climate change. Reducing the rate of surface water discharge from developed sites is one of the most effective ways to manage flood risk.

Traditional piped systems remove surface water from developments as quickly as possible. However, this approach causes several problems:

• It increases downstream flooding and causes sudden rises in flow rates and water levels in local watercourses.
• It reduces groundwater levels and dry-weather flows in watercourses.
• It harms amenity and biodiversity, because the run-off carries contaminants such as oil, organic matter and toxic materials.

So how does SuDS work instead? SuDS use a sequence of management practices and control structures to drain surface water more sustainably than conventional piped drainage. A good SuDS scheme harnesses the environmental, ecological and social benefits of a site. These include:

• Better water quality – SuDS provide on-site attenuation and also treat the water. As it passes through fine soils and the roots of selected plants, the system safely removes pollutants washed off hard landscaping before the water reaches the natural watercourse.
• A sympathetic environmental setting – SuDS create habitats for flora and fauna in urban watercourses and open spaces.
• Amenity and social value – attractive green spaces meet the needs of the local community and residents.

The various types of SuDS include:

• Permeable paving
• Soakaways and other infiltration devices (so that no surface water run-off leaves the site)
• Swales and basins
• Bioretention and raingardens
• Green or sedum roofs
• Living / green walls
• Rainwater re-use
• Infiltration trenches and filter drains
• Ponds and wetlands
• Attenuation tanks

Benefits of SuDS

You shouldn’t view sustainable drainage as merely a ‘mandatory system’ or a box to tick. Implementing SuDS brings a huge range of benefits:

• Planning consent: Considering SuDS early can significantly improve your chances of gaining planning consent first time. It also confirms whether SuDS are achievable on your site from the outset. Leave it to a post-planning condition and you risk discovering there is no viable discharge point — which can make the whole development unviable if the SuDS condition cannot be discharged.
• Reduced flood risk: SuDS reduce the risk of flooding to your development and to surrounding areas.
• Cost reduction: Well-sited SuDS at the planning stage can cut the cost of any attenuation required. They also use landscaping areas to provide above-ground SuDS, which treat surface water and reduce its peak flow rate.
• Biodiversity: Increasing urbanisation has raised flood risk and reduced wildlife. SuDS enhance biodiversity on site, attract wildlife and create amenity value, making the site more attractive to those who live and work nearby.

What is the Purpose of a SuDS Report or Assessment?

You carry out a SuDS report or assessment as part of a planning application. It shows that you have considered the surface water run-off from your proposed development and followed the SuDS hierarchy. The aim is to manage surface water as close to its source as possible, and so prevent flooding within new developments and in the surrounding area.

SuDS reports play a crucial role in supporting planning applications, from concept through to discharging planning conditions after approval. Involve a SuDS specialist from the outset and you’ll enjoy a smoother planning process, fewer delays and lower costs later in the development.

You may need a SuDS report:

• To validate planning applications at the pre-planning or planning application stage
• To discharge planning conditions relating to SuDS and surface water run-off
• To show that the development manages its surface water run-off through SuDS, reducing flood risk on site and elsewhere
• To gain BREEAM surface water run-off credits

What are planning conditions? When a local authority grants a planning application, it usually attaches a number of conditions. These can cover building materials, timescales, ecology, mitigation, SuDS, and foul and surface water drainage. You cannot start construction until you have met and formally discharged these conditions.

Are SuDS and SuDS Reports Compulsory?

In England, Government policy has required developers to consider SuDS in planning applications for all major developments since April 2015. Major developments generally means those of 10 or more dwellings, or equivalent non-residential or mixed-use schemes. Because LLFAs are statutory consultees on these applications, a SuDS report is effectively expected in practice. Many local authorities also request one for smaller developments through local planning policy.

Are SuDS reports in Wales compulsory? Wales takes a stricter approach. Under Schedule 3 of the Flood and Water Management Act 2010, construction work with drainage implications cannot begin until a SuDS Approving Body (SAB) has approved the drainage system. This applies to most new developments of more than one dwelling, or where the construction area is 100m² or more.

You can incorporate SuDS into most developments, provided you consider them early in the planning and design process. If a site cannot accommodate a sustainable drainage system, the development may not be viable until it can.

What is Included in a SuDS Report or Assessment?

At Nimbus, we take a holistic approach to SuDS reports. We carry out all the necessary work from beginning to end to make the process as straightforward as possible for developers. Each report is bespoke to your development and follows the relevant local authority’s requirements.

A Nimbus SuDS report can include:

• Flood Risk Assessment: we assess flood risk from all sources and provide mitigation measures where necessary (required for major developments in Flood Zone 1, and any development in Flood Zones 2 and 3)
• Pre- and post-development surface water run-off calculations and hydraulic modelling using industry-standard software
• Pre-development contributing area and exceedance route plans
• Post-development contributing area and exceedance route plans
• Percolation / infiltration tests to BRE365 standards
• BRE365 soakaway sizing calculations and hydraulic modelling
• Surface water attenuation calculations and hydraulic modelling
• Assessment of the SuDS hierarchy in line with local and national policy
• A proposed SuDS strategy in line with local and national policy
• Outline SuDS design layout plans for your development
• Detailed SuDS design plans for the LLFA, Building Regulations and construction
• Management and maintenance plans for the SuDS over the lifetime of the development

How We Build Your SuDS Report

Beyond these deliverables, our process follows a clear sequence. First, we describe the existing and proposed drainage arrangements for the site. Next, we calculate the existing and proposed surface water run-off using industry-standard software.

We then assess the suitability of the various SuDS options by working through the SuDS hierarchy. We propose appropriate elements — such as green roofs, raingarden planters, raingardens, rainwater harvesting, soakaways, ponds and attenuation tanks — to meet the run-off rate requirements for your site. Finally, we provide an indicative drainage layout, then a detailed design of the surface water network and SuDS systems through layout plans and detailed drawings. We can deliver the report as a standalone document, or combine it with a Flood Risk Assessment for the site.

Will I Need to Carry Out Any Other Drainage Reports?

You may need a second-stage report, depending on the size, location and type of your project. Authorities sometimes request these once they have accepted or conditioned the SuDS strategy. Known as ‘Detailed SuDS and Surface Water Network Design’, these reports provide comprehensive layouts, long sections and construction details.

Which Properties Require a SuDS Report?

It is good practice for all new developments, whatever their size, to incorporate SuDS in their design. Whether a minor development needs a SuDS report to validate its planning application depends on each Lead Local Flood Authority’s requirements.

However, flood risk increases year on year, so most LLFAs now request these reports, even for minor extensions. Since 2015, national policy expects major developments — those of more than 10 units, or equivalent commercial or industrial schemes — to provide a Flood Risk Assessment and SuDS strategy with the planning application. The level of detail required varies by application type.

Nimbus’ Approach to Sustainable Drainage System Reports

Every professional on a construction project needs a solution that ensures compliance, controls cost, meets timescales and suits the individual site. Through a holistic approach, Nimbus Engineering handles your SuDS report from beginning to end, so you don’t have to worry about it.

As experts in this field, we take account of the critical elements of your site — including whether it sits in an at-risk or critical drainage area. We then give you clear guidance and ensure compliance with the relevant authorities and bodies.

Conclusion

As you can see, there is a lot to digest when it comes to SuDS and SuDS reports. We hope this guide has answered your key questions, so you can approach your next development project with confidence.

Many regulations surround SuDS reports and surface water management. The best advice is simple: prepare early and consider these systems from the start of your development process. All too often, developers leave little space on site for sustainable drainage. With the right preparation and knowledge, you can avoid complications and unexpected costs later on.

Do you still have questions about SuDS and SuDS assessments? Please get in touch with a Nimbus expert today. We’re always happy to help.