Understanding Drainage Hierarchy in the role of SuDS
When planning a new residential development, bespoke home, commercial building or industrial site, surface water drainage needs to be considered from the earliest stages of design.
Understanding the UK drainage hierarchy can help architects, developers and contractors make informed decisions about how rainwater should be managed and where it should ultimately be discharged.
The hierarchy is an important part of effective SuDS design and can influence site layout, landscaping, levels and the amount of land required for drainage infrastructure. Bringing a civil engineer or SuDS engineer into the project at the right stage can help ensure these considerations are addressed before designs become too advanced to change easily.
What is the Drainage Hierarchy?
The Drainage Hierarchy sets out a preferred order for managing surface water. This applies to any development, whether it’s a residential housing scheme, new build home or commercial unit. The principle is that water should be managed as close to where it falls as possible, using the most sustainable option available.
The hierarchy generally prioritises rainwater reuse first, followed by infiltration into the ground. If infiltration is not technically feasible, discharge to a watercourse should be considered, followed by a surface water sewer. Connecting to a combined sewer sits at the bottom of the hierarchy and should only be considered as a last resort.
This approach forms an important part of sustainable drainage systems and should be considered when developing a SuDS drainage design.
The five stages of the drainage hierarchy
1. Rainwater reuse
The first option is to collect and reuse rainwater wherever practical for ‘non potable use’. This could include rainwater harvesting for toilets or irrigation, as well as smaller measures such as water butts.
Reusing rainwater can reduce the volume of surface water leaving a development while also reducing demand for mains water.
2. Infiltration
If rainwater cannot be reused, the next option is to allow it to infiltrate naturally into the ground.
Examples include soakaways, infiltration trenches and permeable paving. Infiltration can provide an effective and sustainable way of managing surface water, but its suitability depends on factors such as geology, soil permeability and groundwater levels.
This is why early investigation of site conditions is an important part of SuDS design.
3. Discharge to a watercourse
Where infiltration is not technically feasible, the next option is generally discharge to suitable surface water above ground such as a river, stream or canal.
Any proposed discharge will need to consider the capacity and environmental sensitivity of the receiving watercourse, as well as any relevant permissions or restrictions.
4. Discharge to a surface water sewer
If rainwater cannot reasonably be managed through the higher levels of the hierarchy, connection to a dedicated surface water sewer may be considered.
A suitable connection point and available capacity will need to be established and the proposed discharge rate may need to be restricted through attenuation or other SuDS measures.
5. Combined sewer
Connecting surface water to a combined sewer is the least sustainable option and should generally be considered only when the alternatives above have been demonstrated to be unsuitable.
Combined sewer systems carry both foul and surface water. During periods of heavy rainfall, additional surface water can contribute to capacity issues and increase the risk of sewer flooding and storm overflow events.
Why does the Drainage Hierarchy matter?
The hierarchy is more than a design preference. It helps planning authorities and Lead Local Flood Authorities assess whether a proposed drainage strategy represents an appropriate and sustainable approach to surface water management.
Importantly, a project cannot simply select the easiest or cheapest connection option. Where a higher level of the hierarchy is not feasible, the design team may need to provide evidence explaining why.
For example, if infiltration is being discounted, a developer may need site specific evidence such as BRE 365 infiltration testing to demonstrate that the ground cannot accommodate the proposed drainage strategy.
How does the hierarchy affect SuDS design?
Understanding the hierarchy early can prevent drainage from becoming a constraint later in the design process.
For example, if infiltration is likely to be feasible, the site may need suitable space for soakaways or other infiltration features. If infiltration is unlikely to work, the project may instead require attenuation, a controlled watercourse discharge or a connection to the surface water network.
These decisions can affect build construction, roads, parking, landscaping and finished levels. Incorporating them during RIBA Stages 0-2 is therefore much easier than trying to retrofit drainage once the site layout has been established.
What information is needed?
An early desktop assessment can provide useful information before detailed SuDS drainage design begins. The British Geological Survey’s geological mapping can help indicate likely ground conditions, while flood risk mapping can identify potential surface water and river flood constraints.
Existing drainage asset plans can also help establish whether public surface water or combined sewers are present nearby.
However, desktop information is not a substitute for site investigations. A civil engineer or SuDS engineer can assess the available evidence, identify what further testing is required and develop a drainage strategy appropriate to the site.
When should you involve a civil engineer?
The earlier a civil engineer is involved, the more opportunity there is to influence the overall design.
At concept stage, a civil engineer can help assess drainage constraints, identify potential discharge routes and advise on the space required for SuDS features. This can help architects and developers avoid layouts that later prove difficult or expensive to drain.
As the project progresses, the civil engineer can develop the detailed SuDS drainage design, undertake calculations and provide the technical information required to support planning and construction.
Final thoughts
Understanding the Drainage Hierarchy is an essential part of planning sustainable development. By considering rainwater reuse, infiltration, watercourses and sewer connections in the correct order, project teams can develop drainage strategies that reduce flood risk, protect water quality and minimise pressure on existing infrastructure.
Most importantly, drainage should be considered alongside the wider design rather than treated as an engineering detail at the end of the process.
If you’re planning a new development and want to understand the information available before engaging a civil engineer, explore our practical guide to Sustainable Drainage Systems. It brings together key SuDS resources, including geological and flood risk mapping, to help architects, developers and contractors make better informed decisions at concept stage.