The SuDS Management Train: A Practical Design Walkthrough

The SuDS management train is one of those concepts that every engineer learns early in their career but many struggle to apply consistently in practice. It is the organising principle behind sustainable drainage design — a hierarchical approach that manages surface water as close to its source as possible, treating and attenuating runoff through a sequence of stages before it reaches the receiving watercourse or sewer.

Done well, the management train delivers drainage that looks nothing like drainage. It becomes landscape, habitat, amenity, and place-making. Done poorly — or ignored in favour of oversized underground tanks — it results in missed opportunities and, increasingly, planning objections.

This article walks through the management train from first principles, with practical guidance on applying each stage to real development projects.

What Is the Management Train?

The SuDS management train is a treatment sequence that manages surface water runoff through four successive stages. Each stage provides attenuation (slowing the flow) and treatment (removing pollutants) before passing the water to the next stage. The principle is that runoff is managed in small, distributed components close to where the rain falls, rather than in large, centralised structures at the end of the pipe.

The four stages are:

1. Prevention — avoiding runoff generation in the first place.
2. Source control — managing runoff at or very near the point where rain hits the ground.
3. Site control — managing runoff from multiple source control components across the site.
4. Regional control — managing runoff from the entire site before discharge to the receiving water.

Not every site will need all four stages, but the design should follow the hierarchy and demonstrate why any stage has been omitted.

Stage 1: Prevention

Prevention is the first and most effective step in the management train. It involves designing the development to minimise the generation of surface water runoff in the first place.

Practical prevention measures include:

• Reducing impermeable area: Challenge the assumption that every surface must be hard. Can car parking areas use permeable paving instead of tarmac? Can footpaths be gravel or resin-bound rather than concrete? Can private gardens be soft landscaped rather than paved?
• Green roofs: Replacing conventional roofing with green (sedum or biodiverse) roof systems reduces runoff from the roof area by 50-80%, depending on the system and rainfall intensity. Green roofs also provide thermal insulation, biodiversity value, and visual amenity.
• Rainwater harvesting: Collecting roof runoff in water butts or larger cisterns for reuse (garden irrigation, toilet flushing, vehicle washing) removes that volume from the drainage system entirely.

Prevention is often overlooked in drainage design because it sits at the interface between the drainage engineer and the architect. The drainage engineer may not feel empowered to challenge the architectural design, and the architect may not be aware of the drainage benefits of their design choices. Good SuDS design requires collaboration between disciplines from the outset.

Stage 2: Source Control

Source control manages runoff at or very close to the point where it is generated — typically individual buildings, car parking areas, or road sections. Source control components are small, distributed, and close to the surface.

Common source control features include:

Permeable Paving

Permeable paving allows rainfall to infiltrate through the surface and into a sub-base reservoir, where it is stored temporarily before infiltrating into the ground or being conveyed slowly to the next stage. Permeable paving is particularly effective for car parks, driveways, and low-traffic roads.

Design considerations: sub-base depth (typically 300-500mm), infiltration rate of the underlying soil (a soakaway test is essential), structural loading requirements, and long-term maintenance (annual sweeping to prevent clogging).

Rain Gardens and Bioretention

Rain gardens are planted depressions that receive runoff from adjacent impermeable surfaces. The runoff infiltrates through the planting substrate, which provides filtration and pollutant removal, before draining into the sub-soil or to a perforated pipe collection system.

Rain gardens are excellent source control features because they combine attenuation, treatment, and amenity in a compact footprint. They work well in residential front gardens, along road verges, and in car park margins.

Filter Strips and Swales

Filter strips are gently sloping vegetated surfaces that receive sheet flow from adjacent impermeable areas. They slow the flow, promote infiltration, and filter out sediment and pollutants. Swales are shallow, vegetated channels that convey runoff slowly while providing treatment through filtration and biological uptake.

Both features require space, which is why they work best on larger sites or sites with generous landscape buffers. On constrained urban sites, rain gardens or tree pits may be more appropriate.

Stage 3: Site Control

Site control components receive runoff from multiple source control features and manage it at the site scale. They provide additional attenuation and treatment and begin to consolidate distributed flows into a manageable number of discharge points.

Detention Basins

Detention basins are dry (normally empty) depressions that fill with water during storm events, storing runoff temporarily before releasing it at a controlled rate. They are effective for large volumes of runoff and can double as recreational open space during dry weather.

Design considerations: basin geometry (gentle side slopes for safety, typically 1 in 4 or shallower), inlet and outlet structures, emergency overflow routes, and landscape integration.

Ponds and Wetlands

Permanent water features provide the highest level of treatment in the management train. Ponds and wetlands retain water permanently, allowing sediment to settle, pollutants to be processed by biological activity, and ecosystems to develop. They also provide significant amenity and biodiversity value.

Ponds and wetlands require careful design to maintain water quality, manage fluctuating water levels, and ensure safety (particularly where children have access). They work best on larger sites where the permanent water area is proportionate to the overall development.

Underground Storage

Where surface space is limited, underground attenuation storage (geocellular tanks, oversized pipes, or concrete tanks) can provide the required attenuation volume. However, underground storage provides no treatment, no amenity, and no biodiversity benefit. It should be considered a last resort within the management train — used only when surface features cannot provide sufficient storage.

LLFAs and SABs (in Wales) are increasingly pushing back on designs that rely heavily on underground storage without demonstrating that surface SuDS features have been maximised. If your design includes a large geocellular tank, expect questions about why above-ground alternatives were not used.

Stage 4: Regional Control

Regional control is the final stage in the management train, managing the combined runoff from the entire development before it is discharged to the receiving watercourse, sewer, or ground. In practice, the distinction between site control and regional control is often blurred, particularly on smaller sites.

Regional control features include large ponds, wetlands, or flood storage areas that serve the entire development. On strategic sites, regional control may involve off-site features such as flood storage reservoirs or constructed wetlands that serve multiple developments.

The key function of regional control is to attenuate the total discharge from the site to the agreed discharge rate, typically the greenfield runoff rate for the site. The flow control device — usually a vortex control or orifice plate — is located at the point of discharge and limits outflow to the permitted rate.

Applying the Management Train in Practice

The management train is not a rigid formula but a design philosophy. The art of good SuDS design is in selecting the right combination of features for the specific site conditions, development type, and receiving environment.

Residential Development

For a typical housing development, a good management train might include: green roofs on apartment blocks (prevention), permeable paving for private driveways and parking courts (source control), roadside swales and rain gardens (source/site control), and a landscaped detention basin or pond at the low point of the site (site/regional control).

Commercial Development

For a retail or industrial development with large roof and hardstanding areas, the management train might include: rainwater harvesting for non-potable use (prevention), permeable paving for car parks (source control), filter strips along building perimeters (source control), and an attenuation pond or wetland before discharge (site/regional control).

Constrained Urban Sites

For dense urban sites where space is at a premium, the management train must be creative: green roofs and blue roofs for roof attenuation (source control), tree pits with engineered soil volumes along streets (source control), bioretention planters integrated into public realm (site control), and underground storage only where surface features cannot provide sufficient volume (regional control).

Common Mistakes

1. Jumping straight to underground tanks. The most common mistake is skipping the upper stages of the management train and going directly to a geocellular tank. This provides attenuation but misses the treatment, amenity, and biodiversity benefits that planning authorities increasingly expect.

2. Designing SuDS as an afterthought. SuDS features need space, and that space must be allocated in the masterplan from the outset. Trying to retrofit SuDS into a fully resolved layout is difficult, expensive, and usually results in a compromised design.

3. Ignoring maintenance. Every SuDS feature requires maintenance, and the maintenance regime must be designed at the same time as the feature itself. Specify who will maintain each feature, how often, and at what cost. A beautiful rain garden that is never maintained becomes an ugly, clogged ditch.

4. Not testing infiltration. Infiltration is the preferred discharge method for SuDS, but it depends entirely on the soil conditions. Always conduct soakaway testing (BRE365 or equivalent) before assuming infiltration is viable. Many sites have impermeable clay soils where infiltration is not an option, and the design must account for this.

The management train is not complicated, but it does require thoughtful application. If you need help designing a SuDS scheme that follows the management train and satisfies LLFA requirements, get in touch. We design drainage systems that work — above ground and below.