Posted by Callum Vallance-Poole, on April 8, 2025.
With heavier rainfall and more paved surfaces than ever before, flooding is an increasing concern across the UK. Whether you’re building a new home, managing a commercial site, or planning a local infrastructure project, managing surface water runoff is critical to protect both people and property.
One of the most effective tools in modern sustainable drainage is the stormwater attenuation tank. This underground system helps manage excess rainwater by temporarily storing it and releasing it slowly, reducing the risk of flooding and protecting local infrastructure.
What is a Stormwater Attenuation Tank?
A stormwater attenuation tank is a sealed underground structure that stores rainwater during heavy storms and releases it at a controlled rate over time. Rather than allowing water to rush straight into sewers or streams (which can overwhelm them and cause flooding), the tank slows the flow – a process known as attenuation.
Attenuation tanks are commonly used in areas where natural drainage is limited due to impermeable surfaces like roofs, roads or patios. By acting as a temporary reservoir, they reduce peak flow rates and help developments meet local drainage regulations, particularly when infiltration (like a soakaway) isn’t suitable.
While they’re often out of sight and out of mind, these tanks are a quiet but crucial part of many drainage systems across the UK – especially in new housing estates, commercial parks, and urban regeneration projects.
How Stormwater Attenuation Tanks Work
At their core, attenuation tanks manage the quantity of water during and after rainfall. Here’s how:
- Capture: Rainwater from roofs, roads or other hard surfaces is directed into the tank via drains or gullies.
- Storage: The tank temporarily holds this water underground. Depending on the site, this can range from a few cubic metres to several hundred.
- Controlled Release: A flow control device (like an orifice plate or vortex flow controller) regulates how quickly the stored water exits the tank. This slows the discharge to match allowable rates set by local authorities – often the greenfield runoff rate, or how much water would naturally run off the undeveloped land.
Attenuation tanks are often paired with silt traps and filter chambers to prevent blockages and protect the system from debris or pollution. They don’t treat water for quality – that’s the role of other SuDS components like swales, ponds or permeable paving – but they are essential for volume control.
Types of Attenuation Systems
There are several forms of attenuation tanks, each suited to different site conditions:
- Modular Geocellular Systems (plastic crates): Highly configurable and lightweight, ideal for residential or commercial sites. These are wrapped in an impermeable liner to create a sealed tank.
- Oversized Pipes (concrete or plastic): Common in infrastructure projects, they provide robust storage and are easy to inspect.
- Concrete Vaults: Used in heavy-duty applications like airports or rail yards, where structural load is a major concern.
- Above-Ground Basins: Not technically tanks, but they serve a similar function and offer additional biodiversity and landscape value where space allows.
- Dual-Purpose Tanks: Some systems combine attenuation with rainwater harvesting, allowing non-potable water reuse (e.g., for garden irrigation or toilet flushing).
Why Are Attenuation Tanks Important?
1. Flood Protection
The most obvious benefit is flood prevention. By slowing the rate of runoff entering the drainage network, attenuation tanks reduce pressure on public sewers and help prevent surface water flooding – both on-site and downstream.
2. Planning Compliance
UK planning policy increasingly requires developments to include Sustainable Drainage Systems (SuDS). Attenuation tanks are often critical to demonstrating compliance, particularly where infiltration is not feasible. Local authorities assess whether the post-development runoff rates match pre-development conditions – and tanks help achieve that.
3. Space Efficiency
Being buried underground, attenuation tanks preserve valuable surface land. This is especially useful on dense urban sites where space is at a premium. Car parks, gardens or playgrounds can all be placed above a tank without affecting its performance.
4. Climate Resilience
As storms become more intense due to climate change, the ability to buffer and delay runoff is increasingly important. Attenuation tanks provide that buffer, helping developments remain resilient even during extreme rainfall.
Common Applications
Stormwater attenuation tanks are used across a range of sites:
- New Homes and Extensions: Particularly where permeable ground is limited or connection to a sewer is necessary.
- Housing Developments: Often installed beneath roads or communal green areas to manage estate-wide runoff.
- Commercial Sites: Under car parks or service yards in retail parks, office campuses and logistics centres.
- Infrastructure Projects: Highways, rail stations, hospitals and schools all use attenuation to protect critical assets from flooding.
Some systems are even designed to be adoptable by water companies or highway authorities – though this requires careful adherence to design standards and maintainability.
Limitations and Considerations
While attenuation tanks are highly effective, they aren’t a one-size-fits-all solution:
- No Water Quality Treatment: They don’t remove pollutants. Pre-treatment (like silt traps or oil separators) may be needed upstream.
- Maintenance Required: Over time, sediment and debris can build up. Routine inspections and occasional cleaning are essential.
- Structural Considerations: Tanks must be designed to support the loads above – especially in trafficked areas.
- Not a Complete SuDS Solution: They manage flow, not amenity or ecology. Best practice is to combine tanks with green SuDS features where possible.
- Adoption and Ownership: There must be a clear plan for who maintains the tank, especially on shared or public land.
Stormwater attenuation tanks play a vital role in modern drainage strategies, helping developments meet planning requirements, protect against flooding, and build climate resilience. They store excess rainwater and release it in a controlled way – a simple concept with powerful benefits.
For homeowners, developers, and planners alike, understanding how attenuation tanks work is key to delivering sustainable, compliant projects that respect both the built and natural environments.
Want to Learn More?
If you’re planning a development or retrofit and need advice on attenuation or SuDS design, get in touch with a member of the team today.