Stormwater management refers to the strategies and systems used to handle rainwater runoff in developed areas, preventing flooding and environmental damage. In the UK, heavy rainfall events are becoming more frequent and intense due to climate change. At the same time, ongoing urbanisation means more roofs, roads, and car parks that don’t absorb water, leading to greater volumes of runoff. Effective stormwater management is therefore essential, not only to protect properties from flood risk, but also to meet strict sustainable drainage requirements in modern construction.
When rain falls on natural ground, much of it soaks into the soil or evaporates. But on paved or built-up sites, water can no longer infiltrate as it used to, which creates several problems. First, less infiltration means less groundwater recharge, potentially lowering the water table over time. Second, if runoff is not drained quickly or is left unmanaged, it can pond and cause surface flooding on the site or in surrounding areas. Third, diverting a lot of stormwater into sewers can overwhelm drainage networks, leading to sewer overflows and downstream flooding. Climate change is exacerbating these issues: the UK saw about 5% more rainfall in the 2009–2018 period compared to 1961–1990. Without proper stormwater management, intense downpours can quickly translate into flash floods and property damage.
Sustainable Drainage Solutions (SuDS)
Modern developments address these challenges through Sustainable Drainage Systems (SuDS) – design approaches that mimic natural water cycles to manage runoff locally. The idea is to slow down the water, store it, and either let it infiltrate into the ground or release it gradually at a controlled rate. Wherever space permits, developers are encouraged to include “green” drainage features like retention ponds, swales (shallow vegetated channels), rain gardens, or permeable pavements. These features allow water to soak into the soil or evaporate, thus reducing the volume entering drains. In fact, UK planning policy and Building Regulations (Part H) promote a clear discharge hierarchy: deal with runoff on-site via infiltration first, and only use sewers as a last resort. This means features like soakaways or permeable landscaping are preferred wherever ground conditions allow.
In practice, sustainable drainage is about more than just moving water; by definition SuDS approaches consider water quantity and quality alongside amenity and biodiversity goals. A well-designed SuDS feature can therefore serve multiple purposes. For example, swales and ponds are often integrated as attractive landscape elements in public open spaces or road verges, with wildflower plantings that add visual interest for the community while also providing habitat for wildlife. The vegetation and stored water in these systems create new ecological niches offering shelter, food and breeding grounds for organisms ranging from pollinating insects and amphibians to birds and other small animals. In short, good SuDS design manages runoff and enhances the environment, making developments more pleasant for people and more supportive of urban biodiversity
However, many projects, especially in dense urban areas, cannot rely on open SuDS features alone. Space is often limited, or the local soil is heavy in clay and impermeable, making full infiltration impractical. Studies show that newer housing developments may have significantly less green space than older neighbourhoods (up to 40% less), so there’s simply nowhere for all the water to naturally soak in. That’s where engineered solutions come in. Developers can install underground stormwater storage systems to capture excess runoff and prevent flooding when above-ground options are insufficient. One common approach is building an underground attenuation tank, essentially an empty reservoir that temporarily holds stormwater and then releases it slowly. This allows even a fully paved site to handle heavy rains without overwhelming the sewers or flooding the area.
What Is a Stormwater Attenuation Tank?
A stormwater attenuation tank is a sealed underground structure that stores rainwater during storms and releases it at a controlled rate over time. Instead of water rushing straight into sewers or rivers (which could overflow them), the attenuation tank holds the water back (a process known as “attenuation”) and drains it out gradually. Attenuation tanks are usually installed beneath car parks, gardens, or open spaces in a development. While they’re out of sight, they play a crucial role in preventing flood peaks and ensuring compliance with drainage regulations. In many UK housing estates, commercial parks, or road projects, these tanks are now a standard feature to manage surface water runoff.
Most attenuation tanks today are built using modular plastic units commonly called attenuation crates (also known as stormwater crates or geocellular crates). These are strong, lightweight plastic structures that interlock to create a large void space underground. Over 95% of the volume inside a crate structure is empty space, allowing it to hold a significant amount of water relative to the excavation size. The modular design means engineers can configure a tank of almost any shape or size by clipping together more crates, making this approach very flexible for different sites. Once assembled, the crate matrix is typically wrapped in special liner materials to complete the tank.
Attenuation vs. Infiltration: It’s important to note there are two main ways to set up an underground crate system, depending on the project’s drainage strategy. For attenuation tanks, the crate structure is wrapped in an impermeable geomembrane (like a thick plastic liner) to form a watertight tank. Collected rainwater is stored and later released in a controlled outflow through a connected pipe or outlet. This controlled discharge is often limited to the pre-development (greenfield) runoff rate allowed by local authorities, greatly reducing the risk of overwhelming the sewers and causing localised flooding. By contrast, an infiltration system (often called a soakaway crate setup) uses the same crates but wrapped only in a permeable geotextile fabric. In that case, there is no outlet pipe, the water slowly soaks into the surrounding soil instead, at whatever rate the ground can accept. Both attenuation and infiltration setups look similar (and use similar components); the difference is simply whether the water is being held and released to a drain, or directly returned to the earth. In many cases, the decision comes down to soil permeability and regulatory preference: if the ground percolation is good and not polluted, infiltration is ideal for recharging groundwater, but if not, an attenuation tank with a controlled outlet is used.
How Do Attenuation Crate Systems Work?
Once installed, an attenuation crate system manages stormwater through a simple three-step process:
- Capture: Rainwater runoff from hard surfaces (like roofs and roads) is collected via gutters, drains, and pipework and directed into the underground attenuation tank. Typically, gully pots and filters are used at inlets to trap debris or silt so that mostly clean water enters the tank.
- Storage: During the storm, the water fills up the hollow spaces within the crate structure, effectively turning the underground void into a temporary reservoir. Thanks to the high void ratio of the crates, a tank can store a large volume, from a few cubic metres for a small car park, to hundreds of cubic metres for a big development. The modular design means the tank’s dimensions (length, width, depth) can be tailored to available space on site.
- Controlled Release: As the storm subsides, the stored water is released slowly through an outlet pipe or orifice. A flow control device (for example, a vortex flow controller or an orifice plate) is installed at the tank’s outlet to regulate the discharge rate. This device ensures that water exits no faster than the pre-agreed rate (often the natural greenfield runoff rate set by the authorities). By throttling the outflow, the attenuation system prevents any sudden surge of water downstream. The excess water might be released over several hours or days in a controlled manner, typically into a nearby watercourse, storm sewer, or drainage ditch. In an infiltration setup, this third step is replaced by soaking into the ground around the tank (no outlet pipe).
In both cases, the outcome is that peak rainfall is buffered, the site only releases water slowly, significantly reducing the risk of flooding on-site and downstream. While some sediment may naturally settle within the tank, these systems are not designed to filter or treat pollutants. To protect water quality, it’s good practice to include upstream silt traps or pre-treatment measures (such as catchpits or oil separators where required) to remove debris and contaminants before water enters the tank. Regular inspection and maintenance, typically on an annual basis, helps ensure that silt traps and inlets remain clear and that the system continues to function effectively.
Common Stormwater Storage Solutions
Attenuation crate systems are one of several tools available for managing excess runoff. Other stormwater storage solutions that engineers may consider include:
- Oversized Pipes: Large-diameter pipes (plastic or concrete) laid underground can act as linear storage chambers. These are essentially oversized drains that hold water during a storm and release it slowly. They are robust and often easy to inspect, but require space in the ground and are less modular in shape.
- Concrete Tanks or Culverts: Pre-cast concrete tanks or box culvert systems can form extremely strong underground reservoirs. They handle heavy loads (like under roadways or airports) and have a long lifespan. However, they usually involve higher material and installation costs and less flexibility in configuration compared to plastic crate systems.
- Modular Geocellular Crates: The focus of this article; these are plastic crate systems which are highly configurable, lightweight, and strong. They can be assembled to almost any size and are ideal for most residential and commercial projects due to their flexibility. Many modern developments use crate-based attenuation because it maximises storage volume in a given footprint and is relatively quick to install.
- Above-Ground Basins or Ponds: Where land is available, creating a retention basin, lagoon or detention pond at the surface is a great SuDS solution. These features not only store water and prevent flooding, but also can provide amenity and biodiversity benefits (like creating a wetland habitat). The drawback is they occupy valuable land area and need appropriate landscaping; hence, they’re not always feasible on space-constrained sites.
Often a combination of these approaches is used in a drainage design. For example, a site might use permeable pavements and swales to handle minor rainfall, and an attenuation tank for extreme storms. Some tanks are even dual-purpose for instance, an attenuation tank might be combined with a rainwater harvesting system, so that stored water can be reused for irrigation or toilets rather than simply discharged. The choice of solution depends on site conditions, regulatory requirements, and project budget. Geocellular attenuation crates have become especially popular because they “tick all the boxes” for modern developments. They are modular, space-efficient, high capacity, and suitable for trafficked areas.
Benefits of Attenuation Crate Systems
Using attenuation crates as part of a stormwater management plan offers several key benefits for UK development projects:
- Prevents Flooding: By holding back peak flows, an attenuation tank dramatically reduces the risk of flash flooding on-site and downstream. The stored water is released slowly enough that local rivers or sewers stay within capacity, helping protect communities from flood damage. In short, these systems act like a safety valve during heavy storms, smoothing out the flood peaks.
- Regulatory Compliance: Sustainable drainage isn’t just good practice, it’s a legal requirement. The UK’s National Planning Policy Framework mandates SuDS for major developments, and local authorities won’t approve projects without a viable drainage plan. Attenuation crates provide an efficient way for developers to meet runoff rate limits set by planners or Lead Local Flood Authorities. Installing a compliant attenuation system helps ensure your project gets the green light and avoids costly redesigns.
- Space Efficiency: Because crate tanks are buried, they don’t take up usable surface land. You can build car parks, driveways, or parks right above an underground tank without losing developable area. This is especially valuable in urban sites where real estate is at a premium. Instead of a large open pond (which might not be possible on a tight site), the storage is hidden below ground, leaving the surface free for other uses.
- Cost-Effective and Flexible: Modular crate systems are often more cost-effective than traditional concrete or pipe-based solutions. They use lightweight materials and can be transported and installed relatively easily. In fact, projects have saved tens of thousands of pounds by opting for plastic attenuation crates over conventional pipe chambers. The flexibility in sizing also means you only install as much volume as needed. Should the site layout change, the tank shape can be adjusted. Additionally, many crates are made from recycled plastic, supporting sustainability goals while often being cheaper than concrete structures.
- High Capacity, Low Excavation: With void ratios of 95% or more, geocellular crates make very efficient use of the excavation volume. In practical terms, this means less digging and less soil to dispose of for a given storage requirement, compared to stone-filled soakaways or large pipes. Reducing excavation not only cuts disposal costs (which are high in the UK) but also speeds up construction. A large crate tank can often be installed in a matter of days with a small crew and machinery, minimising site disruption.
- Structural Strength: Don’t be fooled by the plastic material attenuation crates are engineered for strength. When properly installed with the right cover depth, they can support substantial loads, including vehicular traffic. Different crate models are available for various load ratings (from light garden use to heavy HGV loads). This means you can confidently put crate tanks under car parks or access roads without compromising on structural integrity. The modular grid disperses loads effectively, and many systems have been proven to withstand 30+ tonne lorries when installed to spec.
- Climate Resilience: As climate patterns shift towards more intense rainfall, having an attenuation system is a forward-looking measure. It gives a development a buffer against extreme storm events that might become more frequent. By providing extra storage and slowing runoff, attenuation crates help communities adapt to climate change and remain resilient during extreme weather. They are a key tool in designing infrastructure that can handle future conditions as well as present ones.
Planning and Design Considerations
Implementing stormwater management on a project isn’t just about digging a hole and dropping in crates, it requires careful planning from the early stages of design. In the UK, developers typically must submit a surface water drainage strategy as part of the planning application, detailing how runoff will be controlled on-site. Local planning authorities, often guided by the regional Lead Local Flood Authority (LLFA) or water company, will specify the maximum rate at which the site is allowed to discharge stormwater. This is usually set to mimic the natural greenfield runoff rate (the rate of water runoff prior to development) or another conservative limit to protect the catchment. For example, an authority might say that even after you build 100 houses, the runoff leaving the site during a storm should be no more than, say, 5 litres per second per hectare. Meaning you need to attenuate any excess on-site.
Once the allowable discharge rate is known (and if infiltration alone won’t handle all the water), engineers calculate the required storage volume for the attenuation system. This involves analysing local rainfall data and storm scenarios, commonly a design storm like a 1-in-30 year or 1-in-100 year event plus a climate change factor, depending on policy. Using hydrological modeling tools, the team will determine how many cubic metres of water need to be stored so that the outflow can be throttled to the permitted rate without flooding. (For instance, a 1-hectare site might need to temporarily store tens or hundreds of cubic metres during a cloudburst.) The UK Environment Agency or LLFA may require these calcs to ensure the design meets safety margins.
Early coordination is key: The drainage design should be integrated into the site layout from the beginning – it’s much easier than retrofitting later. Planners will look for evidence that you considered SuDS options (like infiltration tests to see if soakaways work, percolation rates, etc.). If soil tests show poor infiltration, then an attenuation tank is justified. The next step is deciding where to locate the tank and how to integrate it with the site. Common locations are under car parks, under landscaped areas, or beneath roadways on the development. The spot must have enough room for the tank footprint and adequate depth (allowing for soil cover on top for protection and landscaping). Engineers will choose a solution that fits both the space and loading requirements, for example, using heavier-duty crates or concrete sections if the tank sits under a road carrying heavy trucks, versus lighter modules for a garden area.
During design, attention is also given to inlet and outlet details. Inflow pipes should connect to the top of the tank to fill it, and an outlet (with flow control) at the bottom to drain it at the set rate. An overflow might be included for exceptional storms (so that if the tank ever fills beyond capacity, excess water can safely route to a controlled overspill area rather than cause damage). It’s also wise to incorporate inspection chambers or silt traps before water enters the tank, making maintenance easier. Local authorities often require observation points to be built in, to verify the system is working and to allow for cleaning out debris periodically.
Finally, there’s the question of ownership and maintenance. Who will maintain the attenuation tank over its life (which can be well over 50 years)? Will it be adopted by the county’s SuDS approving body or managed by a private management company? A maintenance plan is usually needed in the drainage strategy, committing to inspections (e.g. annual checks for sediment). The good news is, when designed and installed properly, attenuation crate systems need minimal upkeep, mostly just keeping the upstream gullies clean and occasional jet-washing if sediment builds up. Many installations in the UK have now been in service for years, quietly doing their job every time it rains.
Stormwater management might not be the flashiest aspect of a new development, but it’s absolutely vital for the long-term sustainability and resilience of our built environment. By thoughtfully managing runoff, using both natural infiltration where possible and engineered attenuation where needed, developers and engineers can protect homes and businesses from flooding, comply with regulations, and even save on construction costs with efficient designs. The UK’s emphasis on Sustainable Drainage Systems means that features like attenuation crates are no longer optional add-ons; they’re a core part of responsible construction in the face of changing climate patterns and urban growth.
In summary, stormwater management is all about keeping rainfall under control: ensuring today’s downpour doesn’t become tomorrow’s flood. With the right combination of green infrastructure and modern attenuation systems, even the heaviest rains can be handled in a safe, controlled way that safeguards communities and the environment. As you plan your next project, consider stormwater solutions early, consult drainage experts, explore modular attenuation tank options, and coordinate with local authorities on SuDS requirements. By doing so, you’ll be investing in a development that stands the test of time (and weather!), delivering both peace of mind and compliance.
Posted by Callum Vallance-Poole, on October 13, 2025.
