In Part 3 of our Sustainable Urban Drainage Systems (SuDS) Series, we explored how offsite-built packaged plant rooms integrate the pumping and control systems that keep urban drainage projects moving. But even the most advanced pumping solution needs somewhere to hold stormwater when infiltration isn’t viable. That’s where attenuation tanks become essential.
For sites with poor soil permeability, high water tables, or no space for ponds and swales, engineered below-ground storage isn’t just a design choice, it’s the only way to achieve SuDS compliance, protect against flooding, and secure planning approval.
In this article, we’ll show how correctly specified attenuation tanks work alongside pumps and flow controls to manage peak runoff, keep developments flood-safe, and meet stringent planning requirements, even on the most constrained urban plots.
What Are Attenuation Tanks?
Attenuation tanks are engineered underground storage systems that temporarily hold excess stormwater during heavy rainfall. In a SuDS context, they intercept and store runoff that would otherwise overwhelm the downstream drainage network, releasing it at a controlled rate once peak flows subside.
Typically built from modular geocellular units or large-diameter pipes, they offer high void ratios, structural strength, and the flexibility to fit within challenging site layouts. In urban SuDS schemes, they are especially valuable because they provide storage without taking up surface land, helping developments meet water quantity objectives under the four pillars of sustainable drainage.
In design, engineers consider:
- Capacity and flow control: ensuring the tank volume matches site runoff and discharge rate requirements.
- Structural loading: selecting a specification suitable for installation under trafficked areas.
- Site constraints: adapting layout to avoid underground services, foundations, or irregular plot shapes.
- Ground conditions: using sealed or lined tanks where soils are impermeable, contaminated, or affected by high groundwater.
When combined with pumps, filtration, or treatment stages, attenuation tanks can also support water quality improvement and enable rainwater reuse as part of an integrated SuDS strategy.
The Role of Attenuation Tanks in SuDS
In a SuDS strategy, attenuation tanks act as the system’s primary buffer, managing the timing and volume of runoff entering the downstream network. Their placement and configuration are calculated to work seamlessly with pumps or flow control devices, ensuring discharge occurs at a rate the receiving system can accommodate without increasing flood risk. This mimics the natural, slow release of water into the ground, fulfilling the “water quantity” pillar of SuDS.
When are attenuation tanks needed in SuDS?
For projects where traditional, landscaping-led SuDS are unworkable, a drainage design must overcome significant site challenges. Below-ground attenuation tanks become the primary engineered solution in the following scenarios:
When there is little or no space for green infrastructure:
On high-density residential or retrofit developments, valuable surface space is often a luxury. Attenuation tanks offer a powerful, underground solution that enables flood resilience and compliance without consuming the land required for ponds, swales, or infiltration trenches.
When soil conditions prevent soakaways:
Sites with impermeable clay, contamination, or a high water table cannot rely on infiltration. A robust, sealed attenuation tank provides a controlled storage mechanism, ensuring water is managed effectively regardless of ground conditions.
When drainage networks are ageing or already overloaded:
Many urban drainage systems are not equipped to handle the increased runoff from modern developments. Attenuation tanks provide a critical buffer, slowing the flow of water and preventing the site from contributing to downstream flooding.
When planners require a clear mitigation strategy:
Local planning authorities demand robust, measurable solutions to approve development. Attenuation tanks provide a clear, quantifiable method for managing runoff, offering the performance certainty required to secure project approval.
Because they operate out of sight below ground, attenuation tanks free up valuable surface space for other uses, making them well-suited to:
- Car parks, access roads, and other multi-use paved areas
- High-density residential or mixed-use schemes
- Retrofit and brownfield developments with limited open land
- Sites facing strict planning or discharge consent conditions
Properly designed tanks work in tandem with flow control systems and pumps to ensure discharge rates are both effective and compliant, especially under AMP8-aligned flood resilience goals. Dutypoint supplies bespoke attenuation tanks that are purpose-built for these kinds of sites.
Horizontal versus Vertical Attenuation Tanks in SuDS Design
Both vertical and horizontal tanks are used in SuDS, but each is better suited to different site scenarios based on their structural design and footprint.
Horizontal Attenuation Tanks
Horizontal tanks are the most common choice for SuDS on urban and brownfield sites due to their shallow profile and large footprint. They are particularly well-suited for the following situations:
- Sites with a high water table: Their shallow depth minimises the need for deep excavation, which is a major advantage where a high water table would cause issues with buoyancy or stability.
- Integration under hard surfaces: They are ideal for installation beneath car parks, access roads, and playgrounds. Their shape allows them to be buried under wide, low-load surfaces, maximising the use of a site’s footprint without compromising valuable surface space.
- General attenuation and flow control: They provide a large storage volume with a relatively low profile, which works well for passive gravity-fed systems or in conjunction with pumps for controlled discharge.
Vertical Attenuation Tanks
Vertical tanks, which have a smaller footprint and greater depth, are a more specialist solution used when space is at a premium. They are best for:
- Extremely restricted footprints: On sites where the available surface area is very limited and a deep excavation is feasible, vertical tanks offer the maximum storage capacity for the smallest possible footprint.
- Avoiding existing underground services: Their compact plan view allows them to be positioned carefully between other underground utilities, foundations, or services that a longer, horizontal tank could not navigate.
- Sites with minimal surface space: If a site is entirely built upon and the only space available is a small, deep area, a vertical tank can provide the required attenuation without needing to use valuable land.
Overall, horizontal tanks suit most SuDS applications, offering large storage volumes at shallow depths, ideal under hardstanding and in high water table areas. Vertical tanks are a niche option for sites with very limited surface area or complex underground constraints, providing high capacity in a small footprint where deep excavation is possible.
Achieving SuDS Compliance with Dutypoint’s GRP Attenuation Tanks
Dutypoint’s range of Vortech™ Plus GRP tanks offers engineered storage options to suit both horizontal and vertical SuDS designs.
Horizontal GRP tanks: Available in diameters from 1,200 mm to 4,000 mm and lengths up to 26,500 mm (capacities up to 300,000 litres), these one-piece units are ideal where high-volume attenuation is needed at shallow depths, such as beneath car parks or access roads.
Vertical GRP tanks: Compact in footprint but high-capacity, these are suited to constrained sites where deep excavation is feasible and horizontal layouts are impractical.
Both configurations can be specified with integrated pumping, flow control, and monitoring, allowing them to handle surface water and foul water where flow rate restrictions apply. Optional features such as baffle walls, separate storm/foul compartments, or mixing/aeration systems further extend their performance, enabling integration into complex SuDS schemes that demand reliability, durability, and compliance.
Store, Control, Comply – Unlock More from Your SuDS
For urban and brownfield sites where infiltration isn’t an option, attenuation tanks provide the compliant, below-ground buffer that keeps projects flood-safe and planners on-side. Correctly specified and integrated with pumps and flow controls, they help you manage peak flows without sacrificing valuable surface space – a critical win on high-density developments.
In Part 5 of our SuDS series, we’ll explore how to add value to your project by recycling stormwater. We will show how a correctly specified rainwater harvesting system can provide a sustainable source of non-potable water, saving costs and enhancing a building’s green credentials.
Need a compliant attenuation solution for a space-restricted or high water table site? Speak to Dutypoint’s expert team today to find out how we can design and deliver the right storage system for your project.
SuDS Attenuation Tanks: Frequently Asked Questions
Sealed GRP attenuation tanks provide a robust solution by preventing groundwater ingress and floatation, ensuring stormwater storage is secure and effective even in challenging, waterlogged ground conditions.
They provide a powerful, out-of-sight solution for sites with no space for traditional features like swales or ponds, offering critical stormwater storage in a compact, below-ground footprint.
Yes. By holding back peak runoff and releasing it at a controlled rate, attenuation tanks prevent a site from overwhelming the downstream drainage network, directly contributing to wider urban flood risk management.
A bespoke GRP tank is purpose-built to a project’s specific spatial and volume requirements. This ensures a perfect fit for complex layouts and optimises performance on sites with unique constraints.
Yes, tanks can be partitioned into separate compartments. This allows one section for flood attenuation while another stores rainwater for non-potable applications like irrigation or toilet flushing.
Yes. Properly specified GRP attenuation tanks can be installed beneath trafficked areas, providing high-volume stormwater storage without reducing usable surface space – ideal for high-density developments.
