What is a Detention Tanks

Detention tanks are engineered storage structures integrated into a sewerage or drainage system to temporarily hold excess water during periods of peak flow. Unlike retention systems, which store water permanently, detention tanks are dry most of the time, filling only when rainfall or wastewater flow exceeds the capacity of downstream systems. The stored water is then released in a controlled manner once flow levels subside, reducing the risk of surcharging, surface flooding, and sewer overflows.

Detention tanks are critical components in urban flood mitigation, combined sewer overflow (CSO) reduction strategies, and sustainable drainage system (SuDS) designs. They can be constructed from a range of materials and installed either above ground or underground, depending on spatial constraints and project requirements.

Purpose and Function

The primary objective of a detention tank is to mitigate hydraulic overload within a sewer or drainage network by temporarily attenuating flow. During heavy or prolonged rainfall, or when peak discharge from impervious surfaces exceeds sewer capacity, detention tanks intercept excess water to prevent immediate release downstream.

This function serves several key purposes:

  • Preventing surface water flooding in urban areas 
  • Protecting wastewater treatment works from hydraulic shock or bypass 
  • Reducing the frequency and volume of combined sewer overflows (CSOs) 
  • Allowing controlled discharge of stored water at rates that do not exceed network design limits 
  • Providing compliance with discharge consents and planning regulations 

In many systems, detention tanks are complemented by flow control devices, such as orifice plates or vortex regulators, which manage the release of stored water back into the main sewer or outfall after the storm has passed.

Types of Detention Tanks

Detention tanks come in various forms and configurations, designed to suit different applications, flow conditions, and spatial limitations. The most common types include:

1. Traditional Concrete Tanks

These are large, robust, in-situ or precast concrete chambers, typically constructed underground. They are well suited to major infrastructure projects or urban developments where long-term durability and high-volume storage are required. Their internal surfaces may be smooth or profiled to encourage self-cleansing.

2. Modular or Geocellular Tanks

Geocellular systems consist of interlocking plastic units with high void ratios, assembled to form a subterranean tank. These are lightweight, quick to install, and ideal for areas with limited space. They are commonly used beneath car parks, roads, or landscaped areas and may be wrapped in impermeable geomembrane to create a sealed detention tank.

3. Oversized Pipes or Pipe Tanks

In some cases, large-diameter pipes (e.g. concrete or plastic) are laid horizontally and act as detention tanks. These offer the advantage of simplicity and are particularly cost-effective for linear storage needs, such as alongside roads or beneath industrial yards.

4. Steel or Fibreglass Tanks

Used in sites with tight constraints or where modular construction is necessary, steel or GRP tanks can be prefabricated and installed with minimal disruption. These are often used in retrofitting or temporary works but can also serve permanent applications.

Each system may be equipped with inlet screens, flow control chambers, telemetry, and access manholes to facilitate inspection and maintenance.

Hydraulic Performance

The performance of a detention tank is evaluated based on its ability to:

  • Accommodate a predefined design storage volume during peak inflow 
  • Delay discharge to the downstream system through controlled release mechanisms 
  • Support hydraulic efficiency by minimising stagnation, dead zones, and sediment build-up 
  • Integrate with real-time control systems in smart drainage networks 

The design process typically begins with hydrological modelling to determine the peak flow rates and storm durations the tank must handle. Design storms, often based on return periods of 1-in-30 or 1-in-100 years, are used to establish worst-case scenarios. Tools such as MicroDrainage, InfoWorks ICM, and SWMM simulate flow accumulation and storage dynamics, allowing engineers to size the tank appropriately.

Key parameters include:

  • Storage volume (m³) 
  • Inflow rate (L/s or m³/h) 
  • Permissible outflow rate 
  • Time to empty post-event 
  • Sediment transport and self-cleansing velocity 

Advanced installations may include real-time monitoring and adaptive control, enabling dynamic response to changing rainfall patterns and system conditions.

Role in Sewerage and Surface Water Systems

Detention tanks are used in a wide range of drainage and sewerage contexts. Their applications include:

  • Combined sewer networks: Providing buffer capacity to reduce overflows during wet weather 
  • Separate surface water systems: Managing urban runoff in areas where downstream capacity is limited 
  • Highway drainage: Storing road runoff to prevent flooding of carriageways or downstream culverts 
  • Housing and commercial developments: Meeting planning conditions for surface water attenuation 
  • Wastewater treatment plants: Preventing hydraulic overload at the headworks 

They are often used in conjunction with other SuDS components such as swales, ponds, and infiltration systems, forming part of a management train that controls runoff close to source and across the catchment.

Regulatory and Planning Context

In the United Kingdom, detention tanks are subject to guidance and standards issued by planning authorities, water companies, and national bodies. Key regulatory considerations include:

  • Sewerage Sector Guidance (SSG): For adoptable assets, sets standards for tank sizing, access, materials, and performance 
  • Lead Local Flood Authority (LLFA) requirements: Mandate attenuation to greenfield runoff rates or specific discharge limits 
  • Environment Agency permitting: May apply where tanks discharge to surface waters or sensitive environments 
  • Building Regulations Part H: Covers drainage design for buildings and requires flood risk mitigation 
  • Non-Statutory Technical Standards for SuDS: Provide national design criteria for surface water management in England 

Planning applications must often be accompanied by a Drainage Strategy or Flood Risk Assessment (FRA) that demonstrates how detention tanks, along with other infrastructure, will meet regulatory requirements and environmental targets.

Advantages and Limitations

Detention tanks offer a range of functional and logistical benefits, though there are also constraints to consider.

Advantages:

  • Allow for attenuation without large surface footprint (especially underground systems) 
  • Reduce flood risk and CSO frequency 
  • Flexible in form and scalable for various site sizes 
  • Compatible with both new build and retrofit applications 
  • Can be automated or linked to smart drainage systems 

Limitations:

  • Require regular inspection and maintenance, particularly for sediment and debris removal 
  • Can become ineffective if not cleaned or if control devices are blocked 
  • Underground tanks are harder to inspect and more expensive to maintain 
  • Do not provide infiltration or water quality treatment unless designed as part of a hybrid system 
  • Risk of anaerobic conditions or odour if water is retained too long 

Despite these challenges, detention tanks remain a widely accepted and effective solution in modern stormwater management.

Maintenance and Asset Management

Detention tanks must be managed to ensure long-term functionality and compliance. Maintenance plans should consider:

  • Inspection frequency: Visual checks and CCTV surveys for structural integrity and sediment build-up 
  • Sediment removal: Scheduled clearance, especially in locations with high pollutant loads 
  • Control device maintenance: Ensuring flow regulators, penstocks, and valves operate correctly 
  • Access safety: Confined space entry protocols, gas detection, and rescue planning 
  • Monitoring equipment: Calibration and data management for telemetry systems 

The owner or adopting authority must also maintain accurate records of inspections, interventions, and performance to satisfy regulatory and legal responsibilities.

Integration with Sustainable Drainage Systems

Although traditionally associated with engineered infrastructure, detention tanks are increasingly being integrated into SuDS schemes, where they function as below-ground attenuation features supporting green surface elements.

In SuDS design, detention tanks may serve as:

  • Emergency storage beneath rain gardens or permeable paving 
  • Overflow tanks linked to swales or filter strips 
  • Final-stage attenuation before discharge to a watercourse 

Their use must be balanced against the broader goals of SuDS—namely, to mimic natural hydrology, enhance water quality, and support biodiversity. As such, tanks are often paired with above-ground features to meet both engineering and environmental objectives.

Conclusion

Detention tanks are essential tools in modern urban water management, offering a reliable means of controlling peak flows and reducing the risk of flooding and sewer overload. Whether constructed from concrete, plastic, steel, or modular geocellular systems, they provide flexible and scalable solutions for surface water and wastewater systems.

With growing pressure on drainage infrastructure due to climate change, urbanisation, and regulatory demands, detention tanks continue to play a pivotal role in delivering resilient, compliant, and sustainable drainage solutions. Their successful implementation requires careful design, regular maintenance, and integration within a broader strategy of catchment-based water management. For engineers, planners, and asset managers alike, detention tanks represent a vital component in the evolution of smarter, safer, and more sustainable drainage networks.