What is a Detention Basin

A detention basin is an engineered, vegetated depression designed to temporarily hold stormwater runoff during and immediately after precipitation events. Unlike retention basins or ponds, detention basins are typically dry under normal weather conditions. Their primary function is to attenuate the flow of surface water by providing temporary storage and controlled release, thereby reducing peak discharge rates and mitigating flood risk downstream.

As part of sustainable drainage systems (SuDS) and modern stormwater infrastructure, detention basins are widely implemented in both urban and rural developments to manage the quantity of runoff generated by impervious surfaces and changing land use. When designed correctly, they also offer co-benefits such as water quality improvement, habitat creation, and landscape integration.

Purpose and Function

Detention basins are installed to achieve several hydrological and environmental objectives. The central purpose is to delay the entry of runoff into downstream drainage networks, thereby:

  • Reducing peak flow rates during storms 
  • Protecting urban drainage systems from overload 
  • Minimising downstream flooding risks 
  • Allowing for sedimentation and pollutant settlement before discharge 
  • Promoting passive infiltration where feasible 

The principle of operation is straightforward: during a storm event, water is diverted into the basin, where it is temporarily stored. A flow control structure, such as an orifice or weir, regulates the rate at which water exits the basin, ensuring that downstream systems receive a moderated flow over an extended period rather than a sharp surge.

Key Design Features

Detention basins vary in form and complexity depending on site conditions, catchment size, design standards, and intended function. However, most share a common set of physical characteristics:

  • Dry base: The floor of the basin is designed to remain dry between rainfall events. 
  • Inlet and outlet structures: Designed to control flow entry and discharge, often including energy dissipation features. 
  • Side slopes: Typically graded between 1:3 and 1:5 for stability and ease of maintenance. 
  • Vegetation cover: Grass or native plantings help to stabilise soil, slow runoff velocity, and provide aesthetic and ecological benefits. 
  • Sediment forebay: A designated area near the inlet to trap sediment and debris, simplifying maintenance. 
  • Emergency spillway: A safety feature to manage extreme rainfall events that exceed the basin’s storage capacity. 

Some basins are also integrated with permanent water features or biodiversity zones, but these are more characteristic of hybrid designs such as wet detention ponds or retention wetlands.

Applications in Drainage and Flood Risk Management

Detention basins are a key component in both new developments and retrofit projects aimed at managing surface water sustainably. Their use is especially prevalent in:

  • Residential developments: To manage runoff from roofs, driveways, and streets. 
  • Industrial and commercial sites: Where large areas of impermeable surfacing generate significant runoff volumes. 
  • Highway and transportation projects: Managing water from road surfaces and embankments. 
  • Parks and green infrastructure: Dual-purpose basins providing both drainage and recreational/landscape value. 
  • Flood attenuation schemes: Reducing flow into rivers, culverts, or combined sewer networks during peak rainfall. 

By temporarily detaining water, these basins lower the burden on piped infrastructure and receiving water bodies, contributing to broader flood resilience strategies.

Hydrological Performance

The performance of a detention basin is determined by its ability to:

  • Capture and store runoff: The volume of water it can hold before discharge begins. 
  • Control the discharge rate: Ensuring compliance with greenfield runoff rates or local planning authority standards. 
  • Facilitate sedimentation: Allowing suspended solids and pollutants to settle before outflow. 
  • Handle design storm events: Typically sized for storms with return periods ranging from 1-in-10 to 1-in-100 years, depending on regulatory requirements. 

The hydrological behaviour of a detention basin is influenced by:

  • Catchment area and land use 
  • Soil type and infiltration rate (if infiltration is permitted) 
  • Rainfall intensity and duration 
  • Groundwater level 
  • Shape and depth of the basin 
  • Efficiency of flow control devices 

Computer modelling tools such as MicroDrainage, InfoDrainage, or SWMM are typically used during the design process to simulate stormwater flows and ensure the basin performs to specification under various rainfall scenarios.

Vegetation and Landscape Integration

Although detention basins are primarily functional drainage assets, they are increasingly designed with landscape integration and ecological value in mind. Vegetation selection, planting layout, and basin shape can all contribute to the basin’s success from both an engineering and aesthetic perspective.

Common planting schemes include:

  • Grassed basins: Easy to maintain, robust under variable water conditions. 
  • Native wildflower mixes: Enhance biodiversity and pollinator habitat. 
  • Reed beds or sedges: Occasionally used in low-flow zones or around the outlet to improve water quality. 

Landscaping helps prevent erosion, improve visual appeal, and increase community acceptance. In public spaces, detention basins may be designed for dual use—such as playing fields or parks—provided safety and drainage performance are not compromised.

Operation and Maintenance

For detention basins to function effectively over the long term, they require a structured and ongoing maintenance regime. This is critical for both hydraulic performance and environmental quality.

Typical maintenance activities include:

  • Periodic mowing of vegetation to maintain grass height and visibility 
  • Removal of sediment and debris from the forebay and outlet 
  • Inspection and cleaning of inlets, outlets, and control structures 
  • Repairing erosion or damage to basin banks or spillways 
  • Monitoring vegetation health and controlling invasive species 
  • Safety inspections, especially for basins in public areas 

The frequency and scope of maintenance should be outlined in a maintenance plan, often required by planning authorities or adopting organisations such as local councils or water companies. Maintenance responsibility may fall to local authorities, private developers, management companies, or adoption bodies, depending on ownership and legal agreements.

Regulatory and Planning Context

In the United Kingdom, detention basins form part of the recommended suite of SuDS measures encouraged by:

  • National Planning Policy Framework (NPPF) 
  • Non-statutory Technical Standards for Sustainable Drainage Systems 
  • Local planning authority drainage guidance 
  • Lead Local Flood Authority (LLFA) strategies 

Where surface water runoff from new development is required to be attenuated to greenfield rates, detention basins are often the preferred method. They are generally encouraged over underground tanks due to their visibility, maintainability, and potential for multifunctional land use.

If a basin is to be adopted by a water company under the Sewerage Sector Guidance (SSG), it must meet specific construction, access, and performance criteria.

Advantages and Limitations

Detention basins offer a wide range of benefits in the context of stormwater management, but they are not without limitations.

Advantages:

  • Simple and cost-effective to design and build 
  • Visually unobtrusive and can blend into the landscape 
  • Provide flood risk reduction and water quality benefits 
  • Minimal mechanical components reduce operational risks 
  • Contribute to biodiversity and public amenity when well designed 

Limitations:

  • Require relatively large land areas compared to underground solutions 
  • Generally ineffective in removing dissolved pollutants without additional treatment 
  • May be less suitable in areas with high groundwater tables or impermeable soils (if infiltration is desired) 
  • Risk of sediment build-up if not maintained 
  • Safety considerations required in public-facing locations 

Despite these limitations, detention basins remain a cornerstone of sustainable water management and are widely supported in planning and design guidance.

Detention Basins vs Retention Basins

A common point of confusion lies in distinguishing between detention and retention basins:

  • Detention basins: Dry outside storm events, temporarily store water, and discharge it slowly to the downstream system. No permanent pool of water. 
  • Retention basins: Always contain a standing pool of water and provide both storage and treatment of stormwater. Often designed as ornamental ponds or wetlands. 

Both systems may include similar components, such as sediment forebays and controlled outlets, but serve different hydrological and ecological roles.

Conclusion

Detention basins are a vital tool in modern drainage design, offering a passive, robust, and environmentally responsive method for managing surface water runoff. Their ability to temporarily store and gradually release stormwater makes them highly effective in reducing flood risk, protecting infrastructure, and supporting sustainable development.

When thoughtfully designed and maintained, detention basins can provide far more than just hydraulic functionality. They can enhance the visual landscape, support biodiversity, and contribute to climate resilience in both urban and rural environments. For planners, engineers, developers, and regulators, detention basins represent a practical and multifunctional approach to managing the challenges of water in the built environment.