What is a Storage

In the context of drainage, flood control and water resources, storage refers to the deliberate containment or impounding of water, either temporarily or over the long term. It may take the form of surface storage, such as ponds, basins, and reservoirs, or subsurface storage, such as underground tanks, gravel layers or aquifers. The primary objective of storage is to manage the quantity and timing of water flow — particularly surface water runoff — and to mitigate flooding, support groundwater recharge, improve water quality, and ensure water availability during dry periods.

Storage is a key principle in both traditional drainage infrastructure and modern Sustainable Drainage Systems (SuDS). In engineered systems, storage allows for the collection of stormwater or wastewater during peak flows and its gradual release during low-flow periods, reducing pressure on treatment works, sewer networks and natural watercourses.

Types of Storage

Storage in water management can be categorised by its location, duration and function. The two broad types are:

1. Surface Water Storage

This involves above-ground features that capture and hold runoff. Common examples include:

  • Detention basins: Dry ponds that temporarily store stormwater during rainfall events

  • Retention ponds: Permanent bodies of water that also store and treat runoff

  • Balancing ponds: Engineered basins that equalise flow variations before discharge

  • Reservoirs: Large-scale impoundments used for water supply and flood control

2. Subsurface Storage

This refers to underground systems designed to capture water either temporarily or for long-term use:

  • Tanks and chambers: Modular or precast concrete structures installed below ground

  • Gravel beds and infiltration layers: Promote percolation into the soil while storing water

  • Aquifers: Natural underground reservoirs replenished by infiltration-based systems

Each storage type plays a distinct role in controlling peak flows, managing runoff and supporting long-term sustainability of drainage systems.

Functions and Benefits of Storage

Storage systems provide several essential functions in both urban and rural environments. Their benefits extend across hydrological, environmental, infrastructural and public safety dimensions.

Key Functions

  • Flow attenuation: Reduces the rate of water discharge to downstream systems, lowering flood risk

  • Volume control: Limits the total volume of runoff leaving a site

  • Water quality improvement: Allows sediments and pollutants to settle out before water is released

  • Groundwater recharge: Supports the replenishment of aquifers via infiltration-based storage

  • Supply management: In the case of reservoirs or harvesting systems, provides water for later use

Broader Benefits

  • Infrastructure protection: Reduces stress on sewers, culverts and treatment plants

  • Regulatory compliance: Helps meet drainage design standards and environmental targets

  • Ecological support: Some storage features create habitats and support biodiversity

  • Public amenity: Well-designed ponds and basins can double as green or recreational spaces

Storage is therefore not only a technical component but also a strategic asset in integrated water management.

Storage in Sustainable Drainage Systems (SuDS)

In SuDS design, storage is one of the four core components, alongside prevention, infiltration and treatment. The purpose of storage within SuDS is to mimic natural processes by controlling the volume and rate of runoff close to where rainfall occurs.

Storage features in SuDS often combine multiple benefits:

  • Detain water on site: Prevent immediate runoff to receiving water bodies

  • Enable evaporation and infiltration: Return water to the natural cycle

  • Provide treatment opportunities: Remove sediment and associated pollutants

  • Enhance landscape: Incorporate visually appealing and biodiverse areas

Examples of storage features within SuDS include swales with check dams, rain gardens with storage layers, extended detention wetlands and underground attenuation crates.

Design Considerations for Storage Systems

Designing an effective storage system requires careful analysis of site conditions, rainfall patterns and runoff characteristics. Key factors include:

  • Design storm event: Typically based on a 1 in 30 or 1 in 100 year return period, depending on flood risk

  • Storage volume required: Calculated from runoff coefficients, rainfall intensity and site area

  • Inflow and outflow controls: Structures such as weirs or orifices are used to regulate discharge

  • Freeboard allowances: Ensure the system can accommodate extreme events or blockages

  • Access for maintenance: Essential for long-term performance and safety

Hydraulic modelling is often employed to simulate system performance under various rainfall scenarios. Where infiltration is part of the strategy, soil infiltration rates and groundwater levels must also be assessed.

Examples of Storage Applications

Storage systems can be found across a wide range of sectors and site types. Examples include:

1. Urban Developments

New housing estates, business parks and retail developments incorporate surface or underground storage to meet planning conditions and manage their runoff responsibly.

2. Transport Infrastructure

Roads and railways often include linear storage features such as swales, soakaways and attenuation tanks to manage runoff from impervious surfaces.

3. Agricultural Areas

Field-edge storage ponds help manage runoff and reduce sediment and nutrient loss into nearby watercourses.

4. Industrial and Commercial Sites

Where runoff may contain pollutants, storage systems can be combined with oil separators or filters to improve water quality before discharge.

Storage solutions must be tailored to the specific site, risk profile and receiving environment to function effectively.

Maintenance and Operation

Storage systems, particularly those that also provide water treatment or infiltration, require regular maintenance to retain their effectiveness. Typical maintenance activities include:

  • Sediment removal: Particularly in forebays or at inlets

  • Vegetation management: Ensuring planted areas remain healthy and do not obstruct flow

  • Inspection of control structures: Checking for blockages, damage or wear

  • Debris clearance: Removing litter or obstructions that may affect inflow or outflow

For underground systems, periodic inspections via manholes or access points are essential. Asset owners should maintain records of inspections and any remedial work undertaken.

Storage and Climate Change Adaptation

As rainfall patterns become more variable and extreme events more frequent due to climate change, the importance of storage systems is growing. Climate-resilient drainage designs increasingly include:

  • Additional storage volume: To accommodate more intense storms

  • Flexible outflow control: Allowing adjustments as conditions change

  • Hybrid solutions: Combining storage with infiltration, evaporation and reuse

Adaptive management of existing systems may also involve retrofitting additional storage or upgrading outflow structures to cope with future conditions.

Regulatory Context and Guidance

In the UK, storage is governed by several planning and technical documents, including:

  • National Planning Policy Framework (NPPF): Encourages developments to include SuDS, including storage where appropriate

  • CIRIA SuDS Manual (C753): The definitive technical guide on SuDS design, including sizing and configuring storage

  • Building Regulations Part H: Requires drainage designs to consider capacity and prevent overload of public sewers

  • Lead Local Flood Authority (LLFA) standards: May provide local requirements for runoff rates and storage volumes

Meeting these standards is essential for obtaining planning approval and ensuring long-term environmental compliance.

Challenges and Limitations

While storage is a proven and widely used drainage solution, it is not without challenges:

  • Space limitations: Surface storage requires land, which can be scarce in urban areas

  • Construction costs: Excavation, lining and structural work can be expensive

  • Pollution risk: Poorly maintained systems may become a source of water quality issues

  • Public safety: Deep basins may present drowning or health hazards without proper design

  • Ground conditions: Sites with low permeability or high water tables may limit infiltration-based storage

Mitigation strategies, including multi-use spaces, modular tank systems, or shared infrastructure, can help overcome some of these issues.

Conclusion

Storage is a cornerstone of effective water management, whether used to mitigate flood risk, improve water quality, support ecosystems or ensure the sustainability of urban development. By temporarily or permanently holding water in controlled environments, storage systems help balance the extremes of the hydrological cycle and provide resilience in the face of growing environmental pressures.

Whether integrated into a SuDS scheme or deployed as a standalone infrastructure component, well-designed storage plays a critical role in protecting property, people and the environment. As water management becomes increasingly important in planning and development, storage will remain a key strategy for delivering safe, functional and sustainable drainage systems.