What is a Reservoir storage

Reservoir storage refers to the temporary retention of water within a drainage system during periods of elevated flow, typically associated with storm events. This stored volume may occupy dedicated tanks, oversized pipes, natural depressions, open channels, or surface storage areas, depending on the system design. The primary objective of reservoir storage is to reduce the risk of flooding and to moderate the rate at which water is conveyed downstream.

As flow rates and water levels increase during heavy rainfall, part of the runoff or combined sewage is detained within the system instead of being discharged immediately. Once inflow rates reduce — typically after the storm has passed — the stored volume is gradually released or drained. This process helps to protect both upstream infrastructure and downstream receptors such as treatment plants, watercourses, and urban areas.

The Role of Reservoir Storage in Urban Drainage

Modern urban drainage systems are designed not only to collect and convey water but also to manage peak flows and protect sensitive areas from flooding and pollution. Reservoir storage plays a vital role in this strategy by:

  • Attenuating peak flow rates: Holding back water during high-intensity rainfall prevents surcharging in pipes and overflows at outfalls.

  • Delaying discharge: Temporarily storing water allows downstream systems to recover capacity before release.

  • Preventing sewer overflows: In combined systems, storage can delay or eliminate discharges to receiving waters during storm events.

  • Protecting treatment works: By smoothing inflow profiles, reservoir storage reduces the risk of shock loading at wastewater treatment plants.

This function is particularly important in developed areas with high levels of impermeable surfaces, where natural infiltration and percolation are limited.

Types of Reservoir Storage Systems

Reservoir storage may take various physical forms depending on the site conditions, system type, and available space. Common implementations include:

  1. Underground storage pipes: Large-diameter pipes laid horizontally within the sewer network to provide temporary volume. These are often installed as online storage — integrated within the existing pipe path — or as offline storage connected via a diversion structure.

  2. Attenuation tanks: Purpose-built underground structures designed solely for flow detention. These may be constructed from concrete, plastic crates (geocellular units), or modular chambers.

  3. Stormwater ponds and wetlands: Open surface features that provide both hydraulic storage and water quality treatment benefits.

  4. Basins and swales: Engineered surface depressions designed to fill during rainfall and drain slowly.

  5. Surface car parks and green roofs: Dual-purpose infrastructure where surface water is stored temporarily before gradual release.

  6. Open channels or artificial culverts: Channels that can hold water during peak flows and drain over time.

The selection of a storage type depends on available space, maintenance requirements, land use constraints, and environmental considerations.

How Reservoir Storage Works

The principle behind reservoir storage is relatively simple. As rainfall occurs, runoff enters the drainage system. When the volume exceeds the system’s immediate conveyance capacity, the excess water is detained in the designated storage facility. This volume is held temporarily and released in a controlled manner once downstream capacity is available.

The process typically involves:

  • Inlet control: A flow regulator or weir directs excess water into the storage zone.

  • Storage phase: Water fills the available volume during the storm event. The system must be sized to accommodate design rainfall intensities and durations.

  • Outlet control: Once the inflow reduces, a flow control device such as an orifice or vortex flow controller gradually releases the stored volume back into the system.

In some systems, pumps are also used to control the timing and rate of release. The entire process must be carefully modelled to ensure the desired level of service and flood protection.

Hydraulic Considerations and Design Parameters

The design of reservoir storage elements requires a detailed understanding of site-specific hydrology and system hydraulics. Key factors include:

  • Design storm event: Engineers must define the rainfall event for which the storage will be effective — typically a 1-in-30 or 1-in-100-year storm.

  • Runoff volume: Calculated based on surface area, permeability, rainfall intensity and duration.

  • Inflow rate: Determines how quickly the storage will fill.

  • Available storage volume: Must be sufficient to hold excess flow without causing flooding.

  • Discharge rate: Must not exceed downstream capacity or cause overloading of connected systems.

  • Time to empty: Indicates how quickly the system can reset for subsequent rainfall events.

Storage systems are often modelled using software tools such as InfoWorks ICM or MicroDrainage to simulate system performance under various scenarios.

Applications of Reservoir Storage

Reservoir storage systems are employed in a wide range of drainage and flood management contexts. Examples include:

  • Combined sewer systems: To reduce overflow frequency during storms and limit pollution of receiving waters.

  • Separate stormwater networks: To limit discharge rates to watercourses or public sewers.

  • Sustainable Drainage Systems (SuDS): Where above- or below-ground storage is integrated into site design to mimic natural drainage.

  • Urban regeneration schemes: In densely built environments, underground storage tanks are used to mitigate surface runoff from redeveloped plots.

  • New housing developments: Where planning conditions require flow rates not to exceed greenfield runoff rates.

Storage can be implemented at different scales, from individual buildings to regional infrastructure projects.

Storage Efficiency and Behaviour

Reservoir storage is dynamic — meaning that it is both filled and emptied as part of its normal operation. The efficiency of the system depends on:

  • Timely outflow: To ensure the system does not remain full before the next storm.

  • Properly sized outlet controls: Oversized orifices release water too quickly, while undersized ones may delay emptying.

  • Maintenance: Siltation or blockages can reduce capacity and restrict discharge.

Engineers may use hydrographs to compare inflow and outflow over time and ensure the system achieves the intended attenuation. Performance monitoring is also used on larger schemes to validate modelling assumptions and regulatory compliance.

Benefits of Reservoir Storage

The strategic use of reservoir storage in urban drainage delivers multiple benefits:

  • Flood prevention: Reduces the risk of surface water flooding by intercepting and delaying runoff.

  • Network protection: Prevents surcharging of sewers and manholes, improving service levels.

  • Pollution reduction: In combined systems, lowers the frequency of CSO discharges by detaining flows.

  • Supports SuDS compliance: Helps developments meet planning and environmental requirements.

  • Reduces downstream erosion: By smoothing peak discharges to natural watercourses.

  • Adaptability to climate change: Offers resilience in coping with more intense rainfall events.

These benefits make reservoir storage a key component in sustainable urban water management.

Maintenance and Operational Considerations

Although reservoir storage is typically passive in operation, it requires regular inspection and maintenance to ensure continued effectiveness. Maintenance tasks may include:

  • Silt removal: Especially in basins and tanks where sediment can accumulate and reduce volume.

  • Inspection of flow control devices: To confirm correct operation and absence of blockages.

  • Vegetation management: In surface features such as ponds or swales to maintain hydraulic function.

  • Structural checks: For underground tanks and pipe storage systems, including access covers and joints.

  • Telemetry systems: Where installed, sensors and alarms should be tested to ensure data accuracy.

Without proper maintenance, storage systems may lose capacity, become ineffective, or pose environmental risks.

Limitations and Design Challenges

Despite their advantages, reservoir storage systems are not without limitations:

  • Space requirements: Surface-based storage features may require large land areas, which can be difficult in urban locations.

  • Cost: Below-ground tanks and large-diameter pipes involve significant capital investment.

  • Complexity: Systems with multiple inlets, controls and monitoring may require specialist design and operational skills.

  • Overflow risk: If not properly sized, storage can be exceeded in extreme rainfall, leading to flooding.

  • Maintenance burden: Especially in poorly accessible or neglected systems.

Successful storage schemes must balance performance, cost, space, and long-term serviceability.

Conclusion

Reservoir storage is a foundational principle in modern drainage design, providing temporary volume to manage excess flow during rainfall events. Whether through underground pipes, tanks, surface ponds, or engineered basins, storage systems help prevent flooding, protect infrastructure, and maintain compliance with environmental regulations. By attenuating peak flows and enabling gradual discharge, they play a vital role in creating resilient and sustainable drainage networks. With growing urbanisation and climate pressures, effective storage design and maintenance will remain critical to future-proofing urban water systems.