What is a Storm tanks / storm drains

Storm tanks and storm drains are critical components of urban drainage systems, specifically designed to manage large volumes of stormwater during periods of heavy rainfall. While both structures serve the purpose of handling excess water, they operate at different stages within the drainage network.

  • Storm drains (also known as surface water drains or gullies) are the first point of entry for rainwater runoff from roads, roofs, and paved areas. They collect water and direct it into the underground drainage system.

  • Storm tanks are large-volume storage facilities located within sewers or treatment works. They temporarily hold stormwater when the flow exceeds the capacity of the network or treatment facility.

Together, storm drains and storm tanks form an integrated system that protects urban environments from flooding, sewer overload and water pollution.

The Role of Storm Drains

Storm drains are surface-level features designed to collect and convey rainwater. They are typically found at the side of roads, in car parks, and around buildings. Water enters the drain through grates or inlets and is channelled via a network of pipes.

Functions of storm drains include:

  • Capturing runoff quickly: Preventing surface water accumulation on streets and public areas

  • Conveying water safely: Directing runoff into the underground drainage system or watercourses

  • Protecting infrastructure: Reducing hydrostatic pressure on foundations and pavements

  • Supporting public safety: Minimising standing water that could lead to slips, traffic hazards or damage

Storm drains are usually connected to separate surface water systems or to combined sewer systems, depending on the age and design of the area.

The Role of Storm Tanks

Storm tanks are large-volume containers or chambers that store excess stormwater temporarily. They are typically used in:

  • Combined sewer systems: To hold back flows that exceed sewer capacity during storms

  • Wastewater treatment works: To prevent untreated overflows during high inflow periods

  • Industrial facilities: To manage runoff before treatment or controlled release

By storing water during rainfall events and releasing it gradually afterwards, storm tanks:

  • Reduce the risk of sewer flooding

  • Prevent combined sewer overflows (CSOs) into rivers and streams

  • Protect treatment facilities from hydraulic shock

  • Allow retained water to be treated when capacity becomes available

Storm tanks are typically constructed from reinforced concrete, steel, or modular plastic units, and may be installed underground or above ground, depending on site constraints.

How Storm Drains and Storm Tanks Work Together

The drainage process begins when stormwater enters the system through storm drains. From there:

  1. Water is conveyed into sewers or surface water networks.

  2. During heavy rainfall, flow increases rapidly.

  3. If the sewer’s capacity is reached, water is diverted into storm tanks.

  4. Once the storm subsides, water in storm tanks is gradually returned to the sewer or treatment facility.

  5. In some cases, controlled overflow to the environment occurs via consented outfalls.

This staged system ensures that peak flows are buffered and that the downstream infrastructure is not overwhelmed.

Types of Storm Tanks

Storm tanks can take several forms, depending on their location and function:

  • Offline tanks: Divert excess flow into a side chamber when thresholds are exceeded

  • Online tanks: Located directly in the flow path; always carry some flow

  • Passive tanks: Fill and empty by gravity

  • Active tanks: Include pumps, valves and telemetry to manage flows automatically

Features often include level sensors, flushing systems to prevent sediment build-up, and odour control mechanisms.

Materials and Construction

Both storm drains and storm tanks must be built to withstand variable flow conditions, heavy loads, and exposure to contaminants.

Common materials include:

  • Precast concrete: Durable and widely used for chambers, manholes and tank walls

  • HDPE and polypropylene: Used in modular underground tanks

  • Cast iron or galvanised steel: Used for gully grates and stormwater inlets

Installation typically requires civil engineering works, especially for larger storm tanks, and must comply with Building Regulations and local water authority specifications.

Design Considerations

When designing storm drains and storm tanks, engineers must consider:

  • Catchment area: How much rainfall will be collected and over what surface area

  • Design rainfall event: Often based on 1-in-30 or 1-in-100 year storms

  • Inflow and outflow control: Use of weirs, orifices and valves to regulate flow

  • Water quality: Inclusion of oil separators or silt traps to improve effluent quality

  • Maintenance access: Ensuring inspection and desilting are feasible

In urban areas, space constraints may require the use of compact or modular tank systems, while in rural settings, larger open tanks may be feasible.

Environmental and Regulatory Considerations

Storm drains and tanks play a vital role in environmental protection by reducing direct discharges of polluted runoff into rivers and streams. However, if not properly maintained or if capacity is exceeded, they can become sources of contamination.

In the UK, regulations governing stormwater management include:

  • The Water Industry Act 1991

  • Environment Agency permits for storm overflows

  • Planning requirements under the National Planning Policy Framework

  • SuDS guidance in the CIRIA SuDS Manual (C753)

Water companies are required to monitor and report spill events from storm tanks and CSOs, with increasing emphasis on transparency and public reporting.

Maintenance Requirements

Both storm drains and storm tanks require ongoing maintenance to function effectively:

Storm drains:

  • Regular removal of leaves, litter and sediment from inlets and gullies

  • Jetting and CCTV inspection of pipework

  • Clearing of blockages to prevent ponding or localised flooding

Storm tanks:

  • Desilting to maintain storage volume

  • Inspection of pumps, sensors and flushing systems

  • Checking for structural defects and access safety

Neglecting maintenance can lead to flooding, pollution incidents and regulatory penalties.

Challenges and Future Developments

As urbanisation and climate change increase the frequency and intensity of storm events, challenges for drainage systems include:

  • Aging infrastructure: Many systems were not designed for current storm intensities

  • Limited space: Especially in dense urban developments

  • Pollution risk: From first flush runoff and misconnected foul sewers

  • High capital costs: Particularly for large underground tanks

To address these challenges, modern trends include:

  • Use of smart sensors and automated flow control

  • Integration of green infrastructure and SuDS upstream

  • Retrofitting storm tanks in existing networks

  • Publicly accessible dashboards showing overflow activity

Conclusion

Storm tanks and storm drains are essential elements of modern urban water infrastructure. Storm drains collect rainwater at the surface and direct it into underground networks. Storm tanks provide temporary storage when those networks become overwhelmed. Together, they prevent flooding, reduce pollution, and protect both people and the environment.

Effective design, operation and maintenance are key to ensuring these systems can cope with increasingly severe weather patterns and expanding urban development. As part of integrated drainage strategies, they offer a robust and proven solution to the challenge of managing stormwater in the 21st century.