What is a Control Structure

A control structure is a hydraulic device or engineered feature that regulates, restricts, or redirects the rate of water flow within a drainage or sewerage system. These structures are critical elements in the design and operation of both natural and artificial water management systems, including stormwater networks, foul water systems, Sustainable Drainage Systems (SuDS), and flood protection infrastructure.

By modulating flow rates, control structures ensure that downstream systems are not overloaded, that treatment plants operate within their design parameters, and that water is retained, diverted, or released in a manner that aligns with environmental and operational objectives. Their correct specification, installation, and maintenance are vital for ensuring hydraulic stability, protecting infrastructure, and achieving regulatory compliance.

Purpose and Function

The principal role of a control structure is to manage flow. Whether it involves slowing down runoff, diverting water to storage, or ensuring consistent discharge to a receiving body, the key objective is to control hydraulic behaviour.

Control structures perform various functions, including:

  • Flow regulation: Restricting peak flow to prevent downstream flooding or surcharging. 
  • Storage control: Managing the rate at which water is released from attenuation tanks, ponds, or reservoirs. 
  • Level control: Maintaining upstream water levels for ecological or operational reasons. 
  • Diversion: Redirecting water to alternative flow paths during high-intensity storms or maintenance. 
  • Backflow prevention: Blocking reverse flows that may enter a system due to surcharging or tidal effects. 
  • Energy dissipation: Reducing flow velocity to prevent erosion or scouring at outfalls and transitions. 

Control structures are usually passive and rely on hydraulic principles such as gravity, head pressure, and orifice flow. However, in some applications, particularly in treatment works and complex flood control systems, active control systems using sensors, actuators, and programmable logic are employed to provide dynamic flow management.

Common Types of Control Structures

There is a wide variety of control structures, each designed for specific hydraulic conditions and performance requirements. The selection depends on factors such as flow characteristics, site constraints, maintenance accessibility, cost, and long-term durability.

1. Orifice Plates

Orifice plates are flat metal or plastic discs with a precisely sized circular opening. Installed at the outlet of a chamber or pipe, they restrict flow by forcing water through a small orifice, creating a controlled pressure drop. They are particularly effective where a consistent upstream head can be maintained.

2. Vortex Flow Controls

Vortex flow control devices, such as hydro-brakes, use centrifugal force to restrict flow without relying on moving parts. As flow increases, a vortex forms within the unit, creating back pressure and restricting discharge. These devices are widely used in SuDS and stormwater attenuation systems due to their self-cleansing characteristics and ability to handle varying flow rates efficiently.

3. Weirs

Weirs are simple barriers placed across the width of a channel or chamber. Water flows over or through them once it reaches a specified height. Weirs can be:

  • Rectangular 
  • Triangular (V-notch) 
  • Compound or stepped 

The shape and configuration determine how flow is modulated at different levels.

4. Penstocks and Gates

Penstocks are sliding gates that open or close to control flow manually or automatically. Often used in treatment plants or flood control applications, they provide high levels of control and are useful in isolating sections of a system.

5. Flap Valves

Flap valves are non-return devices that open under forward flow and close under backflow pressure. They are commonly installed at outfalls or tidal interfaces to prevent water ingress while allowing free discharge.

6. Float-Controlled Valves

Used in tanks and reservoirs, these devices adjust flow based on the level of water. The float mechanism rises or falls with water level, adjusting the valve position to regulate inflow or outflow.

7. Baffle Chambers

Baffles or deflection plates are used to change the direction or velocity of flow. In stormwater systems, they can reduce turbulence or direct sediment to settlement zones for removal.

Applications in Drainage and Sewerage

Control structures are used throughout drainage infrastructure, from localised attenuation systems in housing developments to regional-scale flood defences. Some of the key applications include:

  • Sustainable Drainage Systems (SuDS): Flow controls are central to SuDS, where they limit runoff from attenuation features like tanks, swales, ponds, and permeable surfaces to greenfield or permissible discharge rates. 
  • Combined Sewer Systems: In combined networks, flow control structures help prevent surcharging and reduce the frequency of combined sewer overflows (CSOs) by throttling discharge into treatment works or storage. 
  • Flood Alleviation Schemes: Large-scale detention basins and flood reservoirs are equipped with control structures to manage how much water is released downstream during and after storm events. 
  • Wastewater Treatment Works (WwTWs): Control valves and gates regulate the flow through different treatment stages, ensuring optimal retention times and hydraulic loading rates. 
  • Highway Drainage: Flow controls in gullies and interceptors reduce peak discharge to receiving watercourses, minimising erosion and pollution transport. 
  • Agricultural Drainage: In irrigation and drainage schemes, simple weirs and gates control flow through ditches, culverts, and retention ponds. 

Design Considerations

The effective design of control structures requires a detailed understanding of hydrology, hydraulics, and site-specific conditions. Key design criteria include:

  • Design flow rate: The target maximum flow the structure should allow. 
  • Headwater level: The upstream water level during peak flow conditions. 
  • Downstream constraints: Limits imposed by pipe capacity, receiving watercourse, or treatment works. 
  • Sediment and debris handling: Susceptibility to clogging and ease of maintenance. 
  • Structural integrity: Ability to withstand hydraulic forces, weathering, and vandalism. 
  • Accessibility: For inspection, adjustment, and cleaning. 
  • Compliance: Adherence to local authority or sewerage undertaker standards and guidelines, such as those in the Sewerage Sector Guidance or CIRIA SuDS Manual. 

Control structures are often tested using computational hydraulic models to simulate their performance under various rainfall events and flow scenarios.

Operation and Maintenance

Although many control structures are passive and require minimal mechanical intervention, regular inspection and maintenance are essential to ensure reliable performance. Typical maintenance tasks include:

  • Clearing debris and sediment 
  • Inspecting for corrosion or damage 
  • Checking for blockages or misalignment 
  • Calibrating and adjusting flow control parameters (for active devices) 
  • Verifying correct operation after storms or high-flow events 

Failure of a control structure—whether due to blockage, mechanical failure, or design flaw—can result in severe consequences, including upstream flooding, environmental pollution, structural damage, and non-compliance with discharge regulations.

Regulatory and Planning Context

In the United Kingdom, flow control structures are regulated within the planning and sewer adoption framework. Planning applications that involve drainage must typically demonstrate how flow is controlled using:

  • Greenfield runoff rates: For new developments, flow to the public sewer or watercourse must not exceed pre-development levels. 
  • Discharge consent limits: For direct discharges to the environment, the Environment Agency or local council sets flow and quality parameters. 
  • Sewerage adoption requirements: Water and Sewerage Companies (WaSCs) require approved flow control devices in adopted SuDS or attenuation features, following Sewerage Sector Guidance (SSG). 

Designers are also encouraged to follow industry standards such as:

  • BS EN 752 (Drain and sewer systems outside buildings) 
  • CIRIA C753 (The SuDS Manual) 
  • Sewers for Adoption (now replaced by Design and Construction Guidance under SSG) 

Conclusion

Control structures are fundamental to the safe, efficient, and environmentally responsible operation of drainage and sewerage systems. By regulating the rate and direction of water flow, they enable designers and operators to mitigate flood risk, protect infrastructure, and comply with legal and environmental standards.

From simple orifice plates in a suburban soakaway to complex vortex controls in urban attenuation schemes, the principles remain the same: to manage flow reliably, predictably, and sustainably. As urban development intensifies and climate variability increases, the intelligent deployment of control structures will be essential to resilient water infrastructure in both the public and private realms.