Flow control devices and choosing between them

A flow control device is the one component in a surface water drainage design that a lead local flood authority will always look at, because it is what actually limits the discharge rate the whole strategy promises. Choosing a flow control device therefore comes down to three questions in order: how much head is available, how small the restriction has to be, and who will maintain it. Get the first two wrong and the attenuation volume you calculated is fiction. Get the third wrong and the device silts up in year three and the site floods.

What a flow control has to achieve

The control exists to hold back runoff so that discharge from the site matches an agreed rate, usually the greenfield rate for the design event or a betterment on the existing rate for a brownfield site. Everything upstream, the attenuation volume, the pipe sizes and the freeboard, is sized against the discharge curve of that device.

So the control is a hydraulic component with a performance specification, not a fitting chosen at the end. The specification needs the design discharge, the head at which that discharge occurs, the discharge at lower heads, and the behaviour as the chamber fills.

The point that catches designers out is the shape of the head-discharge relationship. A simple opening passes much less than its design flow at low head, which means the storage fills faster than a single design-point calculation suggests. The attenuation volume has to be modelled across the whole event, not computed from a peak flow.

The national non-statutory technical standards for sustainable drainage set the framework for discharge rates and for the events to be considered, and the recognised design reference for the hydraulics is the industry SuDS manual. A Drainage Strategy should state the control type, the design discharge, the head and the chamber arrangement rather than noting that flows will be restricted.

Vortex units against orifice plates

The two common options behave differently and the difference matters most on flat sites.

A vortex flow control uses the geometry of the chamber to induce rotation, which throttles the flow. For the same design discharge it has a larger physical opening than a plate would need, and its head-discharge curve is flatter, so it passes closer to its design rate across a wider range of water levels. The larger opening is the practical advantage: less blockage risk and less dependence on screening.

An orifice plate is a plate with a hole, which is inexpensive, easily understood and easily verified on site. Its weakness is that achieving a low discharge needs a small hole, and a small hole blocks. Many lead local flood authorities set a minimum orifice diameter, commonly in the region of 75 mm, to limit that risk, and check the local standard rather than assuming.

A penstock or a throttle pipe sits alongside both. A long small-diameter pipe laid at a shallow gradient can provide the restriction through friction, which avoids a discrete opening altogether, and it suits some outfall arrangements where the available head is generous.

Blockage risk and screens

Blockage is the failure mode that matters, and it is managed by sizing, by screening and by access.

Screening is a trade-off rather than a solution. A fine screen protects a small opening and then becomes the blockage itself, usually with leaf litter after the first autumn. The conventional arrangement is a coarse screen with a bypass route, so that if the screen blocks, water passes over or around it rather than backing up uncontrolled.

What tends to work in practice: size the opening above the local minimum, put the control in a chamber that can be reached without entering confined space, provide a bypass weir set above the design water level, and keep the chamber clear of the first point where silt will drop out. Where the catchment includes a car park or a road, a silt trap or catchpit upstream pays for itself.

Note: the screen and the bypass have to appear on the drawing. A strategy that mentions blockage risk in text and shows a bare plate in the chamber detail will attract a comment.

Head available and its effect on storage

Available head is the vertical difference between the top water level in the attenuation and the outfall, and it is often the binding constraint on an infill or a flat site.

Low head has two consequences. It forces a larger opening for the same discharge, which can make the target rate unachievable with a simple plate. And it reduces the depth of storage you can use above the control, so the attenuation footprint grows.

Where head is tight, the options are to raise the storage, to lower the outfall where the receiving system allows, to accept a shallower, wider storage arrangement such as a geocellular tank or permeable paving sub-base, or to use a control type that performs better at low head. Raising finished levels is usually the least costly of those, and it needs resolving before the architectural levels are fixed rather than after.

On sites where the outfall is to a watercourse, the tailwater level during a flood event is part of the head calculation. A control that works in dry weather and drowns at the design event has not been assessed properly, which is one of the connections between the drainage design and the Flood Risk Assessment.

Pumped systems and when they are accepted

Pumping surface water is the last resort in the hierarchy, and most authorities treat it that way. The objections are power dependence, maintenance liability, long-term adoption and the consequence of failure during the event when the pump is most needed.

Where a pumped solution is proposed it needs justifying: evidence that gravity discharge is impossible, duty and standby pumps, storage volume sufficient to cover the design event with the pumps out of action, power supply arrangements including any standby provision, telemetry and alarm, and a named party responsible for maintenance for the life of the development.

Basements are the common case, and there the interaction with flood risk and sleeping accommodation is what the authority will focus on, as set out in basements, flood risk and sleeping accommodation.

Pumping foul and surface water in a shared system is generally resisted, and combining them is a separate problem again.

Access, maintenance and adoption

Whoever adopts the control decides what is acceptable, so establish the adoption route before specifying.

A water and sewerage undertaker adopting the system under a sewer adoption agreement will apply its own design and materials requirements, which may rule out some proprietary units or require particular chamber arrangements. Where the system stays private, a management company or a householder carries the maintenance, and the design has to be simple enough that it will actually be done.

The maintenance specification belongs in the strategy: inspection frequency, what is being looked for, how silt is removed, the access arrangement, and who holds the record. Where the outfall discharges to an ordinary watercourse, the consenting position is separate from planning, and land drainage consent for works to an ordinary watercourse covers that route.

Specification detail in the strategy

Reviewers ask for the same handful of things, so put them in the first submission.

1. The control type, the design discharge and the head at which it is achieved.
2. The full head-discharge curve or table, not a single point.
3. A chamber detail showing the control, the screen, the bypass and the access cover.
4. The invert levels of the inlet, the control and the outfall, with the assumed tailwater level.
5. Modelled results for the design events including the climate change allowance, showing storage used and discharge achieved.
6. The maintenance schedule and the party responsible.

Where a proprietary unit is proposed, name the performance requirement rather than relying on a product reference alone, so that an equivalent can be substituted at construction without a variation.

Drainage design is work councils expect from a suitably qualified engineer, and professional review before submission is strongly advised. PlanScope can draft the strategy narrative, the policy sections and the maintenance schedule from your project data, and the hydraulic modelling remains the engineer's. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.