Flow control devices: orifice plates and vortex units

The whole of a site's surface water strategy passes through a device the size of a dinner plate. A suds flow control device sets the discharge rate that every storage calculation depends on, and the choice between an orifice plate, a proprietary vortex unit and a penstock changes the storage volume, the blockage risk and whether anyone will adopt the system. Reviewers who check the device against the model, rather than taking the design flow on trust, find mistakes regularly.

What each device does and where it suits

An orifice plate is a fixed opening in a plate or wall. Discharge varies with the square root of the head above it, so flow rises steeply as water backs up. It is simple, has no moving parts and nothing proprietary about it, which is why adopting bodies like it.

A vortex flow control uses the geometry of the chamber to induce rotation, which limits discharge across a range of heads while keeping a much larger throat than an equivalent orifice. For the same design flow, the opening is bigger, so the blockage risk falls. The cost is a proprietary unit with a manufacturer-specific head-discharge relationship that has to be modelled from the certified curve.

A penstock is a sliding gate, hand-wound or actuated. It isolates rather than controls. Penstocks belong upstream of a control for maintenance access and pollution containment, and they are not a substitute for a flow control device, although drawings sometimes present them as one.

Throttle pipes, long runs of small-diameter pipe, and weirs also appear. Both work, and both are less controllable than the two main options.

The minimum practical opening

The small orifice problem is where most designs come unstuck. Restricting a site to a greenfield rate can imply an opening of 40 or 50 mm, and an opening that size blocks with a crisp packet.

Designers commonly treat around 100 mm as the minimum practical orifice diameter for a gravity system, with anything smaller avoided or accepted only with screening and a maintenance regime. Standards differ between authorities and adopting bodies, so confirm the figure that applies before fixing the design rather than after.

Where the required flow cannot be delivered through a 100 mm opening, the options are limited and should be presented explicitly:

1. Use a vortex device, which achieves the same discharge with a larger throat.
2. Accept a higher discharge rate and provide more storage, with the case made on the basis that a blocked small orifice is worse for flood risk than a slightly higher controlled rate.
3. Provide the control in stages, with an upstream device at a manageable size.
4. Use infiltration for part of the site so the controlled flow serves a smaller area.

Option two is often the right engineering answer and the hardest to sell, so support it with the blockage argument and a comparison of storage volumes.

Head-discharge behaviour and modelling

Model the device, not the intended flow. Entering a fixed 5 litres per second into a simulation, when the installed device will pass 3 litres per second at low head and 8 at high head, produces storage volumes that do not reflect what will be built.

For an orifice, use the orifice equation with a sensible discharge coefficient and state the coefficient. For a proprietary device, use the manufacturer's certified head-discharge curve for the specific unit reference, and include that curve in the appendix. Check the flow at the 1 in 1, 1 in 30 and 1 in 100 plus climate change events, and look at the behaviour at low head, where some vortex devices deliberately pass a higher flow before the vortex forms.

Then check the tailwater. A device with a downstream water level above the orifice is drowned, and its discharge is governed by the difference in head rather than by the upstream depth alone. On sites discharging to a watercourse with a high water level in the design event, this changes the answer materially, and our note on what a lead local flood authority checks in a drainage strategy sets out how that review is usually conducted.

Chamber design, access and screens

A control device that cannot be reached will not be maintained. The chamber needs safe access from a hard surface, with the device removable from ground level wherever possible, because anything requiring confined space entry will be inspected less often than the maintenance schedule claims.

Details that matter:

• A silt trap or sump upstream, sized so that grit settles before it reaches the control.
• A bypass screen rather than a full-flow screen, so that partial blockage does not stop all discharge.
• Step irons and a chamber large enough to work in, or a device on a lifting frame.
• An overflow or emergency weir with a defined route, so that blockage causes controlled flooding to a known area.
• A clear label in the chamber giving the device reference and the design flow.

That last item costs nothing and saves arguments years later, when nobody can remember which unit was installed.

Choosing a device for a very low discharge rate

Small sites with a greenfield rate below 2 litres per second are the hardest case, and the honest position is that precise control at that rate is not achievable with a simple orifice.

Many lead local flood authorities recognise this and accept a practical minimum, either by allowing a stated minimum discharge or by accepting a vortex unit with a larger throat. Some accept infiltration for part of the site instead. What does not work is a drawing showing a 32 mm orifice with a note that it will be maintained, and a Drainage Strategy that offers that will come back with a request for a redesign.

Raise the point at pre-application stage, because it affects the developable area through the storage volume, and the answer varies between authorities. The statutory position on sustainable drainage approval is covered in our note on Schedule 3 and where the law stands.

Verification after installation

The commonest defect found on a completed site is an installed opening larger than the designed one, which quietly raises the discharge rate and leaves the storage undersized in the event it was designed for.

Verification should include a photographic record of the installed device with its reference plate visible, a measurement of the orifice or throat, the manufacturer's delivery documentation, a check of the invert level against the drawing, and confirmation that the screen and silt trap were installed. Where the system is being handed to a management company or an adopting body, that record is the handover document.

Adoption implications of a proprietary unit

Adopting bodies are cautious about proprietary devices for reasons that have nothing to do with performance. Spare parts, long-term availability, the need for a specific manufacturer's replacement, and the risk of a unit being replaced with something different during a repair all sit behind that caution.

Ask early whether the adopting body accepts the device you intend to specify. Where it does not, the choice is between redesigning around an orifice with more storage, or keeping the system in private management, which brings its own funding questions as set out in our note on SuDS adoption and long-term maintenance. Either way, the decision belongs at concept stage, because it changes the storage volume and therefore the layout.

This is general guidance, not legal advice; drainage design and flow control selection should be carried out by a qualified engineer and confirmed with the lead local flood authority and the adopting body. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.