Vortex Flow Control Selection Helper
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Vortex Flow Control Selection Helper
Decide whether a vortex flow control (Hydrobrake-type) beats a plain orifice for your consented discharge, see the head–discharge benefit, and get the parameters to specify to any supplier. Vendor-agnostic, based on The SuDS Manual (CIRIA C753, Chapter 28).
Is a vortex control warranted?
Enter the consented discharge and the design head. The tool sizes the equivalent plain orifice and advises whether a vortex control is the better choice (mainly for blockage resistance).
Head–discharge: vortex vs orifice
How the release changes as the storage fills. A plain orifice keeps rising with √head; a vortex control holds the flow much closer to the design rate — releasing less at high head, which can reduce the storage you need.
What to specify & how to select
A vortex control is chosen from the manufacturer's certified head–discharge curve — not a generic equation. Give the supplier these.
| Parameter | What it is |
|---|---|
| Design flow, QD | consented discharge to release |
| Design head, HD | head above the control at top of storage |
| Min. flow / kick-off | flow at low head, if critical |
| Chamber & pipe sizes | to suit the control and access |
| Verified curve | independently tested head–discharge data |
Why a vortex. For the same design flow it offers a much larger clear opening than an equivalent orifice (less blockage), and its head–discharge curve stays nearer the design rate as head rises – so it releases less in big events, often reducing the required attenuation storage. It needs no power or moving parts.
When. Use a vortex where the consented flow is so low that a plain orifice would be too small to resist blockage (commonly below ~50 mm), or where reducing storage volume is valuable. Below very low flows even a vortex has limits – combining sub-catchments to a single larger control may be better.
Selection. Pick the unit from the manufacturer's certified curve for your QD at HD; many LLFAs require an independently verified head–discharge curve. Provide a bypass/penstock and access for maintenance (C753 Chapter 28).
This vortex flow control helper decides whether a vortex device (the Hydrobrake type) is a better choice than a plain orifice for limiting your site's discharge to its consented rate. Enter the design discharge and head and it sizes the equivalent orifice, shows the head–discharge benefit, and lists the parameters to specify to any supplier. It is vendor-agnostic and based on The SuDS Manual (CIRIA C753). Surface water runoff has to be restricted to an agreed rate under the national standards for sustainable drainage systems.
What a vortex flow control does
A vortex flow control releases water through a chamber that makes the flow spin, which holds the discharge close to the design rate even as the head rises. A plain orifice, by contrast, lets the flow keep increasing as the storage fills. The practical benefit is a larger opening for the same restricted flow, so a vortex control is far less likely to block than the small orifice that would otherwise be needed.
Vortex or orifice — which should you use?
For very low consented discharges, the orifice needed to hit the rate can be so small that it clogs; this is where a vortex control earns its place. For higher flows, or where head is limited, a plain orifice or penstock may be simpler and cheaper. The helper sizes the equivalent orifice for your flow and head so you can see whether it falls below the practical blockage threshold and a vortex device is worth specifying.
What to specify to a supplier
Because vortex controls are proprietary, you specify the duty, not the product: the design (consented) discharge in litres per second, the design head at top of storage, the minimum acceptable outlet diameter for blockage resistance, and the upstream storage arrangement. The helper sets these out so you can request comparable quotes from any manufacturer.
Further reading: the 2025 national standards for SuDS · source control: SuDS at plot level · gov.uk SuDS technical standards.
Frequently asked questions
Hydrobrake is one well-known brand of vortex flow control; there are others. The device and the calculation are the same in principle, which is why this helper is written to be vendor-neutral.
Mainly when the consented discharge is low. The orifice that would hit that rate becomes too small and blockage-prone, so the larger opening of a vortex control is the safer choice.
Whatever the lead local flood authority consents — often the greenfield runoff rate or a betterment on the existing rate. The control is then sized to hold the site to that figure.
One of our experienced Flood Risk Consultants will get back to you within 60 minutes