Choosing a flow control for a very low discharge rate

When the permitted discharge rate on a small site works out at two or three litres per second, the difficulty is not the calculation. It is that a flow control device for a low discharge rate of that order needs an aperture narrow enough to block on a crisp packet. Most lead local flood authorities recognise this and accept a practical minimum, often five litres per second, but you have to ask rather than assume, and you have to show what you tried first.

The device choice then comes down to three variables: the flow you must not exceed, the head you have available above the outlet, and how the thing will be cleaned when it silts up.

Why very low rates are hard to control

A flow control works by restricting the cross-sectional area through which water can pass at a given head. The relationship is not linear. Halving the flow does not halve the aperture, it reduces the area by half and the diameter by about 30 per cent, so small reductions in permitted flow produce apertures that shrink fast.

Run the orifice equation and the problem is obvious. The illustrative figures below use the standard form Q equals Cd multiplied by the area multiplied by the square root of twice gravity times head, with a discharge coefficient of 0.6 and one metre of head, which is a common enough arrangement on a small attenuation tank.

Those numbers are illustrative arithmetic showing the method rather than a design. A 31 millimetre hole in a drainage system will block, and when it blocks the tank surcharges and floods the thing it was built to protect.

So the honest position on a very small site is often that the greenfield calculation gives a rate below what can be reliably controlled, and the strategy has to say so and propose the alternative.

Orifice plates and the blockage limit

An orifice plate is a flat plate with a drilled or cut opening, fixed across the outlet of a chamber. It is inexpensive, has no moving parts, is easy for an adopting body to understand, and its performance can be checked on site with a tape measure.

The limit is blockage. Practice across most drainage authorities sets a minimum aperture in the region of 75 millimetres for a plate serving a piped system, and some go further. Below that, the plate needs either upstream screening it will not get, or a maintenance regime nobody will follow for the next 60 years.

There are two ways to keep a plate and still hit a low rate. Reduce the head over the plate by raising the outlet, which increases the aperture needed for the same flow. Or use a two-stage arrangement: a larger plate controlling the higher flows and a separate low-flow route sized to pass the base flow, with the combined performance curve submitted.

Both need checking against the head available, which is usually the reason neither works on a flat site.

Vortex devices and the head discharge curve

A vortex flow control induces a rotating flow inside a shaped chamber so that the outlet aperture can be considerably larger than an equivalent orifice for the same design flow. That is the whole point of the device on a low-rate site: a proprietary vortex control passing five litres per second may have an outlet several times the diameter of the plate it replaces.

The trade-off is the shape of the head-discharge curve. A vortex control passes proportionally more flow at low heads, then flattens as the vortex forms, which means the design flow is achieved at or near the design head and not below it. The curve is not something you can derive with a hand calculation, so the manufacturer's certified curve for the specific unit has to be in the appendices, with the design head marked on it.

Three practical points follow. The unit is specific to the flow and head combination, so a later change to tank depth invalidates it. The chamber has to be large enough for the unit and for someone to reach it. And the flatter curve interacts with storage volume, which is the next problem.

The head available and what it rules out

Head is the difference between the design water level in the storage and the water level at the outfall. On a sloping site with a deep tank you have plenty. On a flat site discharging to a shallow highway drain, you may have 400 millimetres, and that changes everything.

Vortex controls generally need a design head in the region of a metre to develop their characteristic performance, and units designed for low heads exist but pass more flow for a given outlet size. With very little head the realistic options narrow to an orifice plate with a large aperture and a correspondingly low flow, a throttle pipe, or a pumped discharge.

A throttle pipe is a length of small-diameter pipe laid at a controlled gradient, using friction rather than a single restriction to limit flow. It avoids a pinch point, and it needs length and a rodding regime, which is why it appears more often on industrial sites than on housing.

Pumped discharge should be the last option. It introduces a failure mode, needs a power supply, needs an emergency storage volume for pump failure, and adopting bodies are reluctant. Where the alternative is no development, state the case explicitly rather than presenting a pump as a neutral choice.

On a very small site, the better answer is often to avoid the discharge altogether. Infiltration, where the ground permits it, removes the flow control question, and our note on drainage evidence for a single new dwelling sets out the testing that supports it.

Silt, screening and access for cleaning

Every low-rate control needs a silt management strategy, and the instinct to fit a screen in front of the aperture is the wrong one. Screens block more readily than orifices, and a blocked screen produces the same surcharge with less warning.

What works instead is a catchpit or silt trap immediately upstream with a sump deep enough to hold a realistic annual accumulation, a chamber at the control large enough for a person to enter safely with step irons and a removable cover, a penstock or isolation valve so the control can be worked on without draining the system, and an overflow or bypass weir set above the design water level so that a blocked control causes a controlled overflow rather than an uncontrolled one.

Then write the maintenance requirement down in the management plan with an inspection frequency and a named responsible party. Adopting authorities read that section. Management companies rarely do, which is the argument for a device that fails safe.

What a low rate does to storage volume

Lower discharge means more storage, and the increase is not gentle. Storage has to hold the difference between inflow and outflow over the critical duration, so cutting the outflow raises both the volume and the critical storm duration, pushing the design towards longer, less intense events.

Two further effects catch people out. Half-drain-down time gets longer, and some authorities set a limit on it, which can rule out a design that otherwise works. And the flatter head-discharge curve of a vortex control means the outflow is lower through the early part of the event than an orifice would give, so substituting a vortex for a plate late in the design process increases the volume needed rather than leaving it unchanged.

Run the model with the actual device curve rather than a fixed outflow. A design sized on a constant two litres per second and built with a vortex control will be short of volume.

Also check whether the authority separates long-term storage from attenuation storage, because on a low-rate site the two can overlap and be double counted.

What the drainage strategy should state

A Drainage Strategy that a lead local flood authority can approve without a round of questions says all of this on one page.

1. The permitted discharge rate, its derivation, and confirmation of any minimum rate the authority allows.
2. The device type and, for a proprietary unit, the model reference with its certified head-discharge curve appended.
3. The design flow, the design head, and the invert levels of the control and the outfall.
4. The aperture or outlet size, with the blockage risk addressed rather than ignored.
5. Silt trap, chamber size, access arrangements, isolation and the bypass or overflow route.
6. Storage volumes for each modelled return period, including the current Environment Agency climate change allowance applied, with the allowance version named.
7. Ownership, adoption route and the maintenance schedule with frequencies.

Where the rainwater strategy also claims volume, keep the accounting separate. Our note on rainwater harvesting and whether it counts as attenuation explains why authorities discount it. And where the site sits in Flood Zone 2 or 3, the control design also has to be consistent with the Flood Risk Assessment, including the behaviour of the outfall when the receiving watercourse is in flood.

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