Fish Friendly Flap Valves
A conventional flap valve does one job well: it shuts. That is exactly the problem for eels and fish, which need to move upstream through the same outfall. Aquatic Control Engineering designs, manufactures and installs fish friendly flap valves and side hung doors that keep full flood protection while giving aquatic life a route through — using eel dampers, reset flaps and low-opening-head construction in stainless steel 316 and HDPE. Standard sizes run from DN100 to DN1800, with square, rectangular and fully bespoke units built to suit your headwall, pipe or culvert.
Passage Without Losing Protection
Eel dampers and float-operated reset flaps let elvers and fish move upstream on a rising tide or flow, then close positively before water levels become a flood risk. You meet your fish and eel passage obligations without weakening the defence.
Stainless Steel & HDPE, Not Cast Iron
Our AQUIKO flap valves are built in stainless steel 316 and HDPE, replacing heavy cast iron designs. Corrosion free in saline and aggressive water, far lighter to install, and they open on a much smaller differential head — which is what makes fish passage possible.
Bespoke To Your Structure
Wall, flange or pipe mounted; round, square or rectangular; single hung flap valve or side hung door sized to your culvert. Because we design and manufacture in the UK, an awkward existing headwall is a drawing change rather than a problem.
About Fish Friendly Flap Valves
A fish friendly flap valve works on the difference between the two water levels either side of it. Light HDPE and PET construction, precise hinge geometry and a resilient seal mean the door lifts on a very small head, so it stays open for longer and passes fish under normal flows. An eel damper — a small, separately hinged aperture in the main flap — opens for elvers when the main door is closed. On tidal outfalls a float-operated reset flap holds a portal open as the tide rises, then closes it at a set trigger level and resets itself as the tide falls.
Our Fish Friendly Flap Valve & Door Options
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Fish Friendly Flap Valves With Eel Dampers
Stainless steel 316 and HDPE construction with a PET flap, integral reset system and eel dampers, supplied complete with all fixings and seals. Rated for 6 m water column short term and 1.5 m constant. Built to your diameter, or to width and height for rectangular openings. The standard choice where an outfall must pass eels and fish year round.
02
Reset Fish Pass Flap Valves
Developed for tidal outfalls. A portal in the HDPE flap valve carries a reset flap and float chamber: as the tide rises the float lifts the portal and fish pass through, and once the chamber fills the portal closes before flood levels are reached. A WaStop non-return valve drains the float as the tide recedes, resetting the valve for the next cycle — no power, no site visit.
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Side Hung Doors For Culverts
Where the opening is a culvert rather than a pipe, a side hung door is the better answer. Hinged at the side instead of the top, it needs only a very small differential to swing open, holds open through low-flow periods for fish and eel movement, and closes positively under reverse head. Wall mounted, stainless steel 316 and HDPE, made to your culvert width and height, rated 6 m water column short term and 1.5 m constant.
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Complete Outfall Packages
Alongside the standard AQUIKO flap valve range (DN100–DN1800, wall, flange or pipe mounted, plus square units), we supply and install eel passes, bristle boards and tiles, WaStop inline check valves, penstocks and headwalls — surveyed, designed, manufactured and fitted by our own teams.
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Technical Specifications (Typical)
- Product ranges — wall mounted (ARF), flange mounted (ARF-F), pipe mounted (ARF-P), square (ARF-S), fish friendly with eel dampers (ARF-ED), pumped (AFP) and side hung doors (SHD)
- Standard sizes — DN100 to DN1800; square, rectangular and fully bespoke openings to order
- Side hung doors — manufactured to culvert width and height, wall mounted
- Materials — stainless steel 316 and HDPE, with PET flaps on fish friendly units
- Design head — 6 m water column short term, 1.5 m constant (standard flap valves rated 6 m water column for up to four days)
- Mounting — wall, flange or pipe mounted; single hung or side hung
- Fish passage features — eel dampers, float-operated reset flap and portal, WaStop reset valve on tidal units
- Options — stroke limiters, vent pipes, pumped configurations, lifting and maintenance provisions
- Supplied with — all fixings and seals, and a 12 month warranty
- Compliance — supports Environment Agency and Eels (England and Wales) Regulations 2009 fish and eel passage requirements
Frequently Asked Questions
Three things. First, it opens on a very small differential head, so it spends far more of the tidal or flow cycle open and passable. Second, it carries an eel damper — a small separately hinged aperture that elvers can use even when the main door is shut. Third, on tidal sites, a float-operated reset flap holds a portal open as the tide rises and closes it at a set level. A conventional heavy cast iron flap does none of this: it seals and stays sealed.
In most cases, yes. The Eels (England and Wales) Regulations 2009 and Environment Agency requirements place a duty on asset owners to allow eel and fish passage past obstructions, and passage provision is now routinely a condition of consent on new and refurbished outfalls. We are happy to talk through what is likely to be expected on your structure and to support the approval process with drawings and design information.
A side hung door is hinged at the side of the opening rather than the top, so it swings rather than lifts. That matters on wide culverts and box openings, where a top-hung flap would be heavy, need a large head to lift and offer only a brief window for fish. A side hung door opens on a very small differential, stays open through low-flow periods and closes positively when the head reverses. We build them to your culvert width and height in stainless steel 316 and HDPE.
Usually, yes — and that is how most of ours are supplied. We survey the existing headwall, pipe or culvert, then design the unit and its mounting arrangement around what is already there, whether that is wall, flange or pipe mounting. Because the valves are stainless steel and HDPE rather than cast iron, they are light enough to install with minimal plant and rarely need new civils. Our own installation teams carry out the works.
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More About Fish Friendly Flap Valves
- Regulatory compliance. Eel dampers, reset flaps and side hung doors help satisfy Environment Agency and eel passage requirements on new outfalls and refurbishments alike.
- Corrosion-free construction. Stainless steel 316 and HDPE throughout, with PET flaps on fish friendly units — no seizing, no scaling and no repainting in saline or aggressive water.
- Low opening head. Light, precisely hinged doors lift on a fraction of the head a cast iron flap needs, cutting upstream levels, reducing debris trapping and extending the passage window.
- Retrofit friendly. Wall, flange and pipe mounted options, plus bespoke frames, mean most units go straight onto the existing structure with little or no new civil work.
- UK design, manufacture and installation. Designed and made in the UK under our own AQUIKO brand, installed and maintained by our own teams, with spares and support for the life of the asset.
We were the first in the UK to supply stainless steel and HDPE non-return valves in place of the outdated cast iron versions, and the same construction is what makes fish passage achievable at an outfall. For the full standard range, sizes and mounting options, see our flap valves page.
