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Pet Flap Valves – Fish Friendly

Fish Friendly Flap valves-Flood defence without compromising fish migration


  • Natural fish movement, with reliable flood defence
  • Quick and Easy to install
  • Lightweight yet robust
  • Low head losses
  • Virtually maintenance-free

Flapped outfalls often pose a barrier to fish migration, particularly at the point of entry from the sea. ACE offers a range of Fish Friendly Flap valves to provide a window of opportunity for fish and eels to migrate naturally. However, without compromising flood defence.

ACE Fish Friendly Flap valves offer the same benefits as the standard flap valve range. An integrated float regulated portal allows a small opening for fish to use as the tide turns. Which then close before a critical water level is reached.

Depending on the application, ACE offer three main types:

Fish friendly “Pet” Flap valve

The ACE “Pet” flap fish-friendly flap valve is a standard flap valve, with a smaller flap valve built-in (nicknamed a pet flap) to allow fish migration. A smaller flap valve is bottom-hung, and fitted with a custom-made float arm that lifts and closes the pet flap as the tide advances. This type is ideal for main river outfalls and has the benefit of an adjustable delay in closure to allow optimisation.

Reset Fish Friendly Flap valve

The ACE Reset fish-friendly flap valve is also a standard flap valve, with a smaller flap built into its main door. The Reset fish-friendly flap valve however is top-hung and incorporates a float to hold it open until desired. As the level reaches the desired trigger level, the float fills with water and ensures positive closure.

As the tide recedes, the float “resets” (hence the name) as the water empties through a Wastop ® non return valve, refilling the float with air for the next high tide. The Reset Fish Friendly Flap valve offers a high level of adjustment, however does require the low tide to clear the bottom of the float to allow it to function.

Side Hung Fish Friendly Doors

Particularly for sea outfalls, mitre doors have traditionally been used as a simple yet effective method of preventing backflow. The advantage of a side hung or mitre door is that once open, it presents no obstruction to the flow, with very little headloss, which in turn is also great for fish migration. The door is then simply pushed closed when the tide advances.

ACE Side hung fish-friendly doors are manufactured in HDPE and Stainless steel 316, so have all the benefits of traditional designs, but with the added bonus that the design vastly reduces the maintenance required.

Our flap valves are available in a wide range of models, chosen to suit our customer’s individual needs, so if you don’t require the flap to be fish-friendly, please see our flap valve product pages.

Installation and Maintenance

Flap valves are either bolted to a wall using resin anchors, or pipe mounted using a number of quick and easy methods depending on applications, all of which do not usually require grouting- typically flap valves take a matter of hours to fit, with minimal setting-dry and no curing time. The benefits to cost and time-saving are clear when compared with more traditional flap valves.

Using self-lubricating hinges, Flap valves only require occasional visual inspection to check for obstruction, and no greasing is required. The EPDM seal also allows for some wear, offering reliable protection throughout the lifetime of the flap valve.

For more information on our previous fish-friendly flap valve projects, please see our case studies.

ACE supply a variety of other flap valves, please click here. 



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