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Fish Friendly Outfalls

Fish Friendly Flapvalves-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


As flapped outfalls often pose a barrier to fish migration, particularly at the point of entry from the sea, ACE offer a range of Fish Friendly Flapvalves to provide a window of opportunity for fish and eels to migrate naturally without compromising flood defence.



ACE Fish Friendly Flapvalves offer the same benefits as the standard flapvalve range, and integrate float regulated portals to allow a small opening for fish to use as the tide turns, which then close before a critical water level is reached.



Depending on application, ACE offer three main types:

  • ACE Fish friendly “Pet” flap- main river, low wave action
  • ACE Reset Fish friendly flap- tidal applications
  • ACE side hung doors– various applications
Pet Fish Friendly flapvalve
Pet flaps on new flaps or retrofitted to existing doors

Fish friendly “Pet” Flapvalve

The ACE “Pet” flap fish friendly flapvalve is a standard flapvalve, with a smaller flapvalve built in (nicknamed a pet flap) to allow fish migration. The smaller flapvalve is bottom-hung, and fitted with a custom-made float arm which 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 flapvalve
Reset flaps for tidal applications-easily adjustable

Reset Fish Friendly Flapvalve

The ACE Reset fish friendly flapvalve is also a standard flapvalve, with a smaller flap built into it’s main door. The Reset fish friendly flapvalve 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 Flapvalve offers a high level of adjustment, however does require the low tide to clear the bottom of the float to allow it to function.

How it works- click here to see a short video

Side hung Fish Friendly flapvalve
Side hung doors- no obstruction to fish

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.

Flap Valve Render

ACE KRK Flapvalves

Our flapvalves 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 flapvalve product pages.


Installation and Maintenance

KRK Flapvalves 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 flapvalves 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 flapvalves.


Using self-lubricating hinges, KRK 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 flapvalve projects, please see our case studies.

ACE supply a variety of other KRK flapvalves, please click here. 


KRK Petflap IOM (Example)


01777 249080

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