Fish passage

Culverts

The use of culverts is common in streams across the Auckland region. Unfortunately many of these structures have been badly designed or poorly installed, and they make up a high proportion of the man-made fish barriers in our streams.

Issues

The most common fish passage problems associated with culverts are:

Vertical drops – culverts that convey high flows can often produce turbulence, resulting in scouring of the streambed at the outflow. Over time, this scouring can result in the culvert becoming ‘perched’, where water falls vertically from the outlet to the stream below. This situation produces a barrier that is impassable to even the most tenacious climbing species.

 

 

Large concrete aprons – these structures are often constructed at the outflow of culverts and result in a wide, thin sheet of water during low flows. This water is often too shallow for larger fish, creating a barrier to their upstream migration.

 

 

 

High water velocities – undersized or high gradient culverts can result in the water velocities within the culvert barrel being too high for fish to swim against. While high flows can be expected during winter floods, culverts should be designed to operate at manageable velocities during the main fish migratory season from September to February.

  

Options

New culverts should be designed with fish passage considerations in mind, as well as the hydrological and physical characteristics of the site. Guidelines for general culvert design principles can be found here:

Four basic designs that can be selected based upon the fish species that are present are outlined in the document below, along with checklists that can be used to make sure that the conditions for fish passage are met.

Existing culverts require retrofitting to allow fish passage, and there are a range of options to accomplish this.

• Ramp fishways – these consist of boulder/cobble or artificial substrate ramps that cover either the whole width or a section of the barrier. They allow fish to swim over obstacles by reducing water velocities, creating resting areas and decreasing the slope.
  
  Ramp fishways (386.6 KB PDF)
  
  
• Spat ropes – these ropes have been shown to be effective for climbing fish species to wriggle up and can be cheaply installed at perched culverts to provide fish passage.
  
  Spat ropes (223.8 KB PDF)
  
  
• Culvert baffling – attaching baffles or artificial substrates to the floor of culverts are effective methods to reduce water velocities and provide resting areas for migrating fish.
  
  Culvert baffling (180.2 KB PDF)

Note: the pdf documents on this page are excerpts from ARC Technical Report 2009/084: “Fish Passage in the Auckland Region – a synthesis of current research".