Fish-friendly culvert design


Northland's freshwater fish species all have different swimming and climbing abilities. The following two fish-friendly culvert designs will suit most of these fish and are simple to build.

The designs use pipe culverts, which are generally suitable for small streams. You will need to be sure that your culvert is large enough for flood flows as well as providing fish passage.

More complex culvert designs may be needed for specific circumstances – depending on location, the species involved and their swimming abilities. For more information, contact our consents team on 0800 002 004.

The ‘low-slope' culvert

‘Low-slope' culverts are suitable if your stream is nearly level. They are suitable for all fish species.

With ‘low-slope' culvert design:

· The culvert is as wide or wider than the stream bed.

· The culvert aligns with the natural stream bed channel.

· Water depth in the culvert is the same as in the natural stream bed.

· Stream bed material is allowed to settle throughout the culvert's length.

· If erosion is likely, the stream banks and bed around the inlet and/or outlet should be protected.

'Low-slope' culvert design diagram.

The ‘natural stream bed' culvert

‘Natural stream bed' culverts are suitable where the stream has a low to moderate slope.

Flow conditions inside them are similar to the natural stream, both upstream and downstream of the culverts.

With natural stream bed culvert design:

Stones, rocks or artificial material are arranged and stuck to the culvert floor to simulate the stream bed.

· The culvert is wider than the natural stream bed channel. You will need to be careful choosing the width of your culvert and the height of any fixed material in the culvert because flood flows must be able to pass through.

· The culvert aligns with the natural stream bed channel.

· The slope is the same as the natural stream bed slope.

· You will need to consider protecting the stream banks and bed around the inlet and outlet as erosion is more likely than with the ‘low slope' culvert.

You will need to check your natural stream bed culverts to make sure the simulated stream bed and the rest of the culvert are performing properly.

'Natural stream bed' culvert design diagram.  'Natural stream bed' culvert design diagram showing culvert width.