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Intake Screens

Our waterways are home to many fish species both indigenous and introduced.  When abstracting water from a stream, river or lake, to protect these species it is important that the aquatic habitat is taken into consideration.

 

Photo of a NZ native trout

 

Rule BW R5 on page 18 in Chapter 8 of the Bay of Plenty Regional Water and Land Plan outlines the permitted activity conditions for surface water intake structures in streams (including modified watercourses), rivers and lakes.   These conditions include where the intake can be located, when works on the structure can be undertaken and the intake screen requirements. If you can meet these requirements you do not need a consent for the intake structure itself. You may still need a consent to take water – we will cover this off in another post.

This post is about the requirements for the intake screens.

Fish, especially juveniles, can be very small and are vulnerable to being caught in the screen or sucked into the intake. The screen needs to be designed to prevent this occurring.

One aspect is the screen material, usually mesh, having openings small enough to exclude fish. The RNRP permitted activity rules state that the intake structure will be screened with a mesh aperture size:

  • Not exceeding three (3) millimetres by 30 millimetres in the tidal areas of rivers and streams.
  • Not exceeding five (5) millimetres by 30 millimetres or five (5) mm diameter holes in any other area that is not in the tidal area of a river or stream.

Another factor is the water velocity across the screen. It needs to be slow enough so that fish don’t get entrained (sucked through or washed over the screen) or impinged (being squashed against the screen). Condition (f) of permitted activity Rule 41 states that the intake velocity through the screen shall not exceed 0.3 metres per second. The intake velocity through the screen is calculated using the screen surface dimensions and percentage of open area, and flow rate so is a unique value for each intake. Your intake structure installer can calculate this for you.

Intake screens need regular maintenance to make sure that they are working effectively, efficiently and continuing to exclude fish. Parts can wear out, sediment and debris can accumulate, floods can cause damage and change in flows can affect the performance of the intake screen.

By making sure that your surface water intake is fitted with an appropriate screen that meets that the RNRP permitted activity rules or the conditions of your consent, and is maintained regularly, helps protect the fish species of Aotearoa New Zealand.

 

Further information can be found here:

https://www.irrigationnz.co.nz/KnowledgeResources/FishScreens

https://niwa.co.nz/freshwater/management-tools/fish-passage-assessment-tool

 

Sue Clark 

Environmental Consultant