Fish screening

Waitaha/Canterbury's waterways are home to several taonga species of native fish, as well as recreationally and economically important trout and salmon species. Making sure they remain in waterways is crucial for them to survive and thrive.

That's why having a compliant fish screening facility that ensures fish are not removed from waterways into races and diversions is an important responsibility for those who take surface water from our waterways.

Screening fish effectively

There are approximately 900 surface water abstraction points in Waitaha. All but a few of the large hydroelectricity generators require an effective and compliant fish screening facility to be in place.

Fish screen requirements are outlined as conditions in the resource consent that authorises the abstraction of water. If you have such a resource consent, it is your obligation to comply with these conditions.

Our compliance programme

We have a compliance programme to check that fish are being effectively screened from surface water takes.

The programme prioritises the largest and most at-risk water takes, but all consent holders will be checked at some point.

If you cannot demonstrate compliance, you will be asked to replace or upgrade your fish screening facilities over the next few years.

Resource consent applications

If your resource consent is expiring and you apply to replace it and cannot demonstrate you have an effective and compliant fish screen in accordance with the Canterbury Land and Water Regional Plan, you will be asked to upgrade it with a compliant design as part of your consent application.

If you do not do this, you run the risk of having your consent application publicly notified because you will not be able to demonstrate you are mitigating the potential effects on fish.

Find out more about the resource consent process.

Designing a good fish screen

What makes a fish screen effective will depend on the waterway and the volume of the water take.

Waitaha has a huge variety of waterways which supply water for hydro-electrical generation, irrigation, stock water, and humans to drink and use.

These water takes vary from a few litres per second to tens of thousands of litres per second. They include everything from lakes, to springs, to stable flow spring fed streams, to hill-fed rivers and finally, to large braided rivers.

There are also a variety of fish that need to be protected, from species that like to hug the bottom and like dark spaces between rocks, such as kōwaro and tuna, to those that move up and down the open water column, like trout and salmon.

Some fish migrate between rivers and the sea, others stay close to home, and others migrate hundreds of kilometres from a headwater stream to the sea.

Design key factors

All of this means that designing and building a fish screen facility to suit different river types and sizes of water takes and ensuring all the different kinds of fish are protected, requires specialist skills and careful consideration when designing a fish screen. It is where biology meets engineering.

Key factors that need to be considered in designing a fish screen:

Intake location: Where in the water column and river the intake and screen should be located to ensure minimum exposure to the facility from the most at-risk species of fish likely to be encountered.
Through screen velocity: The velocity at which water flows through the screen apertures (generally no more than 0.12m/s).
Sweep velocity: The velocity at which water flows past the screen to ensure minimal exposure of fish to the screen (as high as possible but generally three times the through screen velocity).
Fish bypass: A fish bypass is only necessary when a diversion channel is in place. Ideally, there should be no diversion and the fish screen should be located in the natural waterbody. However, where one is essential it should be as short as possible to transport any fish that get washed into it away from the intake and screen and back into the source waterway quickly and safely.
Fish bypass connectivity: The connection to the waterbody should always have sufficient water and be as short and fast as possible to ensure any fish diverted down the bypass are delivered promptly and safely back to the source waterway without any delay.
Gap openings: The aperture size of the screening material is recommended to be 1.5mm wedgewire where there is tidal influence to safeguard whitebait, transitioning to 2-3mm wedgewire further inland where juvenile species are larger.
Operations and maintenance planning: It's important to plan how the fish screen will be operated and maintained to ensure it works properly – including how often it is inspected, cleaned of debris, and repaired when necessary.
Upstream fish passage: Only necessary when a diversion channel is in place. This is to ensure any upstream swimming fish that find their way into the bypass or diversion channel are enabled to move through and back into the main waterbody as safely and efficiently as possible.

Fish screen standards and guidelines

The minimum standard for an effective and compliant fish screen is outlined in Schedule 2 of the Canterbury Land and Water Regional Plan (CLWRP).

The plan aims to ensure fish are kept safe in their natural environment and gives guidance on what a good fish screen facility should look like. It encourages applicants to design wedge-wire mechanical screens that can be easily maintained.

It also encourages designs where key criteria, such as sweep velocity and through screen velocity, can be easily calculated at the design stage to ensure compliance will be achieved without the need for ongoing expensive testing after the facility is installed.

Experience has shown that so called ‘novel’ screens that employ rock barriers or acoustics are not effective at excluding fish and cannot comply.

The following accordions contain the exact detail of Schedule 2 of the CLWRP:

Read schedule 2.1 of the CLWRP

Where the diversion or take does not exceed a maximum rate of 10 L/s and a maximum volume of 100 m3 per day, a fish screen shall be installed to prevent fish from entering the intake. The fish screen shall be designed to the following standard and kept functional at all times that water is being taken:

(a) Water shall only be taken when a fish screen with a mesh size or slot width not exceeding 2 mm for intakes within 2 km of the coast, a coastal lake or estuary, or 3 mm for anywhere else, is operated and maintained across the full width of the intake to ensure that fish and fish fry are prevented from bypassing the screen into the intake; and
(b) The screen area shall be designed to ensure the calculated average through screen velocity does not exceed 0.12 m/s (screens should generally be designed to exceed this area to account for some routine level of clogging of the screen with detritus). The required area (m2) of fish screen should exceed = Flow (L/s)/120.
Example: The minimum required fish screen area for a cylindrical screen can therefore be calculated from:

Area = 2πr(r + h) x z
Where: π = 3.14159
r = radius of cylinder (m)
h = length or height of cylinder (m)
z = proportional open mesh area of screen material (i.e. 0.5 for mesh that is 50% open area)

Note: The above formula holds where the screen is fully immersed in water as is usually the case with pump takes. Where this is not the case, the area will need to be adjusted accordingly. Where 50% of the screen may be exposed, then the area calculation will need to be adjusted to half (or multiplied by 0.5), or the actual screen area would need to be doubled (multiplied by 2) in order to achieve the same area immersed. This example makes no allowance for the area taken up by the end of the intake pipe. Where high levels of detritus and other clogging materials are present, screen areas should be increased to account for reduced effective screen area.

Read schedule 2.2 of the CLWRP

Where the diversion or take does not exceed a maximum rate of 10 L/s and a maximum volume of 100 m3 per day but does not meet the standards in 1 above; or where the diversion or take exceeds a maximum rate of 10 L/s and a maximum volume of 100 m3 per day and the diversion is less than 10 m3/s or the take is less than 500 L/s pumped, a fish screen shall be installed to prevent fish from entering the intake. The fish screen shall be designed with the following features:

(a) The site is located as close to the river source as possible to minimise exposure of fish to the fish screen structure, and minimises the length of stream affected while providing the best possible conditions for (b) - (f) below;
(b) Water velocity through the screen (“approach velocity”) is slow enough (generally <0.12 m/s) to allow fish to escape entrainment (being sucked through or washed over the screen) or impingement (being squashed or rubbed against the screen);
(c) Water velocity across (or past) the screen (“sweep velocity”) is greater than the approach velocity (b) and is sufficient to sweep the fish past the intake;
(d) An effective bypass system is provided that is easily accessible to entrained fish, and fish are taken away from the intake and back into the source channel, or into water which provides the fish with unimpeded passage back into the source channel;
(e) Screening material (mesh, profile bars or other) on the screen needs to have a smooth surface and openings that prevent any damage to fish coming into contact with the screening material; and
(f) The intake structure and fish screen are operated to a consistent, appropriate standard with appropriate operation and maintenance procedures, and this operation and maintenance should be regularly checked or monitored. A record should be kept of all the maintenance and monitoring carried out.

Read schedule 2.3 of the CLWRP

Where the diversion is more than 10 m3 /s or the take is more than 500 L/s pumped, in addition to the features listed in 2 (a) to (f) above, it will be necessary for the intake to be purpose designed and to consider on a case by case basis whether any additional features will be necessary to ensure fish are prevented from entering the intake.

Read the CLWRP notes

Submerged galleries (abstracting water vertically) and galleries in river banks (abstracting water horizontally), or behavioural barriers and devices such as those that use light and sound diversions may not meet all of the engineering features set out in 2 above, but shall be considered to comply with them where it is demonstrated that they are able to exclude fish to the same degree of effectiveness.
In conjunction with a number of stakeholder groups, the CRC has developed good practice guidelines for fish screening in Canterbury. A copy of this guideline can be obtained from the CRC to help in ensuring fish screens are designed, installed and operated to include the features identified in 2 above.

Advice and guidance on fish screen design

There are a variety of experts and resources available that can help provide advice and guidance on fish screen design, installation, maintenance and regulatory obligations.