Stages of the Scheme


Stage 1

Stage 1 has been operational since September 2015.

Overview

Stage 1 of the Central Plains Water Scheme included 23,000 hectares of irrigable area and included construction of a 17-kilometre-long canal delivering water from the Rakaia River into a piped distribution network.  The canal is 30 metres wide and five metres deep.   It is lined with 50 hectares of plastic (HDPE) liner as well as 80 hectares of geotextile underlay.  The 130-kilometre-long distribution network has four lateral pipes from the canal that provide pressurised irrigation water to each farm.

CPWL has been supplying water to Stage 1 since September 2015. The Scheme is successfully delivering reliable alpine-fed river water to our farmers, while enabling a reduction in groundwater abstraction volumes.  For example, in the 2015 -2017 seasons, Stage 1 enabled existing groundwater irrigators to switch off 75% of their groundwater abstraction volume resulting in 80 million cubic metres of water, that would previously have been abstracted, to remain in the ground.

Headrace Canal.
The Headrace Canal adjacent to the Rakaia River looking upstream towards the CPWL Intake and the Rakaia Gorge.

Intake Structures

The river intake is located approximately 8km downstream of the Rakaia Gorge Bridge. It intercepts the stable river braid in this location.  Before entering the main headrace, the intake water is conveyed into flow control structures, sediment retention ponds, and passes through a fish screen.

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An aerial view of the Rakaia River intake structures.

Headrace Canal

Once past the intake structures water is conveyed into a level headrace, and then commences an 8km traverse ‘up’ the northern bank of the river terrace.  The headrace crests the terrace at the 235m contour line around Earlys Pond, near Steeles Road. The terrace headrace was constructed with an engineered fill embankment ‘bench’ rising to around 15m high before water enters a bench cutting into the terrace face for the remainder of its length, and eventually rising onto the upper terrace.

Around two million cubic metres of earth were moved to form the bench and headrace. Once on the upper terrace the headrace travels north for approximately 10km through to Leaches Road, following the most practical route through farmland.  The water level in this section is approximately at ground level. Throughout the length of the headrace, a liner was used to control seepage from the canal.

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The completed Headrace Canal with Pipeline Offtake 5 supplying Stage 2 in the foreground.

Headrace Construction

Contractors took less than 18 months to build the 17km-long headrace canal and 130km-long pipe distribution network.

For the headrace and canal, 2 million cubic metres of earth was moved, including 377,000m³ of topsoil, the equivalent of removing the topsoil from 125 hectares.   550,000m³ of HDPE Liner was also installed.

13 bridges were built, each spanning approximately 25m each. Ten of these were on-farm bridges and three were on public roads.

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Earthworks contractors constructing the Stage1 Headrace Canal adjacent to the Rakaia River.

Reticulation System

A piped reticulation system distributes water to the Stage 1 area, providing water to the farm gate at a pressure equivalent to a head of 40 metres.  Several pump stations have been installed where necessary to boost line pressures.  The reticulation system comprises a network of approximately 100km of pipeline, ranging in diameter from 1600mm to 100mm. The pipeline was trenched on farms and along road easements in order to deliver up to 12m³/s of water through the pipes.

Various control and measuring devices are installed along the pipe route and the entire scheme is monitored and controlled by a central computerized control system.

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A completed Stage 1 Pump Station.

Reticulation Construction

The pipe distribution network consists of several pump stations and four main pipelines extending from the headrace canal down the Canterbury Plains to supply farms requiring irrigation.

The four main pipelines were constructed between May 2014 and August 2015.

With 130km of pipe to lay, contractors worked simultaneously on all main pipelines, and up to six days a week.

The pipes are buried at a depth of no less than 900mm to prevent pipe floatation and to avoid potential impact from agricultural activities.

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HDPE pipeline being installed during Stage 1 construction.

Stage 2

Stage 2 is the most recently constructed Stage and has been operational since October 2019.

Stage 2 Construction

Stage 2 of the scheme covers the area between the Selwyn and Waimakariri Rivers. Construction was carried out by Downer New Zealand Limited and this Stage services 20,000ha, supplying pressurised water to the farm gate.

The key difference between Stages 1 and 2 was the use of a 23km-long large diameter (2.5m) Glass Reinforced Plastic (GRP) pipe on Stage 2 instead of continuing the canal. As a result of this change, all infrastructure on Stage 2 is buried.

This pipe is the largest used in irrigation in New Zealand (to April 2020) and it connects to the Stage 1 Headrace Canal. The GRP was manufactured in Adelaide, Australia and arrived in Canterbury in four shipments.

Around 220 construction and design staff worked on the project.

The Downer team lay around 200km of pipe between May 2017 and May 2018, working with over 150 farming families along the way.

Total project costs for Stage 2 were approximately $200 million.

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Installation of the 23 kilometre long Stage 2 GRP Pipeline.

Sheffield Water Scheme

The construction of the Sheffield Irrigation Scheme enables the supply of irrigation and stockwater to approximately thirty farms in the Springfield/Sheffield area.

Water was first delivered by CPWL for the 2017/18 irrigation season.

Due to the location of Sheffield/Springfield, CPWL developed an alternative strategy to provide water to this area with the goal of avoiding significant pumping costs.

Sheffield

This Scheme is physically discrete from the remainder of the CPWL Stage 1 and Stage 2 infrastructure. As a result, it was developed and is operated in isolation of the main scheme.  The Scheme takes water from both the Waimakariri and Kowai Rivers to irrigate an area of approximately 4,424 hectares.

To access this water, an intake has been constructed at the top of the CPWL Scheme and a 2.5Mw pump station transfers up to 2m3/s of water from the Waimakariri River into a 2.08Mm3 Storage Pond on the corner of Cox’s Road and SH73.

In addition, gravity-fed water is supplied utilising the existing Selwyn District Council (SDC) stockwater network from the Kowai River.  Depending on the level of demand and the Storage Pond levels, water can be taken solely from one water source or from both.

From the Storage Pond, water is reticulated via an HDPE pipe network down the Canterbury Plains in a south-east direction delivering 4mm/day of water at a target minimum pressure of 3.5 bar to individual farms.

CPWL have been working with SDC to increase the year round efficiency and reliability of the Malvern Stockwater Scheme for all customers.  Using CPWL infrastructure to supply stockwater, rather than the current open channel system, enables the closure, with full landowner consent, of a number of small lateral water races fed by the Kowai River.  These closures free up water for irrigation use. It is estimated that between 70-95% would otherwise be lost through ground infiltration and evaporation. No additional water, over and above what is already consented from the Kowai River, can be taken from the river.

In addition, opportunities have been identified to provide raw water to the two rural water treatment plants in the area.  This would improve the reliability of the existing system and ensure there is capacity to keep up with the requirements of a growing urban area.

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An aerial view of the Sheffield Storage Pond immediately after first filling.

Stages of the Scheme


Stage 1

Stage 1 extends from the Rakaia river to the Hororata river and services 23,000 Hectares of farmland.

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Stage 2

Stage 2 commenced operating in October 2018 and services 20,000 hectares between the Hororata and Waimakiriri rivers.

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Sheffield Water Scheme

The Sheffield Water Scheme commenced operation in November 2017 and currently services 4,100 hectares hectares West of Darfield and East of Springfield.

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