CPWL Scheme Overview

The original CPWL philosophy of developing the whole scheme with all shareholders having equal rights, is still the intention. However, given the issues that arose during the consent process such as loss of Waianiwaniwa storage and only moderate shareholder demand  for irrigation in some areas, a staged approach was considered to be the best way to ensure that the scheme proceeds initially and then could be extended to cover the full area. The original intent of providing all shareholders with water at the same cost per share is also confirmed, but this will require a fully cooperative approach from shareholders. It is possible some extreme situations may not be able to be serviced in that way – such as areas requiring pumping above the scheme design parameters.

Whole of Scheme Design

A review of the scheme concept has been completed to take into account the changes that have occurred since the original design was undertaken for consents. Key issues that have changed are:

  • Loss of Waianiwaniwa storage
  • Availability of Lake Coleridge storage
  • Reduced availability of Waimakariri water due to consent conditions
  • Scheme total delivery demand refined and reduced
  • Requirement to transfer water both ways through canal reduced

The revised design has confirmed the best route for the headrace canal is still on the consented alignment but has resulted in significant reduction in the size of the canal through the central and northern stages. The indicative size of the canal shows a reduction from 35 cumecs to 16 cumecs. This will be further refined when detailed design is undertaken. Pipework has been designed for Stage 1 of the scheme and this will be applicable for the pipework for most of the remainder as well.

STAGE 1 OVERVIEW

Overview

Stage 1 of the Central Plains Water Scheme irrigates approximately 20,000 hectares of farmland in the Canterbury Plains in an area bordered by the Rakaia and Hororata Rivers. An intake at the Rakaia River will direct water into a 17km long gravity fed headrace, or canal.  A network of underground pipes distribute water to shareholders downstream of the headrace. The key components of Stage 1 are:

  • An intake and headworks at the Rakaia River to bring water into the headrace.
  • A headrace alongside and traversing ‘up’ the northern bank of the Rakaia River to the top of the main Rakaia Terrace.
  • A level headrace along the plains to convey water north and into the reticulation system.
  • A piped reticulation system providing pressurised water to all shareholder properties in the scheme area.

Stage 1 Infrastructure

Intake Structures

The river intake is located approximately 8km downstream of the Rakaia Gorge Bridge. It intercepts the stable river braid in this location.  The intake water is conveyed into the flow control structures, sediment retention ponds and fish barrier before entering the main headrace. The intake structures and fish screens are scaled appropriately for Stage 1, but designed so that they can be increased in size for future scheme stages.

Headrace

Once past the intake structures water is conveyed into a level headrace, which commences an 8km traverse ‘up’ the northern bank of the river terrace.  The headrace crests the terrace at the 235m contour line around Earlies Pond, near Steeles Road. The terrace headrace was constructed within an engineered fill embankment ‘bench’ rising to around 15m high and then into a bench cutting into the terrace face for the remainder of its length to eventually rise onto the upper terrace.  Around two million cubic metres of earth were moved to form the bench and headrace. Once up onto 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.

Construction

  •  Contractors had less than 18 months to build the 17km-long headrace canal and 130km-long pipe distribution network.
  •  For the headrace and canal, 1.9 million cubic metres of earth was moved, which includes 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 (10 on farm bridges and 3 public road bridges).

Reticulation

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.  A number of 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 5m3/s of water through the pipe.  Various control and measuring devices are installed along the pipe route as well as a telemetry system and central control building for the purposes of monitoring and controlling water flows to each of the farms within the Stage 1 scheme area.

Distribution Network

  •  The pipe distribution network largely consists of a number of pump stations and four main pipelines extending from the headrace canal down the Canterbury Plains to supply the various farms.
  •  These main pipelines were constructed between May 2014 and August 2015.
  •  With 130km of pipe to lay, contractors worked simultaneously on all main pipe lines at the same time, up to six days per week.
  •  The pipes will be buried at a depth of no less than 900mm to prevent pipe floatation and to avoid impact from agricultural activities.