Stage 1 of the Central Plains Water Scheme covers 23,000 hectares of irrigable area and consists 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. A 130-kilometre-long distribution network has four lateral pipes from the canal that provides pressurised irrigation water to each farm.
CPWL is supplying water for its third irrigation season operating Stage 1. The Scheme is successfully delivering reliable alpine-fed river water to its farmers. Stage 1 has enabled existing groundwater irrigators to switch off 75% of their groundwater abstraction volume during the 2015-17 seasons. Environmentally, this has resulted in 80 million cubic metres of water remaining in the ground.
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.
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 Earlys Pond, near Steeles Road. The terrace headrace was constructed with 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.
Contractors had 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, 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).
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 and the entire scheme is monitored and controlled by a central computerized control system.
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 pipelines 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.