Technical elements of the scheme

• With a current estimated construction cost of $409 million, the proposed CPW scheme will be one of the largest construction projects ever undertaken in the South Island.
• The scheme will comprise a water storage reservoir, a headrace canal and a network of irrigation channels capable of irrigating 60,000ha of land between the Waimakariri River, the Rakaia River, State Highway 1 and the Malvern foothills of the Southern Alps.
• A headrace canal will channel water for 56km from intakes in the Waimakariri and Rakaia riverbeds. Water within the canal will be about 5m deep and 30m wide at the surface. Water will be able to flow in both directions however flow velocities will be kept below 1m/second, a rate similar to a slow walking pace.
  
  The exact on ground location of the headrace is still being determined in consultation with potentially affected landowners but will generally be around the 235m (above sea level) contour. This is dictated by the river level at the two intakes and cannot be varied.
• The headrace from each river converges at a point north east of Coalgate. The headrace flow will be augmented at a point north east of Coalgate. There, where the Waianiwaniwa Valley opens out to the Canterbury Plains, a dam will be constructed measuring between 35-55m high and 2km wide to contain a 200-280 million m³ storage lake covering 12km².
• Water will also be sourced from the Waimakariri River, 3-5km upstream from its confluence with the Kowai River. This will run through a 10km long, 3.5m wide tunnel under the foothills into the storage lake.
• Water will leave the system (for distribution to farms throughout the scheme area) via 10 outlet points along the headrace canal. From there it will be channeled into a network of irrigation channels or pipes, which will mostly run along the side of existing roads.
  
  These distribution channels will look very similar to those used by the 60 year old Rangitata Diversion Race system with earthen, grassed sides typically less than 1m high. The water within them will be about 0.5-1m deep and an average 8-10 wide.
  
  Please note that at the time of writing, designs are still preliminary and details or even concepts may change.
• The volume of water passing through the scheme each year has been determined as being the volume that is required to meet the irrigation needs of the shareholding farms within the scheme area 99% of the time.
• The scheme is designed to meet a maximum demand for irrigation purposes of 36m³/second. This means that if all the shareholding farmers want water, the scheme will use 3 million m³/day at times of peak demand, but average daily water use will be about 1.6 million m³. The storage reservoir could be emptied in about 50 days if necessary, but has sufficient capacity to supply the scheme's average requirements for 150 days.
• The scheme will only use the water in the storage lake when irrigation needs are high and river flows are low (typically in the later stages of summer). For a lot of the time, the water will leave the rivers and pass through the scheme without entering the storage lake.
• The low flow levels of the rivers are fully protected by the Rakaia’s National Water Conservation Order and the Waimakariri River Regional Plan.
  Very little water will be taken when rivers approach peak flood levels, as it would mean the scheme has to dispose of large volumes of sediment. This approach preserves the integrity of the braided river systems by allowing the rivers to fully ‘flush’ and prevent a build up of the gravel that flows down from the mountains.
• The CPW Trust has applied to Environment Canterbury for a resource consent to take up to 40 m³/second from the Waimakariri River and up to 40 m³/second from the Rakaia (a joint application with the Ashburton Community Water Trust of which the CPW scheme will be entitled to half, ie. 20 m³/second).
  
  The actual rate of take will be much less most of the time – the average take from the Waimakariri will be 9 m³/second and 4 m³/second from the Rakaia. The total peak intake from all sources at any one time will not exceed 49 cumecs.
  
  It is important to note that:
  • both ‘takes’ are within the statutory limits set by the National Water Conservation Order on the Rakaia River and the Waimakariri River Regional Plan respectively.
  • neither ‘take’ will be continuous and represent the maximum amount of water that the scheme could take at any one time. When either river is in a period of low flow, no water will be taken below the established minimum low flow.

	Waimakariri	Rakaia	Combined take proposed by CPWL
Minimum flow (below which no water can be taken by any irrigator)	41 m³/second (set by the Waimakariri River Regional Plan)	90-139 m³/second, varies by month (set by the Rakaia’s National Water Conservation Order)
Mean annual flood flow	Up to 1,495 m³/second	Up to 2,514 m³/second
Average flow	122 m³/second	221 m³/second
Median flow	89 m³/second	159 m³/second
Maximum potential ‘take’ proposed by CPW	40 m³/second	20 m³/second
Average ‘take’ by CPW (over a 12 months)	9 m³/second	4 m³/second	13 m³/sec
Combined average annual flow	11,000 million m³		410 million m³

© Copyright Central Plains Water Ltd 2004