Friday, December 6, 2019

CANTERBURY RUNNING OUT OF WATER???


I have returned to the world after another time of peace and calm at "The Gorge".

Rakaia Gorge that is and it was somewhat different this time. The river that ruled Mona Anderson's life inspired her to write of her time married to the then manager of Mt Algidus Station, which lies above the confluence of the Rakaia and Wilberforce rivers, the story related in her first book of nine, "A River Rules my Life". That river was in flood for many recent days peaking at over three thousand cumecs at least twice.

A cumec is a cubic meter of water flowing past a point each second. Just absorb that figure,  three thousand cubic meters every second!

Do the maths.

The Rakaia is one of seven major Canterbury rivers that originate on the main divide of the Southern Alps. The Clarence (northern boundary with Marlborough), Waiau, Hurunui, Waimakariri, Rakaia, Rangatata and Waitaki (the southern Boundary). The first six were estimated to have been flowing almost unimpeded at around ten thousand cumecs during the recent rain event.

The Rangitata has the Rangitata diversion race that flows across Mid Canterbury to Highbank where it spills into the Rakaia when water is surplus to the draw-off for irrigation.

The Rakaia feeds the Central Plains scheme that is still developing.

The Waimakariri has some small scale irrigation takes.

The Waiau feeds the Amuri Plains scheme that has morphed from an inefficient border-dyke system to a piped scheme spraying less water over twice the area.

The Clarence is almost untouched.

At the other end of the region the Waitaki is very developed with enhanced lakes Ohau, Pukaki and Tekapo all controlled and the man-made lakes Waitaki, Aviemore and Benmore. These lakes are connected by canals that have left dry the riverbeds that once drained Tekapo and Pukaki - unless the lakes require a spill. The Ohau has now completely disappeared, covered over much of its course by Lake Ruataniwha, with some dry-bed remnant.

If the Upper Waitaki that saves so much water and generates some significant renewable electricity was proposed  in the current climate of no-development-anywhere, it would never make it past the dream stage.

Back to the water flowing to the Pacific Ocean; yesterday Christchurch used 181,165, 000 liters of water which was around 5 seconds of the peak flow of the Rakaia at the gorge bridges.

There is no storage on the Canterbury Rivers apart from the natural lakes on the north branch of the Hurunui - and they have been made sacred as to any controls - and the diversion of the Harper  river to Lake Coleridge.

I accept the high shingle flows make damming the rivers somewhat fraught, but there are still some options for storage away from the streams for summer flow enhancement.

To quote The late John Clarke personna Fred Dagg, "we don't know how lucky we are".

Those were some very impressive flows this week and so much just poured into the sea unhindered while mongs lament any and every use of the resource,  both in situ and planned.

3 comments:

Adolf Fiinkensein said...

GD

They are not mongs. They are highly intelligent activists whose aim has nothing to do with protecting the environment or saving water but everything to do with sabotaging the economy of a western nation.

They need not worry. Your Prime Minister is doing an excellent job for them.

Unknown said...

The Rangitata has high flow diversion and storage at Arundel, for precisely this sort of circumstance. It can and has been done

gravedodger said...

Thankyou Unknown, I had overlooked Rooney's efforts, there have been some issues over leakage but terrific concept