Stopbanks play crucial role


Some $2.5 million spent a decade ago on stopbanks either side of the Ashburton River are paying big dividends for Ashburton town during the current red rain event.

The stopbanks are so crucial that engineers camped overnight at a critical spot on the north branch just to make sure they were doing their job.

Raging floodwaters had washed away a flow recorder and the engineers took turns to be eyes on the ground at Jessop’s Bend, just west of Ashburton, to make sure the stopbanks were not overlapped.

Now, as floodwaters recede, they will still keep inspecting the stopbanks to look for signs of erosion.

Ashburton mayor Neil Brown said the stopbanks had done their job and paid for themselves many times over. The banks are made of compacted gravel sown with grass, and paid for by ratepayers living alongside the Ashburton River.

Environment Canterbury river manager Leigh Griffiths said river engineers would remain in the Ashburton area assessing vulnerable areas and checking stopbanks. ‘‘There have been further breakouts on several rivers today.’’

Floodwaters were reported to be overlapping stopbanks on the southern side of the south branch of the river at Valetta.

If water breaks out in that area it heads towards Tinwald and can go across the main road.

‘‘There is a heightened erosion risk to stopbanks as water levels fall and braids within the river start to move around. Engineers will continue to monitor the situation carefully and carry out damage assessments to critical infrastructure once water levels recede.’’

Mr Brown said it was too early to estimate the cost of the rain event to the district.

Ashburton District Council chief executive officer Hamish Riach said the estimated repair bill would be some weeks away as there was damage council didn’t yet know about.

‘‘It will take time to assess it all.’’

Council staff have been assessing roads and bridges and at 2.30pm today, 17 roads in the district were closed, along with four bridges. Drinking water was being tankered to Mt Somers and Methven.

Council’s wastewater treatment pond at Wilkins Road had also flooded and potentially contaminated water was now mixing with floodwaters heading to Lake Hood, downstream of Tinwald.

Floodwaters were also receding at Hinds, where the residents of two houses were evacuated because of ankle-deep floodwater.

Mr Brown said farmers in the Greenstreet area were bearing the brunt of the event but were being supported by the Rural Support Trust, which was working in the emergency operations centre.

Helicopters had ferried in feed to some stock in isolated pockets.

He said it would take two or three days for the rain to clear the catchment and for rivers to drop back to reasonable levels.

The be prepared precautionary evacuation message was still in force for residents either side of Racecourse Road, and in parts of Allenton and Netherby.

Council’s website has all the latest updates for residents.