The value of the Wairau River Stopbank Scheme in protecting residents on the Wairau Plain from flood was tested to its very limit during the region’s recent storm event.

On July 17 the stopbank network withstood what is now thought to be at or just below the 1 in 100-year design flood for the Scheme.

“The return period for the 17 July flood still requires detailed assessment but based on the 1992 hydrology review it will be at or just below the 1 in 100-year design flood,” Rivers and Drainage Engineering Manager Geoff Dick told Council’s Assets and Services Committee last week.

The flood event’s peak flow has now been assessed as being in the range of 5200 – 5300 cumecs or just below the target scheme design flow of 5500m3.

Mr Dick presented a comprehensive report on the flood event to the committee including an assessment of the scheme’s performance, a summary of the damage, the repairs proposed and capital upgrades

He said the weather event “tested the system to its very limit” in what was a ‘design’ or slightly ‘below design’ flood but the scheme “did its job”.

Four emergency evacuations of residents occurred at Renwick lower terrace, Spring Creek, Tuamarina pocket and Morrins Hollow to ensure safety while the peak of the flood went through the system. For the majority those evacuated returned to dry undamaged homes.

“Overall the scheme performed well but there were three significant stopbank breaches, significant sections of stopbank that either had no freeboard or over topped and significant but expected system damage,” said Mr Dick.

The three key stopbank failures were at Pukaka floodway east bank, Waterfall Creek at Tuamarina pocket and lower Wairau at Morrins Hollow. Preliminary assessment suggests the Pukaka floodway and Waterfall creek breaches were due to a combination of over topping and variable stopbank construction quality over time, and at Morrins Hollow due to piping under the bank.

“In addition to the three breaches we had piping through a stopbank at Spring Creek due to an unsealed but redundant septic tank outfall, overtopping exacerbated by stock crossing wear and tear, overtopping on the Diversion true left bank stopbank that inundated the Blind Creek pump station and complete loss of nearly 400m of edge stabilisation rock work at the Diversion mouth,” he said.

“Of 100-150 flood gates along the network, only two of significance leaked. Overall the scheme is considered to have performed well and consequently saved an enormous amount of associated flood damage across the lower Wairau plains.”

The volume of water lost into the floodplain due to either breaches or stopbank over topping was a small fraction to what a major bank breach or banks down situation would have resulted in, he said.

Repairs are well underway prioritising restoring full flood security to the stopbank system before next winter but a complete programme of recommended works involving more than 100 separate repair jobs at an estimated cost of $11.2M is anticipated to take until June 2024 to complete.

The funding of the local share balance to be found be considered as part of the current 2022-23 annual plan budget process. The estimated flood damage repair cost is about 5 percent of total scheme asset value, well within the expected range of 3 to 10 percent for a design or near design flood event.

The majority of the schedule of repairs are anticipated to be eligible for LAPP (Local Authority Protection Programme) and NEMA (National Emergency Management Agency) funding which will significantly reduce the expected cost burden on scheme ratepayers.

Considerable thought will need to go into the work required at the Diversion to repair the 360m section of eroded bank at the river mouth and a recommendation on how best to approach this will be tabled in the new year.

A more detailed review of scheme performance will be undertaken once the current schedule of repairs nears completion to inform Council of options for ongoing upgrades to further increase scheme security and capacity as part of preparing the next Council Long Term Plan.

Assets and Services Committee Chair Clr Nadine Taylor observed that future upgrades should be seen as an investment into protecting the assets of Marlborough’s ratepayers and said funding this investment was a discussion that will need to be had.

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