Engineering assessments of the latest slump on a section of the riverbank along Anzac Pde in Whanganui, has shown up a bigger - and costlier - problem than first thought.

Reconstruction of several hundred metres of riverbank along Anzac Pde is planned to start in November and covers a stretch of SH4 runs from Portal St to the City Bridge which collapsed twice in a little over 12 months.

The first occasion was in the wake of the June 2015 floods which took out a length of the footpath which runs between the highways and the river. Then a couple of weeks ago another section slumped, taking out the footpath from Taylor St to the City Bridge.

The NZ Transport Agency will foot the bill for the project because it's part of the state highway network. Depending on final design option the repair bill could cost anywhere between $2 million and $5 million.


Ross I'Anson, NZTA highway manager, told the Chronicle this latest washout showed a bigger problem with the riverbank.

"Our further investigations have identified potential issues with erosion of the river bank below the water level, which means we have had to review our original design for the repair," Mr l'Anson said.

"We're now considering using sheet piling, which uses interlocking sheets of steel to form a retaining wall. This retaining wall will allow us to clear the site of water, enabling us to build the permanent retaining structure."

He said NZTA was still working on the design of a final solution for the permanent structure "to ensure it is fit for purpose".

"The agency is aware that this site was at risk of slumping and our repair design has always included the wider area."

He said NZTA has been working with the Horizons Regional Council on a suitable repair solution and plan to start construction in November.

"We're still on track for the work to be complete by the end of April next year," he said.

This stretch of riverbank has been subjected to much more significant slumps before now. They have included washouts from Georgetti Rd to Purua St, when both lanes of Anzac Pde disappeared, and then on Putiki Dr immediately downstream from the City Bridge.

Back in 2003 a riverbank hazards report and mitigation plan was released. The report involved the Whanganui District Council, Horizons Regional Council and NZTA. That report highlighted a 20-year works programme costing an estimated $30 million.

Investigations for the original report in 1998 were followed by a period of intense flooding and major works were necessary because serious bank collapses further threatened road and service assets. The sites were at Kells Ave, Dublin St, Shakespeare cliff and Benefield's near the SH4-Kaimatira Rd intersection.