Environment Waikato plans to relocate the Rawerawe stopbank on the Piako River after it failed in February.
The 120-metre slip happened while the stopbank was being topped up to design levels near Rawerawe East Road north of Ngatea. The slip was on a river bend and was constructed in the 1960s as part of the Piako River Scheme.
The area had earlier been identified as subject to erosion and rows of timber piles were driven into the riverbed along the river edge to reduce erosion and stabilise the foundation.
This week’s Operations Committee meeting heard that immediate remedial works were undertaken to improve the berm stability, using more timber piles along the failed section. The stopbank was reconstructed to 100 year flood design height, but without additional freeboard. A metal buttress was also constructed on the land side while investigations were done into long-term measures.
Low soil strength, a deep channel and limited room had made reconstruction difficult. Options included keeping the stopbank where it is and reinforcing the foundation at a cost of $320,000 to $335,000, or relocating away from the river at a cost of $245,000 to $255,000, which had a better safety factor. This option includes land purchase.
Further investigation was being done into an overall picture of the Piako River Scheme stopbank’s stability and to identify any other areas of risk, the Committee heard.
The immediate works cost $95,000, with the extra $255,000 required over the coming year to complete the repairs to the Rawerawe stopbank to the full scheme standard. The additional funding needed to be budgeted for, as it was not included in next year’s programme.
The Committee agreed the work was urgent to maintain protection to the community on the Hauraki plains.
It recommended approving the capital cost of the immediate works and relocation works, and increasing the Piako Scheme Rates by two percent as an interim measure to cover the additional costs.