All further updates on this project, following the December 2012 decision to abandon engineering options, are available via the link below. Information on this page is presented for historical/archival purposes.
Debris Flow and Landslide Hazards »
Project update - 15 January 2013
Council’s project management team and the Matatā Projects Governance Group believe there is no affordable engineering solution to control future debris flows from the Awatarariki Stream catchment.
Council’s Projects and Services Committee agreed with the recommendation that engineering options should not be pursued on 12 December 2012. The Committee recommended that instead, planning and regulatory options should be developed to identify hazard zones which would take into account the level of risk involved for all properties in the township.
Below are the reports and maps associated with Council's December 2012 decision to abandon engineering options.
Project update - 2 July 2012
- Review of Awatarariki Catchment Debris Control Project - Final Report from Alan Bickers - (PDF, 3 MB)
- Review of Awatarariki Catchment Debris Control Project - Summary (PDF, 861 KB)
In early-March 2012, the Whakatāne District Council placed the planned construction of a ring net debris detention structure in the upper Awatarariki Stream catchment on hold and initiated an urgent review of this project. That followed expressions of concern by the Council’s engineering consultants, and independent advisors, about the viability and cost of the proposed structure.
The project review was led by Alan Bickers, a highly experienced engineer and local government leader and consultant. His report will be received by the Council’s Projects and Services Committee on Wednesday, 4 July.
Project update - 10 March 2012
- Council Report 7 March 2012 - Matata Regeneration Awatarariki Stream - Debris Flow Mitigation Works - (PDF, 7 MB)
Specific concerns raised included:
- A threefold increase in the estimated construction cost for the project, the latest estimate being $5 million to $7 million
- After any debris flow, the material deposited in the debris net would have to be removed to restore the structure’s functional capabilities. The cost of removing the volume of debris deposited in the 2005 event is estimated to be at least $5 million, at today’s costs
- A structure of the scale planned has to be considered “untested” (the Awatarariki ring net debris detention structure would have to detain up to 250,000 cubic metres of debris, a loading that is significantly higher than that achieved by successful debris net structures elsewhere)
- If increased residential development occurred in the Awatarariki Stream “fanhead” area, based on the expectation that construction of the planned structure would prevent future debris flows, the risk to property and to human life would be greatly increased if the structure failed, or was exposed to a larger debris flow than it was designed for
Engineering consultants and the project reviewer have expressed a high level of concern about the viability of the project and recommended that no further work be undertaken on the ring net structure detailed design or resource consent application until a complete review of the project has been undertaken.
Council understands that residents whose properties were subject to the worst impacts of the 2005 disaster will be both disappointed and frustrated at this new delay. However, under the circumstances, we believe there is no alternative other than to urgently gather the information we need to make an informed and responsible decision on the matter.
Council will be appointing an experienced and well-qualified project manager whose sole responsibility will be to ensure Council gets quality information and advice in the shortest possible timeframe. That person will also be available to answer any questions you might have as the review progresses.
How do ring net debris detention structures work?
The flexi-net barrier system was developed in Switzerland, where it has been used extensively. The flexi-net barrier comprises large interlocked rings, which are much stronger than ordinary steel. The concept is that during an event, the net traps the large debris behind the barrier forming a natural dam, while allowing the water and silt to flow through. The effect of the rocks piling together helps slow down and stops the larger debris following behind.
In normal conditions the water from the catchment is able to flow naturally through the flexi-net.
It is estimated that the flexi-net will cost approximately $2.2-2.4 million to construct.
A flexi-net barrier:
- Minimises the level of construction disturbance within the Awatarariki Stream
- Has a very small environmental footprint compared to a dam
- Does not hinder normal to storm level water flows or sediment transport within the stream
- Does not affect fish passage
- Can be installed without negatively affecting cultural sites above the abutments
- Is cost effective for the retained volume compared to a solid dam structure.
The flexi-net has been designed to retain 100,000 cubic metres of material behind the barrier. The majority of the remaining material will be diverted via a spillway to open ground west of Matatā. It is estimated that up to 50,000 cubic metres of predominantly fine-grained (muddy and sandy) material could pass through or over the open structure of the barrier, particularly at the start and towards the end of a debris flow event. The Awatarariki Stream has the capacity to convey this volume of material.