Flood recovery updates
Here you can find regular updates on the status of our rivers and the progress of flood recovery works. We anticipate that flood recovery work will last several years and some repairs can’t be done until the ground dries out or closer to summer.
The May flooding event was an unprecedented weather event with both long and intense periods of rainfall with effects felt from the Waimakariri District through to the Mackenzie. In the Ashburton River/Hakatere catchment area the weather event was of a magnitude greater than everywhere else in the region.
Our initial focus during the flooding was getting on top of emergency works to reduce the immediate flood risk to homes, farms, and lifeline infrastructure.
Region-wide, the damage to Environment Canterbury-owned flood infrastructure included holes in stopbanks, significant stopbank scour and weakening, erosion to riverbanks and large sections of flood protection vegetation - which plays the critical role of slowing down floodwater - was lost.
Repair works have included:
- Temporary erosion control and overflow reduction though temporary bunding
- River fairway, channel, and bridge clearing to re-establish flow paths
- Urgent erosion repairs
- Re-establishing anchored tree protection (ATP) in areas where tree edge protection has blown out
- Providing temporary rock protection in places where urgent river edge protection is required.
Vulnerable to future flow events
All repairs remain highly vulnerable to future flow events and we’re aware that this is a really difficult situation for many landowners. Some rivers, particularly the Ashburton/Hakatere and Orari remain susceptible to rainfall and any significant river flows will likely result in break outs at the same places until permanent repairs can be made.
Temporary repairs prevented further damage
The importance of continuing temporary repairs was highlighted by the rains of 17 July. Although repairs breached at several original flood locations, most of the temporary repairs held up and prevented more wide-spread flooding and further damage. Looking ahead, we expect that ongoing temporary works will be needed in response to erosion of existing temporary structures, and to support implementation of permanent works.
We are in regular contact with MetService and as always flood controllers are on duty 24/7 and fully aware of the vulnerability of our flood protection schemes. If we are alerted to the possibility of out-of-river flows, we will post information on our flood warning page and use our social media channels and the text alert system for proactive messaging if required.
We expect the final cost of damage to our flood protection infrastructure to be somewhere between $15 million and $20 million. This is the replacement cost for infrastructure on a ‘like-for-like’ basis and does not include any additional costs for potential improvements.
Tree edge protection
Replacement of tree edge protection (ATP) is by far the biggest component of permanent flood damage repair, required at 148 sites or groups of sites over 42km across the region.
Stopbank replacement or repair is needed at 46 locations encompassing 5km of stopbank reinstatement.
Effects of climate change
As we consider repairs, we’ll be anticipating the expected effects of climate change and potentially modifying design solutions as part of our recovery work, rather than simply replacing the flood protection infrastructure that was in place.
For example, in the Orari floods of 2017, rather than simply replace a stopbank we modified the design to include building a new part of a stopbank further back from the river. This proved successful during the recent flood event where the modified design helped prevent breakouts that would otherwise have affected large parts of the community.
Longer term repairs may not happen for months to years and will be implemented over time in a staged approach.
When assessing the need for any post-flood river-works we prioritise the protection of community water intakes as critical infrastructure along with roads and bridges. Our primary role is to reduce the probability of surface water overflows from the river affecting these assets.
Immediately following the recent and unprecedented rainfall that resulted in region-wide flooding, there were significant out-of-river flows from the Ashburton River/Hakatere, and we identified three erosion sites near the Methven community water supply. The two upstream sites (sites one and two) have the potential to threaten the intake gallery.
Between 1-15 June we undertook emergency response works to minimise erosion and reduce overflows by building temporary gravel bunds, diverting water away from the large erosion bay at sites one and two.
More recently on 16 July we confirmed that a length of the temporary bund at site one was being threatened by increased flows, and with more rainfall forecast for the weekend we proactively strengthened the bund with additional gravel.
The following day we also created a temporary diversion channel on the other side of the river and successfully diverted flows away from the erosion bay at site one.
Although site three is adjacent to the community water intake, water flows away from the intake gallery and overflows at this location do not pose a threat.
The whole of the Ashburton River/Hakatere catchment remains vulnerable with temporary fixes in place to reduce the likelihood of erosion and overflow. It is likely that we will continue to have similar rain events which will cause more damage and more overflows to occur. We are working on more permanent repairs throughout the catchment and continue to closely monitor river flows and rainfall.