Any invasion of one’s home is particularly upsetting. When the invasion is by the unstoppable rise of water ruining everything it soaks into, it is even more traumatic. For those who live near the Ōpāwaho Heathcote River, the options available to avoid this are narrowing down towards one.
Since the 2011 earthquakes, which lowered the land in several suburbs near the river and raised the river bed with eroded silt, the Christchurch City Council has undertaken a number of actions aimed at reducing flood risk in the lower reaches of the Ōpāwaho Heathcote River.
It has:
- Dredged the riverbed from the Cut to the Ensors Road bridge removing 35,000 cubic metres of silt and mud thereby increasing the capacity of the river.
- Reformed, reshaped and reinforced the banks of the river in the Beckenham area to reduce slumping and to increase the flow
- Built retention ponds in the headwaters of the river to slow the release of stormwater in rain events. Several more of these ponds, and even a dam, are currently being constructed in the Cashmere Valley, Halswell and Hoon Hay areas. When completed in about 2024 their control gates will all be linked to a computer-controlled system to minimise flooding downstream.
- Purchased 21 properties beside the river which were likely to be flooded above floorboards in a 1-in-10 year event.
However, the truth of the matter is that this river has always meandered through a low-lying, swampy wetland, regularly breaching its banks to spread out over the wider area. It is just such a pity that the founding fathers of Christchurch City were so determined to place their city around and on that swamp.
It is society’s downfall to believe that we can engineer ourselves out of such predicaments. Raising the height of the riverbanks, for instance, could be an option, but there would need to be significant exploration of the unexpected consequences, practicality and the low cost-effectiveness of doing so.
Managed Retreat
There is one effective answer to the future flooding of properties on the Ōpāwaho Heathcote River: managed retreat – taking homes away from the worst flood-prone areas. It has already been used as mentioned above in the purchase of properties by the CCC and is particularly cost-effective as a solution. There is the double advantage in managed retreat if the land freed from housing is revegetated with native plants – flood risk reduced while global warming and the need for increased native habitat addressed.
No easy or quick decisions
Whatever might be considered to further reduce flooding, expect the public consultation and decision-making about anything associated with sea-level rise to be lengthy and detailed. If you want a taste of what that sort of decision-making might entail for local government in a time of considerable uncertainty about the effects of global climate change, read “Preparing for Coastal Change – a summary of coastal hazards and climate change guidance for Local Government” a document prepared by the Ministry for the Environment (2017). It outlines a process of ten steps to be undertaken in reaching a decision, and even then there will be multiple possible outcomes which might be triggered for each area depending on the eventual reality of sea-level rise.
The Coastal Hazards Assessment Summary Report 2021 by Tonkin & Taylor makes for interesting reading in this context. A clue to some of the wider issues relating to flooding for the Ōpāwaho Heathcote River in the future can be seen in their comment that sea level rise is predicted to cause a rise in the level of groundwater in places that are up to 1 to 3km inland from the coast and from tidal reaches of rivers.
So, there will be no quick decisions to be made about how the city handles sea-level rise. In the meantime, it will be relying on the works already completed or underway to prevent further flooding of homes in the lower reaches of the Ōpāwaho Heathcote River.
Any further moves will probably have to wait until the eventual enactment of the Climate Change Adaptation Act (CCA) probably in 2023 or 2024. The CCA, in conjunction with the other reforms to the current Resource Management Act, is designed to address complex issues associated with managed retreat, funding and the financing of adaptation.
The question we need to ask ourselves and address in the present time is what do we want the river to look like when our grandchildren grow up?