When I wrote my previous post last week discussing the potentially higher risk of sea flooding to the Hauraki Plains because the foreshore in the Lower Firth of Thames is sinking at around 10 mm a year, I thought that was pretty serious. But it turns out there is a lot of other data confirming subsidence on the land which is equally concerning. This other research confirms that the peatland which makes up a large proportion of the land on the Plains is subsiding at around 18 mm a year. Also, there is evidence that the land beneath the foreshore stop banks (which are meant to protect the Plains from sea flooding) is subsiding at the rate of about 5 mm a year. And finally, there is evidence that tectonic land movement associated with the Kerepehi fault is causing parts of the Plains to subside also.
To recap –
- the recent rise in sea level is around 3 mm – 4 mm a year and this rate is projected to accelerate substantially in coming years with climate warming
2. the foreshore is sinking at around 10 mm a year but this is being offset at the present time by accumulation or sediment of about the same amount. The offsetting rate of sediment accumulation may reduce if better land practices such as planting more forests were to occur in the future
3. the peatland is subsiding at the rate of about 18 mm a year¹
4. the land under the foreshore stop banks is subsiding at the rate of about 5 mm a year
5. subsidence due to tectonic land movement is around 1 mm – 2 mm a year ²
6. much of the Hauraki Plains is already below sea level, and already relies on a complex system of drainage channels, pumping stations and foreshore stop banks to protect it from sea flooding
7. we have evidence from the sea flood of 1938 that if the foreshore stop banks are breached then hundreds of square kilometres would be flooded
8. a Report from the Parliamentary Commissioner for the Environment concludes that 1-in-100 year storm surges such as occurred in 1938, would occur every six months or 12 months with just 0.5 m of sea level rise
When you look at these cumulative effects, the Hauraki Plains clearly has some very serious issues relating to coastal flooding hazard to contend with.
The District and Regional Councils produced sea flooding maps for an invite-only workshop held in 2015 relating to an earthquake on the Kerepehi fault. These showed various sea flooding scenarios ranging from present day Mean High Water Springs, to 0.5m Sea level Rise, to 1-in-100 year Storm Tide, to an extreme 4.3 m Storm Tide with 100-year sea level rise. These maps were produced in the context of an earthquake potentially breaching the foreshore stops banks, rather than the breach occurring due to storm surge, sea level rise and climate change. The hazard was euphemistically described as “Coastal Water Levels” rather than the accurate description of coastal inundation or sea flooding.
With the research confirming the Lower Firth foreshore and the land on the Plains itself is sinking rapidly and the latest projections indicating sea level rise will be more rapid, the authorities will have to significantly revise upward their projections for Relative Sea Level Rise and plan for much more drastic and expensive adaption measures, and may even have to consider managed retreat as a future option. These are not issues which can be pretended away or ignored.
The risk of an earthquake causing a breach in the foreshore stop banks is a real although remote possibility and Councils were correct to recognise and plan for this hazard and hold a Workshop. But why have the sea flooding maps which were presented to the Kerepehi fault workshop not been presented to wider community open days which consider the risks of climate change/storm tide sea flooding- which we know are coming?
For low-lying communities, sea level rise has been described as “a slowly unfolding red-zone”. The subsidence on the foreshore and peat land is happening right now. We know that around 0.3 m of sea level rise is going to happen regardless of attempts to reduce global warming. We have the warnings from the Parliamentary Commissioner about the increased frequency of 1-in-100 year storm surges? And we know from 1938 what the likely consequences will be.
Why have the Councils not obtained expert reports and published accurate sea flooding hazard maps for the Hauraki Plains and done the work necessary to get this information into the District Plan? They were required to do this by Central Government directive back in 2010? Many other Councils with far lesser potential risks to their coastal communities have done so.
The Hauraki Plains have some of the most productive farmland in New Zealand, hundreds of homes and livelihoods, billions of dollars of dairy production and infrastructure are potentially at risk. Such infrastructure includes vital road links from Auckland and Hamilton to the Coromandel Peninsula and a massive investment in drainage works. Hauraki District Council, as well as Central Government and the Regional Council, must urgently make climate-realistic strategic plans, review the District Plan, and engage in extensive public consultation, rather than the ad hoc, limited efforts they are currently taking.
One thought on “Hauraki Plains Land Subsidence Adds To Sea Flooding Risk”
Mapping is likely needed to show what would happen, not just with a 1 m sea level rise, but with an anticipated 3 m high storm surge.
Surely Environment Waikato & NIWA are doing this work, and making it public?
Sea walls are NOT the solution – for they typically increase the erosion nearby.
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