Greater Sea Flooding Risk? – Firth of Thames Mudflats Are Sinking Fast

There is a lot of mud in the Firth of Thames – in some places it is 3 km deep and it is sinking — fast…   New research by Andrew Swales of NIWA has revealed that the mud (sediment) in the Lower Firth of Thames is rapidly subsiding at the rate of close to 10 mm a year.  This rate is within the range of annual sea level rise estimates for the New Zealand coast not expected for another 80 years due to climate warming.  This is a very big deal and must be seriously taken into account by central, local and regional government when assessing the risk of sea flooding to the Hauraki Plains. 

Swales Figure 1, DSCN0878

To put this in perspective, the Port of Auckland has the nearest tide gauge with a long-term record and shows an annual sea level rise of 1.5 mm a year.  Therefore the Firth sediment is sinking at an annual rate six or seven times greater than long-term Auckland sea level rise. 

Relative Sea Level Rise (RSLR) is a combination (plus or minus) of how much the seafloor is sinking (or rising) plus how much the sea is rising due to climate change in a given location.  For the Hauraki Plains, the rate of sinking of the sediment plus the rate of SLR, less the rate of accumulation of sediment must be taken into account in calculating RSLR, and the potential hazard risk to the region.

At the moment the rate of accumulation of sediment in the old-growth mature mangrove forest next to the shoreline is around 10mm, so this is off-setting the rate of subsidence.  However, SLR is projected to accelerate well above current levels into the future meaning that RSLR (SLR plus subsidence) would no longer be offset by sediment accumulation.  Also, sediment accumulation rates in the future may be slowed due to the widespread adoption of land use practices required to lower CO2 emissions, – particularly re-afforestation.   Taking all these factors into account suggests that the Hauraki Plains will face greater potential risks from rising sea levels and coastal flooding than most other coastal regions in New Zealand.

The NIWA research concludes that most of the subsidence is due to compaction of the marine sediment with only a small part (less than 2mm a year) due to tectonic vertical land movement.  The subsidence is likely to continue into the foreseeable future. 

Much of the Hauraki Plains is actually below sea level (New Zealand’s “Holland”) and the area relies on a huge network of stop banks and drainage channels and pumping systems to keep the region from being flooded by the sea.  (land below sea level coloured yellow on map)

HP below MHWS
Yellow areas below sea level

The risk is very real.  In May 1938 a storm surge caused the sea to burst through the stop bank from Waitakaruru to Kopu. The water extended inland to Ngatea, 7.5 km from the coast. About 350 sg km was flooded. About 16 sg km of farmland was flooded up to a depth of 1.2 M.  

Sea Flooding Hauraki Plains – 1938

NIWA has carried out a nationwide study of the sensitivity of the New Zealand coast to both sea flooding and erosion, and has produced maps showing relative sensitivity to these hazards. The Firth of Thames coastline scores the highest possible sensitivity/risk to both sea flooding and erosion (red line) as shown on these maps.

HP erosion sensitivity indexHP flooding sensitivity index

Sea Level Rise is Accelerating –  Severe Storms More Frequent 

It’s reckless to delay because the adverse effects of climate change are already upon us – whether it be coastal erosion, more frequent and more severe storms/surges (2 in April) or more severe floods.  The Parliamentary Commissioner for the Environment has concluded that even a modest 40cm -50 cm of sea level rise (which is already “baked in” to the climate system) will result in “1-in-100 year” severe storms becoming an annual or six-monthly event.  1 m of sea level rise will result in “1-in-100 year” severe storms happening every day.

half M rise ann event Parl Comm

Projections of Sea Level Rise Are Higher

Just how quickly sea levels will rise is still uncertain.  However, projections made only recently have since proved to be conservative. Ice sheets at both poles are melting faster than was predicted only 7 years ago. Many predictions for sea level rise are also much higher than those which underpin the assumptions in the 2010 NZCPS (1m sea level rise by 2100).

 The latest (January 2017) “extreme” prediction from the US National Oceanic and Atmospheric Administration is now 2.5 M of sea level rise by 2100 –  a staggering 1.5 M higher than projections made less than a decade ago. Mid-range projections have levels .5M higher by 2060 – just 43 years away.  Global sea level rise surged between November 2014 and February 2016, with the El Niño event helping the oceans rise by 15mm. This was well above the post-1993 trend of 3 to 3.5 mm per year.

Council’s Response

The Hauraki District Councils Long Term Plan – (2015 – 2025) has recognised the threat from rising sea levels but has adopted a very conservative estimate of just .5m of SLR by 2100.   Many other councils have adopted an estimate of 1 m SLR by 2100.  Even although the Council is aware that peatland on the Plains is subsiding they have seemingly taken no account of this in their estimate of local SLR.

The Council Plan says:-

“The Council is aware that the predicted impacts of climate change could have significant effects on the foreshore and lower river stop bank requirements for the Regional Council’s river schemes” and

“Increased storm intensity projections would likely occur over a period of 100 years, and not within the next 10.” 

“As peat shrinks and the land contour changes, some re-orientation of the drainage network will be required.  Increased drainage pumping capacity may be needed in time, to counteract the possible impact of climate change and rising sea level, in addition to that needed to provide for lower ground levels on the settling peat soils of the Hauraki Plains”

“funding issues will also need to be considered if additional works or assets are required. This could stupidly impact on the land drainage rates for users in the future in the land drainage areas.”

As is the case with the neighbouring Thames Coromandel District Council, the Hauraki District Council has failed to identify coastal hazard zones from inundation (sea flooding) in its planning documents.  This is in clear breach of the Government’s 2010 National Coastal Policy Statement (NZCPS).  The NZCPS requires District and Regional Plans to identify all coastal hazards, including inundation by sea level rise which might occur over a 100-year timeframe – out to 2117.  This work must be done “as soon as practicable,”   Council must also devise policies and rules to restrict use and development in hazard zones, and consider “managed retreat” as an option within such areas.  (See in particular Policies 24 and 25.)  These are not issues which can be left for some future Council to deal with.  Legally the Councils are required to do this work now.

In the 7 years since the NZCPS was issued the only climate- change-related coastal hazards that Hauraki District Council has managed to identify and map are setback lines for coastal erosion at Whiritoa.  Coastal hazard zones around the rest of the District including the coastline on the Firth of Thames have not been identified, even although the ideal opportunity arose when the District Plan was recently reviewed. 

The Waikato Regional Council (WRC)  Coastal Inundation Mapping Toolprovides users with a ballpark estimate to quickly understand the susceptibility of coastal areas to coastal inundation (sea flooding) due to tides, storms and projected sea level rise at a regional scale.”  The tool covers the entire Thames-Coromandel coastline but does not yet have any coverage for the Hauraki Council coastline on the Firth of Thames. With the Firth coastline being identified as one of the most potentially at-risk areas for coastal hazards in New Zealand why has the WRC and Hauraki District Council been so laggard in providing this service?  Perhaps the reason can be explained by this revealing map from the Parliamentary Commissioners for the Environment’s Report?

HP below .5 Parl Comm

Reading through the minutes of the meetings of the Plains Drainage Committees you will find no reference to the potential risks from sea flooding inundation or whether current protection and adaption measures will cope.  It is as if climate scientists don’t exist.

The section in the District Plan relating to Flood Management, River Control, and Coastal Inundation Control concedes that “the (Firth) foreshore stopbanks have the function of providing a barrier to the inundation of the low-lying Plains.”  There is no reference in this section to climate change or sea level rise, or to the requirements in the NZCPS to consider sea flooding on a 100-year timescale.  Does the Council seriously consider that these foreshore stop banks will continue to provide an effective and safe barrier to sea flooding of the Hauraki Plains for the next 100 years and that no specific mapping of potential hazards in the District Plan (such as occurred in 1938) is required in that 100-year time scale?  Really? 

eastern waikato flood

Recent events in Edgecombe and in eastern Waikato are a reminder of how vulnerable stop banks can be.


With the NIWA research confirming the Lower Firth foreshore is sinking at close to 10mm a year and the latest projections indicating sea level rise will be much 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 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 the massive investment in drainage works.  This is yet further evidence that local councils including Thames-Coromandel District, as well as Central Government and the Regional Council, have to take urgently make climate-realistic strategic plans  and to engage in extensive public consultation, rather than the ad hoc, limited efforts they are currently taking.


Methods Used to Measure Subsidence

Three independent and complementary methods were used to take GPS surveys tied to a network of satellite-based geodetic sites over an eight-year period (October 2007 to February 2016):-

  • A set of devices placed atop poles driven18 m into the sediment in the mangrove forest mid-way between the Waitakaruru and Piako River mouths. (see map) These sites recorded subsidence of 7.7mm to 9.4 mm a year – an average of 8.8mm a year
  • The tide gauge at Tararu offshore from Thames Township which had a GPS recorder attached to it. This recorded subsidence of 3.6mm a year.
  • A GPS reference station located on basement rock. This recorded a subsidence rate of 1.6mm a year

Sediment accumulation rates were also measured in cores at 137 sites

The clear conclusion of the study is that the subsidence is largely due to deep subsidence of the Firth’s sediment column by compaction

Relative Sea Level Rates

During the satellite era (1993–2015), RSLR at Auckland has averaged 4.9 mm a year in comparison to the Global Mean Sea Level rate of 3.3 mm a year.  The Tararu gauge near Thames indicates a RSLR of 6 mm a year in the satellite era (1993–2015).