Subsidence Poses a Serious Risk When Combined With Sea Level Rise

The foreshore of Thames west of Queen Street (SH25) between the Shortland Wharf and PaknSave, and the suburb of Moanataiari is subsiding.  This land was “reclaimed” over soft, deep, marine sediment which is sinking due to compression of these deposits.  The rate of subsidence is variable but rates in excess of  10mm a year have been recently recorded in Thames.

 

 

 

Land subsidence can greatly increase the risks of sea level rise. If the land near the shore, or on the foreshore is sinking this adds to the rate of  “relative” or “local” sea-level rise.

 

 

In recent decades there had been numerous surveys taken which record the rate of subsidence. 

 

The most recent of these surveys were undertaken in 2016 by the consultants who prepared the Assessment of Environmental Effects for the proposed new apartment building for Richmond Villas. That Assessment concluded that the Richmond Villas apartment site is subsiding at the rate of 13 mm per year and that most of that subsidence is happening at 10-30m depth.

The Richmond Villas consultants made some highly optimistic assumptions that this rate of subsidence will slow over the next 50 years to a total of around 100 -150 mm if a deep pile foundation is used for the proposed building. This, therefore, presupposes an average annual subsidence rate of just 3mm a year over the next 50 years.

But this optimistic conclusion seems to be contradicted by a recent NIWA study in the mangrove forest near Waitakaruru in the Lower Firth of Thames. The NIWA study concluded that the annual rate of subsidence is around 8 -10 mm a year, but there is no Council rubbish and landfill or “pre-loading” on these sites to help explain away the rapid subsidence rate. Instead, the NIWA report concluded that the subsidence was due mostly to an ongoing long-term geological process causing the marine sediment to compress and sink – especially at depth.  It seems likely that this geological process is the prime cause of subsidence at Thames. 

Thames land subsiding at around 10 mm a year is of extreme concern because this rate must be added to the projected rate of sea level rise to give the local or relative overall rate. Currently, the global rate of sea level rise, confirmed by both tidal gauges and satellite data is around 4 – 5  mm a year.

However tidal gauges at Tararu (Thames) and other tidal gauges around New Zealand have shown a marked increase in the sea level rise rate to around 10 mm a year in the last decade. (Source: LINZ) A decade is too short a time to determine whether this rate is accurate and ongoing.

But there is a high level of confidence amongst climate scientists that around 30 cm of sea level rise by 2050 – 2060 is already “locked in” due to past greenhouse gas emissions and this level of rise is inevitable.

 

Therefore, should the rate of land subsidence at Thames continue at around 10 mm a year, this will of itself add a further 320 mm to the relative or local rate rise by 2050 – to which must be added a further 300 mm (30 cm) due to the locked in global warming induced sea level rise itself projected for 2050 – 2060.

Such a combined local rate of around 500 – 600 mm (0.5 – 0.6 m) within just 30-40 years would be catastrophic for Thames, causing regular storm surge inundation of all the foreshore and much of the central business district of the township. (Map generated by Waikato Regional Council’s coastal flooding simulator.)

Even in a much shorter time frame with 100 – 200 mm of local sea level rise – caused by a combination of land subsidence and sea rise (which could be possible by around 2030 in just 12 – 15  years time) – the Regional Council’s Coastal flooding simulator suggests that there could be serious groundwater flooding in a large area of the Thames foreshore (Groundwater flooding is already a big concern in South Dunedin, but note that subsidence is a minor contributor in Dunedin at around 1mm a year).   100– 200 mm of local sea level rise could also of itself substantially increase the likelihood of more frequent severe storm surges such as Thames experienced on 5 January 2018.

This is why it is absolutely vital that both the TCDC and the Regional Council, commit a very large sum of money to investigate these subsidence and sea level rise processes, carry out the necessary expert research and come up with some possible options as to how these threats might be mitigated. 

These Councils must also encourage an urgent case study of Thames by the NZ SeaRise research project and take advantage of the central government funding.

“Over the next five years, experts working under the NZ SeaRise Programme will try to create accurate estimates of the magnitude and rate of sea level rise for our coastal regions to the end of the century and beyond.

The study, which just received an $8 million MBie grant, would take into account the unique factors of vertical land movements related to ongoing natural ground subsidence.”

It’s not as if subsidence issues for Thames are not already well documented. Back in the 1970s and 1980s the Ministry for Works repeatedly wrote to the Thames-Coromandel District Council warning them that the proposed reclamations on the Thames foreshore would be highly problematic due to the land subsidence which was already occurring. For example, one of these MOW letters from September 1980  warned that the seawall protecting the Danby Field sports ground had already sunk in excess of 1.5 m by 1977 and needed to be continually topped up.

Another MOW letter from 1980 made similar very strong statements about the very high rate of subsidence and the million-dollar financial liability risks that this posed to Thames.

Even without subsidence complications factored in, NIWA’s nationwide risk census placed Thames in the top 10 of all towns and cities from sea level rise. The added risks to Thames from land subsidence are real, and they are very serious.  These issues require urgent investigation.  It is not an exaggeration to say that the whole future of Thames township is at stake. 

An example of the type of research which Thames requires is a March 2018 research project in San Francisco about land subsidence occurring at a rate of about 10 mm a year over similar marine sediment .  This report suggests there are very strong similarities with the situation in Thames.

The conclusion from this report was that land subsidence could increase the area of land inundated from the sea by up to 90% compared to what may occur for the same land with no subsidence.

“However, rates exceed 10 mm/year in some areas underlain by compacting artificial landfill and Holocene mud deposits. The maps estimating 100-year inundation hazards solely based on the projection of sea level rise from various emission scenarios underestimate the area at risk of flooding by 3.7 to 90.9%, compared with revised maps that account for the contribution of local land subsidence. Given ongoing land subsidence, we project that an area of 125 to 429 km2 will be vulnerable to inundation, as opposed to 51 to 413 km2 considering sea level rise alone.”