Photo credit: phys.org
New Maps Reveal Rising Subsidence Rates in New Zealand Coastal Cities
Recent research has highlighted alarming rates of subsidence affecting coastal areas in New Zealand, with some sites sinking by as much as 10 mm per year. This data, derived from high-resolution mapping of five urban centers, indicates a heightened vulnerability to the impacts of sea-level rise.
Dr. Jesse Kearse, who is leading this study as part of his Ph.D. work in geophysics at Te Herenga Waka—Victoria University of Wellington, commented, “The rapid pace of subsidence doubles the effective rise in sea levels that we are experiencing. By the year 2100, the cumulative subsidence could reach one meter, which poses a significant challenge to existing coastal defenses.”
The maps were generated utilizing radar techniques from satellite images taken between 2018 and 2021. By integrating this data with historical records, scientists were able to conduct a comprehensive analysis of land shifts along the coastlines of Auckland, Tauranga, Wellington, Christchurch, and Dunedin.
These findings were recently published in the New Zealand Journal of Geology and Geophysics.
Dr. Kearse further elaborated on the implications, stating, “The threat posed by ongoing sea-level rise is a growing concern for coastal infrastructure in the region. Subsidence—especially in areas altered through land reclamation—intensifies this threat.” The assessment revealed that roughly one-third of the studied sites are subsiding at rates exceeding 2 mm per year. While this may seem minimal on a year-to-year basis, over several decades, it results in significant elevation changes that can greatly affect coastal defenses.
“The subsidence is particularly concerning as sea levels in New Zealand are projected to increase at a rate of approximately 5 mm per year, creating a compounded risk of flooding,” Dr. Kearse added.
Among the areas experiencing notable subsidence, reclaimed land in Auckland’s southern Onehunga coastal region has been observed sinking at rates between 3 and 10 mm per year, which raises alarms given the presence of significant industrial activities there.
Additionally, rates of subsidence have reached as high as 12 mm per year on Auckland’s eastern port margin. On the North Shore, a stretch of 500 meters of highway built on reclaimed land is sinking at 6 mm per year.
The assessment of Wellington revealed that the southern end of the airport runway—situated on reclaimed land—is a subsidence “hotspot,” exhibiting rates of 3 to 4 mm per year. Other localized high subsidence zones were noted, particularly around Wellington’s CentrePort.
Importantly, not all reclaimed zones in Wellington are experiencing elevated subsidence rates. As Dr. Kearse pointed out, “Areas like Evans Bay and most of Wellington’s Thorndon waterfront are subsiding at or below the regional average.”
In Christchurch, significant subsidence was recorded along the coastal region between New Brighton and Southshore, where rates vary from 4 to 9 mm per year. This phenomenon is largely attributed to ongoing geological changes stemming from the 2011 earthquake sequence, rather than simply land reclamation.
Moreover, localized subsidence reaching 6.5 mm per year was identified within Lyttleton Harbor’s reclaimed lands.
Dr. Kearse’s research indicates that existing data underestimated the extent of subsidence within these urban areas. Of all the locations studied, Christchurch had the highest average subsidence rate at 3.6 mm per year, with other average subsidence rates recorded as follows:
- Wellington: 2.4 mm/year
- Lower Hutt: 1.9 mm/year
- Porirua: 1.8 mm/year
- South Auckland: 1.8 mm/year
- Dunedin: 1.6 mm/year
- North Auckland: 1.5 mm/year
- Tauranga: 0.6 mm/year
This comprehensive examination revealed that the most significant subsidence occurred in coastal regions subjected to reclamation in the 20th century and other recent modifications involving excavation and fill. Sites characterized by uncompacted sediment were more prone to rapid subsidence.
Dr. Kearse concluded that these insights underscore the critical need for ongoing research into coastal areas’ susceptibility to rising sea levels. “To effectively address the threats we face from future sea-level rise, having precise data on land movement along coastlines is vital, particularly in densely populated urban areas. Our recent findings provide a snapshot, but continuous monitoring is essential for understanding the long-term hazards along urban coastlines,” he stated.
“The use of remote sensing technologies like those utilized in our assessment represents a cost-effective approach to gathering accurate information about vertical land motion,” he added.
More Information:
Jesse Kearse et al, InSAR measurement of vertical land motion in New Zealand cities, and implications for sea-level rise projections, New Zealand Journal of Geology and Geophysics (2025). DOI: 10.1080/00288306.2025.2470435
Source
phys.org