Nelson has a wide range of slope instability largely due to its varied geology. One area of significant instability involves about 26 ha of hillside overlooking Tahunanui Beach. This area includes some of the most expensive and sought after real estate in the city and is referred to as the Tahunanui Slump. This active slump is part of a complex rotational landslide. It is approximately 700 metres wide and extends from sea level to the crest of the Port Hills. Extensive movements and related damage are documented within the Slump in the 1890s, 1929 and 1962. Surveys of the area confirm that less damaging movements are continuing, at different rates in different areas. The landslide tends to move most during heavy rainfall, or ground shaking caused by earthquakes. Residential development, which commenced in the 1920s, has been restricted since 1985 by the Nelson City Council who assumed responsibility for the Tahunanui area in 1950.
The figure above shows the core area of the landslide where the hazard is known (red), surrounded by a fringe area where the edge of the active slump has not been able to be accurately defined (green) as presented on the planning maps in the Nelson Resource Management Plan (Nelson City Council).
Documented Timeline:
1893: In 1893 construction of Rocks Road across the toe of the slump was well advanced when significant stability problems were encountered. Movement resulted in bulges in the sea wall at the edge of the road which, although the sea wall has since been reconstructed, are still visible.
1920’s: Surveys, for the subdivision of the hillside into residential allotments, began in the 1920’s and subsequent resurveys were to later provide the first hard measurements of movement.
1929: Heavy rains and numerous aftershocks affected the area in the weeks that followed the 1929 Murchison earthquake. Approximately one month later there was landsliding adjacent to major road cracking in which two houses were wrenched off their foundations. It is reported this covered an area of 2 ha and the hillside moved 5 metres down and across Rocks Road at the toe.
1939: Professor Patrick Marshall of Otago University identified the hillside as an active landslide in an unpublished report to the Loans Board. The Loans Board subsequently refused permission to raise a loan for water reticulation only and insisted on adequate stormwater drainage and sewerage simultaneously. Several months later the NCC gave permission to tap a recently completed City Council high pressure water main in the area without the requirement for sewerage or stormwater drainage.
1944: Survey plans recorded up to 1.6 metres of movement on the lower part of the slump since the previous surveys.
1957: A 375 mm diameter high pressure water main failed by tension indicating 200mm of movement since its installation in 1940. Ground cracking developed on several lawns in the vicinity in the upper eastern portion of the Tahunanui Slump.
1962: The area was shaken by earthquakes. Approximately two weeks following these earthquakes, there was further heavy rain in the area. On 1 June damaging movement occurred within the slump and of nine houses that were directly affected, four suffered serious damage with at least one being condemned and torn down. An area of 1 ha immediately above Rocks Road and south of the 1929 movement was affected. Minor movement and damage was recorded elsewhere, being widely distributed throughout the body of the slump. Further movement occurred following wet weather later that year and may have been related to the removal of toe support during the 1 June failures. Acting on the recommendations of the investigators of the 1962 event, the NCC initiated the installation of a new stormwater system on the hillside.
1995: Higher than normal rainfall in Nelson coupled with several intense rain events resulted in surficial failures and several fresh exposures in the headscarp.
Geology:
The western side of the Port Hills consists of a sequence of consolidated well bedded sandstone-mudstone of the Magazine Point Formation which is overlain by the clay bound Port Hills Gravel formation. The Magazine Point Formation, of early Tertiary age, dips southeast into the Port Hills at about 65 - 70 degrees (below horizontal). The contact between the Magazine Point Formation and the Port Hills Gravel is an erosional unconformity dipping about 40 degrees southeast which very approximately coincides with the 50 metre contour. The gravel dips parallel to the contact. The hillside from Magazine Point extending approximately 1.4 km to the south has been involved in a very large, complex and deep seated slope failure. The southern half of this landslide is now regarded as inactive, although there are still minor failures at periodic intervals. This southern inactive area is not believed to have moved since the cutting of the sea cliffs during the post-glacial sea level maximum (about 6,000 years ago). The northern area (slump) is composed of Port Hills Gravel. The rocks of the Magazine Point Formation are well exposed on the shore platform in front of the slump at low tide and in the cliffs above Rocks Road to the immediate northeast. The in situ Port Hills Gravel formation is exposed in the headscarp of the slump. (Denton & Johnson, 1996)
Factors influencing the slump stability
- Seismicity: In 1929 and 1962 seismic movements were closely associated with moderate ground shaking at the site. Earthquake ground shaking may have the effect of “reshuffling the deck” and allowing rapid water ingress resulting in movement.
- Groundwater: Records indicate numerous springs and permanent ponds existed prior to subdivision of the hillside. While some springs have been intercepted by the stormwater system, their distribution is widespread and not all have been controlled. There is also limited capacity for rainfall catchment.
- Earthworks and human related developments: Construction of Rocks Road at the toe of the slump is one of the earliest documented links. This includes undercutting for the construction and/or the widening of the road. There was also some quarrying for road materials. Then there is the major housing developments on the hillsides.
References:
Denton, P. & Johnson, M. (1996). Housing Development on a Large Active Landslide: The Tahunanui Slump Story, Nelson, New Zealand
https://www.gns.cri.nz/Home/RBP/Risk-based-planning/A-toolbox/Setting-the-Scene/General-Natural-Hazard-Guidance/Managing-Uncertainty
https://www.stuff.co.nz/nelson-mail/lifestyle-entertainment/weekend/6225678/Living-on-a-slippery-slope
To log this earthcache:
Please send me your answers to the questions below. You may log this as a find before hearing back from me. I will contact you if there are any issues with your answers and I will delete any logs if the required message has not been received. The first person to send through the correct answers can claim the FTF.
1. Describe what you can see from GZ (or anywhere between there and the beach) that show the features/indications of the slump, influencing factors and resulting landslides.
2. Do you think the biggest influence on the slump has been natural causes or those by human development? Why?