Many landslide monitoring networks have been established throughout the world during the past few decades, with the purpose of defining mitigation measures and to provide warning systems. There are also many software packages for drawing landslide hazard maps, some a few decades old, and the result of several generations of refinements and improvements.
A wide range of techniques are available for investigating landslide movements, including survey marks, extensometers, inclinometers, analogue and digital photogrammetry (both terrestrial and aerial), synthetic aperture radar interferometer (InSAR), and LiDAR surveys (terrestrial and aerial).
Rainfall forecasts and real-time rainfall measurements have been the basis for landslide warning systems in several parts of the world.
However, these techniques suffer from shortcomings in terms of spatial and temporal resolution, and as a result many monitoring programs have been unable to link specific landslides to the triggering factors. There are other shortcomings regarding collection, processing and display of monitoring data.
But a team from GNS Science, a New Zealand government-owned research organisation, claims to have found solutions to these problems by installing a near real-time landslide monitoring network in Taihape, in New Zealand's central North Island.
The organisation operates a hazard monitoring network called GeoNet. It is partly funded by the New Zealand Earthquake Commission, which facilitates research about, and methods for, reducing or preventing natural disaster damage. It also provides insurance under the Earthquake Commission Act.
The project is to build and operate a modern geological hazard monitoring system in New ZealandWhen complete, it will comprise a network of geophysical instruments, automated software applications and skilled staff to detect, analyse and respond to earthquakes, volcanic activity, large landslides, tsunami and the slow deformation that precedes large earthquakes.
GNS team leader Chris Massey says although he knows about near real-time systems in Europe and the US, they are not as detailed as this one. "The increased spatial and temporal resolution of the network is providing a better understanding of landslide movement patterns, and due to the near real-time monitoring system periods of movement can now be linked to the triggering factors," says Massey. "The system allows improved definition of movement triggering thresholds, such as rainfall intensity and duration, which can be used as part of a landslide warning system."
By comparing monitoring data from multiple similar landslides it will be possible to assess the movement patterns of such landslides. It will also help determine whether relationships exist between these patterns, triggering factors and triggering thresholds (whether initiated by periods of prolonged rainfall or ground-shaking due to earthquakes) and properties of the materials forming the landslide slip plane.
Information gathered from this project could be used as the scientific basis for developing alert criteria, which could be used for landslide warning systems, similar to volcano monitoring alert criteria also carried out by GNS.
Massey says the team is working on developing these alert criteria, based on monitoring data from Taihape. It anticipates that these criteria could be applied to similar landslides around New Zealand and overseas, once site specific details are incorporated.
The Taihape project involves a combination of field mapping, sub-surface investigation and near real-time monitoring of rainfall, ground-shaking intensity, groundwater levels and surface movement. A key component of the project is a laser survey network that automatically tracks, at defined hourly intervals, the positions of reflectors placed on the landslide.
The movement data, together with information from piezometers and rain gauges installed on the landslide, are transmitted by radio and internet to GNS, which can be viewed in near real-time via the internet.
The team installed equipment to measure two triggering factors – rainfall and ground-shaking intensity. The movement monitoring system uses the reflectors (currently 30 prisms) located across the landslide to provide a high level of spatial resolution. High temporal resolution is achieved through a robotic total survey station that seeks and measures each reflector location at the hourly intervals.
Reflector positions are based on engineering geological mapping of landslide features (at a scale of better than 1:1000), a review of historical monitoring data and reflectors staggered across pertinent geomorphological features.
Rainfall is recorded by two tipping-bucket rain gauges located on the toe and near the back scarp of the landslide. Groundwater levels are recorded at 5min intervals using vibrating wire piezometers installed in four boreholes across the landslide.
Due to the proximity of the active Taupo Volcanic Zone, any movement triggered by earthquakes is being monitored using a strong-motion accelerograph installed in Taihape Rural Hospital. Data loggers for the piezometers and rain gauges, along with radios and batteries, are housed in custom-built cabinets located on 3m high timber poles. The survey station is located in a weather-proof hut.
Power for all equipment is generated from photovoltaic cells mounted on poles or on the roof of the hut, allowing the system to be remotely operated without any need for mains power.
The monitoring network operates in a near real-time framework, with an approximate site-to-office data transfer delay of one hour. Wireless data transfer from the robotic total station, rain gauges, piezometers and strong motion sensors is achieved via radios to the Taihape Town Hall and via the internet to GNS Science buildings located near the capital Wellington, about 250km south of Taihape. Data is automatically processed, formatted, checked and made available in both human and machine-readable formats. The results are presented in an interactive web-based chart, which is updated at 15min intervals and can be viewed at www.geonet.org.nz.
TAIHAPE LANDSCAPE BEHAVIOUR
The Taihape landslide is a large, deep-seated translational landslide. The landslide area is about 45ha, which includes 209 households, 388 residents and a primary school.
The landslide is closely related to the regional geological setting of the Taihape area. East/west compression and relatively rapid uplift has led to gentle folding of the Tertiary rocks.
As a result the Taihape sandstone has a regional dip of about 7° toward the south-south east. The area is also traversed by a series of north/north east to south/south west striking faults, including the Taihape fault, which forms the west flank of the landslide.
The fault is classified as active. The slip plane has been identified from various ground investigations and ranges from 22m below ground level in the toe of the landslide to 34m near the back scarp of the landslide. It comprises a thin (5mm to 10mm) layer of slickensided clay, thought to represent bedding-plane shear, as the historical direction of landslide movement is coincident with the regional dip direction of bedding.
Latest data has shown the landslide is still moving and at an increasing rate. This has caused significant damage to services, roads and residential properties
Recent monitoring has allowed GNS scientists to identify two main movement patterns: creep movement – characterised by slow displacements with rates typically less than 0.1mm/day sustained over many weeks or months; and surge movement – characterised by more rapid displacements with rates typically greater than 2mm/day (days rather than weeks).
Due to the increased temporal resolution of the monitoring, surge movement patterns have been linked to periods of prolonged rainfall where 60mm or more has fallen within a 48-hour period. This is because rainfall intensity has caused groundwater levels across the landslide to rise, which in turn reduced the strength of material along the landslide slip plane, initiating movement.
From this it has been possible to conclude that for Taihape, rapid displacements of the landslide occur when certain rainfall thresholds are exceeded.
Several other large, deep-seated translational landslides within the Tertiary deposits of central North Island appear to display similar movement patterns to those at Taihape.
Two such landslides located at Utiku and Waikoroa Bluffs appear to have recently reactivated at similar times to the movement recorded at Taihape, suggesting that the landslides may have been triggered by similar rainfall intensities.
GeoNet says: "New Zealanders live on the edge. Depending on their location, it might be the edge of the Australian Plate, or the Pacific Plate. The active Pacific-Australian Plate boundary passes through New Zealand producing earthquakes, volcanoes, steep terrain and active deformation.
"Nowhere in New Zealand is immune from the possibility of damaging earthquakes, and volcanic eruptions could distribute ash anywhere over the North Island.
"A major event almost anywhere in the country would affect the whole society and economy because of the small size of the country and the interdependencies of infrastructure, logistics and business."