Karst Subsidence Karst Subsidence

Karst landscapes are characterised by a high degree of rock solubility in natural waters and are typical of regions underlain by carbonate rocks (limestone and dolomite). Common landforms include small closed depressions (dolines) and larger internally-draining basins called poljes. Tasmania has some of Australia's most significant karst areas, with poljes at Mayberry and Loatta near Mole Creek, and Dismal Swamp near Smithton being the best developed examples in the country. Sinking streams and large springs are associated with complex cave systems, with over 25 km of passage mapped in Tasmania's largest cave system near Lune River.

High rock solubility increases the risk of geohazards associated with ground subsidence and collapse, as well as associated processes of rockfall and landslide, soil erosion, sedimentation and flooding.

Whilst much of Tasmania's karst is found in remote areas managed for nature conservation, well-developed karst systems are also found in agricultural and urban areas, predominantly in northern Tasmania.

The degree of karstification of Tasmania's carbonate rocks is recorded in the Atlas of Tasmanian Karst (Kiernan, 1995), which classifies all known carbonate rock areas into four categoreis (refer to figure). Category A and B karsts are the most highly developed (or have the most potential for karst development), and consequently have the highest likelihood of active karst hazards. Category C and D karst have less likelihood of hazards developing (although many of these karsts are very poorly mapped and documented, so there is the potential for future changes to the classification). 
 
While no karst hazards are currently known in the vicinity of the major urban areas of Tasmania, karst does pose significant engineering challenges in specific rural locations. Major category A and B karsts within regions currently developed for industrial, agricultural or urban use are found in the Mole Creek, Gunns Plains and Smithton-Mella areas.

Many examples of ongoing and instantaneous road collapse have been documented with steep-walled depressions appearing overnight in rural roads, e.g. Mole Creek, Vale of Belvoir and the Lyell Highway at Bubs Hill. Whilst no fatalities have occurred, these hazards pose a significant threat to road safety. Karst has also been an important factor in the location and construction of Tasmania's hydro-electrical infrastructure in the south and west of Tasmania. Whilst subsidence, collapse and associated mass movement are all part of the natural karst system, many of these processes have been accelerated by modification of the land surface and hydrological systems.

Engineering challenges arise in both construction within unstable terrain and also in site remediation following any subsidence and associated mass movement. Pollution of groundwater resources is a linked potential problem, as transmissivity through karst conduits is high.  Planning that addresses karst geohazards also needs to take into account that in many cases there are important natural heritage values associated with cave or surface karst ecosystems.

Karst hazards are often difficult to predict, but extensive experience in the USA and parts of Europe has led to the development of methodologies for site assessment and management. This has produced specialised geophysical techniques for karst assessment, appropriate building and infrastructure development codes, and methods for repairing and restoring karst landscapes and infrastructure following geomorphic or hydrological impacts. Whilst not yet commonly used in Australia, the more intense development of karst areas is requiring a greater focus on assessing, monitoring and managing karst hazards.

The assistance of Ian Houshold (Deparment of Primary Industries, Parks, Water and Environment) is gratefully acknowledged.

Further information on karst can be obtained from the Geoconservation page on the DPIPWE website.

Karst data source: Tasmanian digital karst atlas, version 3, Department of Primary Industries, Parks, Water and Environment.