It may sound like the title for a political thriller or a Doomsday novel but The Rosetta Scenario is not fiction.
In 1984, thirty new houses built in the Hobart suburb of Rosetta began to rupture. Walls gradually cracked apart, concrete driveways split, roofs dipped, all repairs failed and nine houses had to be demolished.
The precipitating cause could not be established, but the underlying reason was found. The houses—newly built and well-constructed—stood on bad ground. Below the surface “boulder-sized rock fragments in a matrix of sand-silt-clay of high plasticity” were infinitesimally rolling down the slope … taking the ground above along for the ride. To geomorphologists these are “deep-seated slide susceptibility conditions”. Irresistible, inexorable and inescapable; geologists categorised the event “the Rosetta scenario”: a new species of landslide.
Does landslip ground exist elsewhere? Yes. About 20% of Tasmania is affected by landslip (search Tasmania’s Landslip Hazard Areas online) and the mountain appears prominently: its entire eastern face—and on it the cable car—lies on ground with “deep-seated slide susceptibility” (Rosetta Scenario).
Landslip is not news. During convict times buildings sometimes failed catastrophically within a short time of construction, some due to the Rosetta Scenario. Taroona has a well-known landslip where housing is now banned. The Hobart City Council was aware of the instability on top of the mountain—so wary they moved the site of their Observation Shelter to avoid it. The MWCC does not have that flexibility.
The best the MWCC could achieve was to position their Pinnacle Centre on the zone’s western edge (Edge? It might well be a precipice). The cableway’s “Tower 3”, however, is smack bang in the zone. Below, Towers 2 and 1 are on fair ground, but the Base Station is almost surrounded by “mountain debris flow susceptibility conditions”, and the Access Road is similarly vexed.
Cable car supporters erupted at the revelations, mocking the maps above as “propaganda” and the landslip scenario as “laughable”, merely “bitsa information, based on indicative map data that, frankly, shows every gully and hill as a potential landslip zone”. In a follow-up post, the Hobart cable car Supporters Group shifted the ground, offering four new rebuttals. That the Pinnacle Road was at a greater risk than the Pinnacle: but that is wrong, the maps show the risks are different, but not greater. Second, that the Rosetta “association” was exaggerated: no, the same deep-seated susceptibility lies under both areas. Third, that the summit Communication Tower foundations caused no damage: correct, but that tower is outside the mountain’s Rosetta Scenario zone. Finally, that the Report cited was “highly cherry-picked”: false. The quotes cited are nowhere contradicted in the Report. What Supporters might find more convincing are the warnings of Tasmania’s foremost geologist David Leaman.
Contrary to Supporters’ flat earth hopes, the landslide mapping is not “bitsa”. Developers, investors and planners need certainty. Mineral Resources Tasmania has mapped the state landslide susceptibility down to 5-meter polygon accuracy, and established four “Hazard Bands”. 1. The Acceptable Band land is where “a landslide may occur in some exceptional circumstances but is such a rare event, development and use is not subject to control.” 2. The Low Band land has no known landslides, but is identified as susceptible: development controls may be necessary. 3. The Medium Band has known landslide features, consequently: “Where there is no compelling reason for development, the land should be zoned for open space, rural, or environmental purposes. Any hazardous or vulnerable use will only be allowed in exceptional circumstances.” 4. The High Band is ground where buildings have failed or ground movement is dramatic, here: “development requires significant investigation and engineered solutions to enable the development to maintain a tolerable level of risk.”
How does the MWCC’s development now look? No MWCC site is on an active or Declared landslip area and deep-seated slides move slower than snails crawl. The sky is not falling. On the other hand, much of the infrastructure is significantly affected. The Pinnacle Centre, with just 7% of the building within the Low Band, could be built subject to ground-truthing. At the bottom, the Access Road crosses Low or Medium Band polygons seven times, but a roadway can be easily repaired and is not threatening. The Base Station is different. Its engine room and cableway anchorage make it a hazardous development, and its purpose of containing crowds of people makes it vulnerable. With 64% of the station within the Medium Band (and a further 25% in the Low Band) approving it would require a compelling reason and exceptional circumstances. Tower 3 is probably the most at-risk infrastructure. It is in the Low Band, but close to the Medium Band and from the top of its 45-metre high tower the entire cableway is suspended. That is a highly Vulnerable use: any failure would be catastrophic.
The Wellington Park Management Plan recognises that “Large areas of the Park have a moderate to high landslip potential” and the management Trust considers landslip and soli erosion “the main natural hazards to be managed in the Park”. It also notes that “the inappropriate location of a building or excessive human activity in an area (causing soil erosion) can increase the hazard.” The MWCC should have rocks in its head but, head buried in sand, it has made no comment on the Rosetta Scenario.
Six months ago (May 2018) the MWCC sought state-level permission to drill into the top of the mountain deep enough to find solid ground, but permission has not been granted. We need hardly wonder why. Respect the Mountain questioned how such permission could even be contemplated. The permission would have to go against the advice of the Department of Premier and Cabinet’s own expert reports.
The MWCC may project confidence, but local planning authorities will tremble. They who approved a development in a Rosetta Scenario zone could not write a get-out-of-jail clause long enough to escape. They would be making themselves liable. An insurance company is likely to be even less sanguine. They would consider not only Rosetta but the more awful scenario of Thredbo. In 1997 a landslide down that mountain in NSW killed 18 people. Water from heavy rain, melting snow and a water main. fractured by soil creep. dislodged thousands of tons of rock. The State Government of New South Wales paid $40 million out-of-court to settle it.
The Rosetta Scenario, Thredbo… what would be the cost of a Pinnacle Scenario?