Information for Investors

Geological Stability and Landslides

One of our greatest concerns for development at this site relate to its geological stability. Landslips and landslides are a well-reported hazard in the tropical north's rugged landscape, and the False Cape area is no exception. According to the Geoscience Australia's database (Commonwealth of Australia 2006), the 1878 flood followed by a severe cyclone triggered many landslides (debris flows). They could be so distinctly heard in Cairns that they sounded like an earthquake. In 1911 torrential cyclonic rain triggered a big debris flow. The landslide brought away trees, rocks and everything from a considerable distance up the mountain side. The rocks were granite. The AGSO site also sates "The removal of vegetation helps to accelerate the hillslope instability and quicken the movement of the landslide." A number of smaller slips have been recorded on the Yarrabah road in between 200 and 2004.

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Figure 10 - Large boulders at False Cape site.

The proposed development is sited on a narrow and steep stretch of coastal land. The steepness of the slope is not conducive to minimising the environmental impacts of erosion and runoff. In addition, the slope poses serious hazards to human safety due to a very large number of large boulders situated throughout the area (see Figure 10), many on very steep gradients, and located above proposed building sites. The likelihood of extreme rainfall events and cyclones during the summer wet season further exacerbates these hazards (there have been two category five cyclones of the Cairns coast within the last two years. The latter, cyclone Larry, caused an estimated $1.5 billions dollars worth of damage to region immediately south of Cairns.)

In 1999, the Australian Geological Survey Organisation published a report into natural hazards associated with the physical environment of the Cairns region (Granger et al. 1999). Included in this report was a description of the hazards associated with landslides, part of which is reproduced below:

[Landslides] can vary in size from a single boulder in a rock fall or topple to tens of millions of cubic metres of material in a debris avalanche.

Landslides can be caused in a number of ways. These include saturation of slope material from rainfall or seepage; vibrations caused by earthquakes; undercutting of cliffs by waves; or by human activity. Almost half the landslides causing injury or death in Australia were the results of human activity including:

  • removal of vegetation;
  • interference with or changes to the natural drainage;
  • leaking water mains;
  • modification of slopes by the construction of roads, railways or buildings on steep terrain;
  • mining activities;
  • vibrations from heavy traffic or blasting; or
  • accidental displacement of rocks.

Certainly the most common trigger for landslides is an episode of intense rainfall. The rainfall threshold values for slope failure are in the range 8 - 20 mm over one hour, or 50 - 120 mm over a day, depending on geology and slope conditions. In Cairns, rainfall intensities of such magnitude have an average recurrence interval (ARI) of considerably less than one year, and landslides are not rare events.

HLA-Envirosciences (2004) describe the False Cape site as being:

"located to a large extent on unconsolidated granite boulder and scree slopes. These slopes form an apron around a precipitous granite escarpment and overlap directly onto coastal sediments with a boulder-strewn coastline."

The underlying granite itself can be expected to be chemically weathered to a depth of many metres as is common around Cairns. The slope and geology suggest significant hazards. The PER does not include a geotechnical report, which we understand is currently being carried out. The State's opinion on this matter is expressed in State Planning Policy 1/03, which says that the land is in a natural hazard management area (landslide) because of slopes greater than 15% and past landslides within the region on similar geology. Unfortunately, this project does not appear to be bound by this state policy due to the way the policy has been implemented.

In line with the State Planning Policy, the Cairns City Council's Hillslopes Development Control Plan indicates that development on such terrain is in a natural hazard management area (landslide) because of slopes greater than 15%, and past landslides on geologically similar areas in the Cairns region. The proponent will be passing on the responsibility of these hazards, as well as the consequences of building in a hazardous zone, to future owners.

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Figure 5 - Aerial view of the False Cape site. The steepness of the site, and the cliffs overhanging much of the "Detached Dwelling" area, are obvious in this picture.

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Figure 11 - Boulders at False Cape site. Some of these boulders are as big as a house.

The development at False Cape will involve, and has already involved several of the causal factors listed above. Most lots have a gradient steeper than 1:3 (18 degrees); many are as steep as 1:2 (26 degrees) (Figure 5). On a 1:3 slope, a 15 metre long home with the front of a house at ground level, would have its rear 5 metres above ground level. The slope above the development is steeper still. Further, according the PER mapping, approximately 70 major boulders are located above residential facilities in lots 901 and 902. To this number must be added a significant number of large boulders above the area of the site mapped. Given the steep slope and extreme mass of some of these rocks, it is possible that vibrations and disturbance of the slope associated with construction works could lead to dislodgement of these boulders, with disastrous consequences for people and structures down slope. Already at least two large boulders have rolled down the slope in late 2005.

A number of boulders intrude into the proposed house sites (proposed lots 6, 9, 11, 18, 28, 29, 40, 43, 44, 56, 83, 86, 89, 90, 96, 98 and 101) (FIGURE 10 AND 11). These boulders will have to be removed in order to allow construction to proceed, leading to possible destabilisation of the boulders on the site.

A high density (R1) residential estate on slopes of this nature is unprecedented in Cairns. Previous development on very steep slopes has been on large allotments with a wide choice of house positions and positions for other features such as pools and driveways. Having room to work around the terrain to some extent allows some of the problems associated with slopes to be avoided. There is concern that the approval of the development at False Cape will undermine the State Policy on landslides by lowering the expected standards. Whilst the development at False Cape is a one-off, and does not create a legal precedent, it could result in more pressure on Council to allow the development of steep slopes elsewhere. This is precisely the situation that the State policy was designed to avoid.

The Reef Cove management plan allows lots to be created and sold before they are assessed for risk. Effectively all responsibility for avoiding the risk of landslip has been passed to future owners. This is irresponsible and bad public administration. On detailed site investigation an owner may find that they cannot build at reasonable cost.

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