Information for Investors

Geological Stability and Landslides

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.  Table 3 provides a listing of landslides reported from the East Trinity area, and recorded on Geoscience Australia’s database (Commonwealth of Australia 2006).

Table 3 – Landslides reported from the East Trinity Area (Geoscience Australia 2006)

YEAR MONTH DAY LOCATION CLASS FATALS INJURIES SYNOPSIS CUSTODIAN Images
2000 02 unknown Yarrabah Road, near Second Beach, Cairns, Queensland debris fall 0 0 A number of very small colluvium debris falls and rock falls, with a total volume of 0.5 cubic metres, occurred near and just south of Second Beach. Small rock falls with a total volume of 0.3 cubic metres happened further south along the road, across Trinity Bay from the airport. These landslides were triggered by heavy rain. GA No Event image
2000 02 unknown Yarrabah Road on eastern side of Trinity Inlet near Koombal turnoff, Cairns, Queensland rock fall 0 0 Very small rock falls and debris falls have occurred in metasediments and colluvium in the road batter. Their total volume is only 1.4 cubic metres. They were triggered by heavy rain. GA No Event image
2004 03 19 Yarrabah Road, just south of Second Beach settlement, east of Cairns, Queensland debris slide 0 0 Debris slides and falls occurred in colluvium (debris flow deposits). Their total volume was about 12 cubic metres. The two largest, debris slides with volumes of 7 and 4 cubic metres, would have blocked at least one lane of the road. Landslides in the road batter have been triggered here by previous rainfall events. GA No Event image
2004 03 19 Yarrabah Road, at foot of climb over Mt Yarrabah, at Koombal turnoff, east of Cairns, Queensland debris fall 0 0 Very small debris falls in metamorphics occurred in the road batter. They have happened here during other rainfall events. GA No Event image
2004 03 19 Yarrabah Road, CH 27.2, on northern slopes of Mt Yarrabah, Cairns, Queensland debris slide 0 0 A batter failure in granite colluvium closed the road for 8 hours. Its volume was approximately 2000 cubic metres, and it was triggered by heavy rain. The rain was associated with a tropical low, subsequently Tropical Cyclone Grace, off the Cairns coast. GA No Event image
1878 03 08 Mt Yarrabah & Nisbet Range, about 5 km E of Cairns debris flow 0 unknown 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. GA No Event image
1911 04 01 Nisbet Range, 5 km E of Cairns debris flow 0 unknown 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. GA No Event image
unknown unknown unknown Cairns, backing the 'House on the Hill' level with about the two mile point up from the Copperlode dam road turn off insufficient information 0 unknown This landslide near Cairns will probably undergo fair sized movement in the near future. The removal of vegetation helps to accelerate the hillslope instability and quicken the movement of the landslide. GA No Event image

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 (LINK TO FIGURE 10), many on very steep gradients, and located above proposed building sites (LINK TO FIGURE 8).  The likelihood of extreme rainfall events during the summer wet season further exacerbates these hazards.

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.  It would be necessary to undertake test drilling both within the proposed residential area and on the slopes above the residential area to determine to underlying geological risk factors.  It would also be necessary to prove that the proposed works, particularly road construction, will not destabilise parts of the hill.  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.

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) (LINK TO 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 (LINK TO FIGURE 7 AND FIGURE 8).  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 downslope.

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) (LINK TO FIGURE 8, 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. Authorities may also feel compelled to deem a potentially unmanageable risk acceptable and give permission to build, because of planning rights.

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