111.jpg Landslides
Get the facts
The Earth’s land moves at least a few centimetres a year. But there are times when it can move metres, even kilometres at a time. This is when landslides occur.
Landslides in Australia have caused fatalities, environmental degradation and millions of dollars damage to buildings, roads, railways, pipelines, communication networks and agricultural land.
Although many of these landslides have resulted from natural phenomenon, almost half of those causing death and injury were caused by human activity.

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What is a landslide?
A landslide is the movement of rock, debris or earth down a slope. They result from the failure of the materials which make up the hill slope and are driven by gravity. The most common types of landslide in Australia are earth slides, rock falls and debris flows. Landslides can be triggered by natural causes or by human activity.

What causes landslid
landslide prone ledge
landslide prone ledge

In general, the factors which influence whether a landslide will occur typically include slope angle, climate, weathering, water content, vegetation, overloading, geology and slope stability.

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The basic types of landslide movement are:

The rapid rate of movement with the descent of material. Falls are commonly triggered by earthquakes or erosion processes.
The tilting of rock without collapse, or by the forward rotation of rocks about a pivot point. Material descends by abrupt falling, sliding, bouncing and rolling.
This is the most destructive and turbulent form of landslide. Flows have a high water content which causes the slope material to lose cosistency. They are channelled by the landscape and move rapidly.
This is one of the most common forms of failure. Slides are sometimes called slumps because they move with rotation. They have a moderate rate of movement and the unity of material is retained, moving largely intact or in broken pieces.
The gradual lateral displacement of large volumes of distributed material over very gentle or flat terrain. Failure is caused when sediment is lost with little or no cohesion such as sands or silts are transformed into a liquid like state. This process is triggered by rapid ground motion most commonly during earthquakes.
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Where landslides occur:

Landslides occur regularly in a variety of landscapes
Fresh Scar
Fresh Scar
across every Australian State and Territory. These landscapes are commonly characterised by cliffs, unstable steep or gentle slopes and slopes subjected to intense or prolonged rainfall.
Landslide prone areas can include locations which have previous evidence of landslide activity. This evidence can be a fresh scar in the landscape.
Other landslide prone areas commonly include slopes made up of low strength, sensitive, collapsible, weathered or loose material.

A few landslide prone areas in Australia include:
  • Landslide Prone areas in Australia
    Landslide Prone areas in Australia
    The Great Dividing Range (NSW)
  • Strezelecki and Otway Ranges (Southern VIC)
  • Mt Lofty Ranges (near Adeleide)
  • Mt Wellington, Tamar Valley and north-west coast of Tasmania
  • New South Wales and Victorian Alps
  • Illawarra Escarpment (near Wollongong)
  • Sydney’s northern beaches and the city’s hinterland
  • Lake Macquarie and Newcastle suburban areas
  • Coastal hinterland of New South Wales
  • Regions of Townsville, Cairns and Mt Tambourine in Queensland.

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What causes landslides:
The factors which influence whether a landslide will
Devastated Homes
Devastated Homes
occur include slope angle, climate, weathering, water content, vegetation, overloading and slope stability.
Humans have an affect on these factors by altering natural processes.

Natural Causes Include:
  • Water saturation of slope material from either intense or prolonged rainfall and seepage
  • Undercutting of cliffs and banks by waves or river erosion
  • Vegetation Erosion from root penetration

Human causes Include:
  • Removal of vegetation
  • Interference with, or changes to, natural drainage
  • Leaking pipes
  • Modification to slopes e.g. roads, buildings, etc.
  • Overloading slopes
  • Mining and quarrying
  • Vibrations from heavy traffic or constuction
  • Excavation etc.
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1996 Western Australia Gracetown Cliff collapse
Gracetown, WA
Gracetown, WA

The community of Gracetown is located about 150 miles south of Perth. With a population of only a few hundred, the community is accustomed to a quiet style of living. But on Friday, the 27th of September, 1996, this community became the center for a terrible tragedy which would cost the lives of spectators at a school surf carnival at Cowaramup Bay, when a 14 metre high limestone sea-cliff collapsed. The spectators had been sheltering from rain under an overhang when about 30 tonne of rock and sand fell.

Environmental Cause:
The cliff face had the capacity to absorb 30-40% water with the more sandy materials having higher absorbing capacities. It was said the trigger for the rockfall was the heavy rainfall on the 27th, which increased the weight of the rock and sand mass by up to 40%’. This caused the overhang to overload and fall.

Economic Impacts:
The hospital contacted, implemented their Disaster Plan, which included the calling of ambulance
Rockfall onto the Shortland Esplanade, Newcastle, 28 October 2002
Rockfall onto the Shortland Esplanade, Newcastle, 28 October 2002
recruits and the Western Australia Health Service and Fire Brigade and State Emergency Services to arrange to set up a field Command Post and crowd control. The hospital morning staff was held back and all off-duty staff members were placed on duty. Heavy digging equipment was brought in the next day and later that same morning, counselors who had responded to the tragedy set up a base in the Community Center to offer counseling to the people affected by the tragedy. The incident ended up costing over $2 million accounting for emergency responses, emergency managment personnel and equipment. The family of the victims could also be facing a legal bill of more than $1 million after failing to prove governments were liable for the tragedy.

The total direct cost of landslides in Australia for the period from 1967 to 1999 is estimated at $40 million.

Environmental Impacts:
Landslide-derived sediment cause interuptions in stream channels that impact water reservoirs or fish habitats. A significant increase in the incidence of landslides is the consequence of the removal of vegetation by a rapidly expanding population of rabbits. Costs of controlling the rabbit numbers and preventing further landslides are estimated to be $24.6 million.

Social Impacts:
Five adults and four children were killed and another three people were injured. One survivor, a 10 year old girl, was dug out from beneath the rubble by the emergency workers after being trapped for 90 minutes. She received relatively minor injuries. Counselling was implemented which included not only friends and relatives of the victims, but also many emergency workers and volunteers who suffered from critical incident stress from working at the scene.

Since 1842 there have been approximately 84 known landslide events, collectively responsible for the deaths of at least 107 people and injury to at least 141 people.

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Management stategies:

The States and Territories have principal
Landslide Progression
Landslide Progression
responsibilitycThe arrangements within each State and Territory are well established and implemented, which have improved cover time. This has been achieved through a move towards integrated whole-of-government response management which focus on community safety.

Your role:
• Request information and assistance from your Local Government Authority prior to land purchase or construction. This information could include past landslide activity and any known landslide risk assessments.
• Consult a geotechnical engineer or engineering geologist for advice concerning development and slope instability.
• Do not undercut steep banks, develop near the top or base of steepslopes, or place pressure or load on steep slopes.
• Do not stand or seek cover below or near coastal cliffs or overhangsand be aware of potential dangers they represent. Take notice of signs giving warning of loose rocks and debris.
• Learn more about the geological hazards in your area and become familiar with tell-tale signs of ground movement.
• Contact local Emergency Services, police or local council.
• Inform affected neighbours.
• Evacuate.

Retaining walls and structures can help retain movement in the earth to prevent future landslides.

Landslide near Dungog - NSW
Landslide near Dungog - NSW

The tell-tale signs of ground movement are:
  • Saturated ground or seeps in areas that aren't usually wet.
  • New cracks and scarps or unusual bulges in the ground.
  • Movement of additional structures such as decks and patios in relation to a house.
  • Sticking doors and windows, and visible open spaces.
  • Tilting or cracking of concrete foundations.
  • Broken water lines and other underground utilities.
  • Leaning telephone poles, trees, retaining walls or fences.
  • Sunken or displaced road surfaces.
  • Rapid increase in creek water levels, possibly accompanied by increased soil content

It is important for engineers and geologists to evaluate slope stability and any landslide threat during development assessments so that effective and timely remedial measures can be implemented.