Appendix C

What are landslides?

1. Landslides are a worldwide phenomenon. They range from extremely minor examples of boulders being dislodged, through to significant catastrophes that result in tens of thousands of deaths,¹ widespread damage to the environment and cultural heritage,² and significant economic loss.³
2. There are a range of definitions used nationally and internationally to describe landslides. Geologists, engineers, planners and emergency management professionals all use slightly different or overlapping definitions.⁴ In general, ‘landslide’ can be defined as the “downslope movement of soil, rock and organic materials”, under the “effect of gravity and also the landform”.⁵ Terms such as landslip, mass movement and slope failure are interchangeable with landslide.⁶

Why do landslides happen?

3. Landslides can be ‘triggered’ by a range of factors, including soil moisture. Indeed, the US Geological Survey considers that “slope saturation by water is a primary cause of landslides”.⁷ Research into known-causes of landslides in the European Union reinforces that conclusion, on the basis that landslides “commonly [occur] due to extreme weather events, such as heavy rainfall and floods”.⁸
4. Slope or soil saturation can also be caused by changes in ground water levels and surface water levels. For this reason, flooding and landslides are often closely related, with landslides occurring as a consequence of floods, because of their links to precipitation, runoff and saturation of the ground by water.⁹
5. Landslides can occur quickly in seconds, or take place over an extended period. They represent a specific hazard in mountainous, coastal and steeply banked areas, with their “impact depend[ent] largely on their size and speed”.¹⁰ The extent or outcome of landslides can also be vastly different. They can vary in size from a boulder falling a few metres down a hill to millions of cubic metres of material travelling kilometres into large towns and cities.
6. There are a range of preparatory factors and features in a landscape that can contribute to slope failure and land sliding.¹¹ These include the basic geology of the environment, steepness of the terrain, vegetation, groundwater conditions, and exposure to weathering, deforestation and erosion. Landslides are also often the result of natural weather events such as earthquakes, bushfires or torrential storms due to their destabilising impact on the terrain.
7. Human activities can also impact the likelihood and severity of landslides. Expanding populations, both into new areas, or enlargement of existing residential areas, have been identified as a primary means through which people contribute to landslides, especially where that change leads to “[d]istributing or changing drainage patterns”.¹² Potential factors that may contribute to landslide susceptibility include, but are not limited to, loading the slope by filling or discharging water (such as from leaking pipes), unsupported cuts or the removal of vegetation.¹³
8. The United States Geological Survey also notes that landslides can occur in areas that were “once stable…due to other human activities such as irrigation, lawn watering...leaking pipes and improper excavating or grading on slopes”.¹⁴ However, an area that may be at risk of landslide or has susceptible geology, may still be considered appropriate to develop or build on, subject to landslide zoning analysis, engineering factors, and a consideration of the impact of building on the natural environment.
9. Importantly, for the purposes of this Report and consistent with scientific and emergency management discourse in Victoria, landslides are related but distinct from avalanches and coastal erosion. Avalanches involve the movement of snow and/or ice from mountain slopes. Coastal erosion involves the removal of sediments to bedrock from the shoreline through waves, currents and land subsidence, which is often expedited by extreme weather events such as coastal storms, surges, flooding and tsunamis.