Landslip and erosion in the Yarra Ranges
The thousands of trees that fell in the June 2021 storms caused extensive damage to homes, roads, drains and the land across the Mount Dandenong (Corhanwarrabul) region.
Many of the areas affected by the storm are also prone to landslide. In some cases, the uprooting of trees and damage to retaining walls may have increased or introduced new risks from landslide.
To help protect residents and property from landslide, Yarra Ranges Council uses a planning control called an Erosion Management Overlay (EMO). This means that special measures need to be undertaken when rebuilding your property as the land you live on is steep and/or prone to landslides. These measures aim to keep you and your property safe.
What landslide risks?
Mount Dandenong is underlain by a rock called rhyodacite which formed through volcanic activity around 370 million years ago.
In its unaltered state, rhyodacite is a very high strength rock and resistant to erosion. The high strength of this rock is the reason Mount Dandenong is a mountain, with the softer rocks around it having eroded down at a faster rate over millions of years leaving Mount Dandenong higher than its surrounds.
However, the rhyodacite undergoes a process called chemical weathering whereby over time and exposure to the atmosphere the minerals comprising the rock transform to red-brown clay which overlies the rock. Within these red-brown clays are remnant rock or ‘corestones’ - rounded boulders of rhyodacite embedded within the clay.
The clay soils or ‘residual soils’ have a much lower strength than rock, and when on steep slopes are prone to landslide.
Mount Dandenong has a long history of landslides, with the first documented landslides occurring in the 1860’s and geological evidence indicating that the soils of Mount Dandenong have been affected by landslides for hundreds of thousands of years.
Landslides in the Mount Dandenong area are a natural geological process.
What factors lead to landslide?
Factors that cause landslide can be divided into Preparatory and Causative factors.
Preparatory factors describe the conditions that make a slope susceptible to landslide and include:
- the steepness of the slope
- lack of vegetation cover
- the thickness and type of soil
- poor drainage.
Causative factors are those that actually trigger the landslide and could include:
- heavy or prolonged rainfall and flooding
We can’t do much about causative factors, so good planning and development in landslide prone areas seeks to address the preparatory factors. This ensures that development does not adversely increase the susceptibility to landslide, therefore Yarra Ranges Council has EMO planning controls. In principle, the more we can preserve the natural landform, vegetation and drainage, the less susceptible the slopes will be to landslide.
Further information can be found in our Erosion and landslide management factsheet(PDF, 4MB) or on our EMO planning pages.