Hallett Cove has been subject to many mild-moderate landslides in the past 100 years although, no official records are kept for mild-moderate graded landslides in this region.7 The most significant landslide that has occurred in Hallett Cove occurred in 1996 at the southern end of the esplanade. Marion council intervened before the public could be harmed although, it cost the council $700,000 in intervention costs.9
Figure 2: Hallett Cove Landslide Front (Source: Travis Hart, 2016)
1996 Hallett Cove Landslide
The 1996 Hallett Cove landslide is the largest known coastal landslide to have occurred within South Australia; in Precambrian rocks (>600 million year old rocks) for over 50 years. The 1995-1996 summer drought caused significant cracks and micro-faults along the esplanade between lots 30 and 31 preventing any usage of the road and walkway.9 Adjacent to the esplanade, the Quartzite and Slate (strong rock) dominant cliff began to show signs of degradation and an instability assessment conducted by Marion council concluded that, an extremely high risk of landslide was present. As a result, the immediate area was cordoned off for safety reasons and Marion council purchased the houses on lot 30 and 31 to demolish them before the landslide occurred, as seen in figure:3. Heavy and prolonged rainfall during the 1996 winter caused ‘swelling’ of the earth, both on the surface and in the cracks on he esplanade allowing the water to soak deep into the ground; this ‘swelling’ was the most probable cause of the landslide although, without the combination of drought prior to this, the effect would have been lesser.9
Figure 3: Hallett Cove Landslide Birdseye View (Source: Google Maps, 2016)
Figure 3 above, shows the mass of rock that was displaced by the 1996 Hallett Cove landslide (shaded red area), and the current cliff edge (purple line). Also noticeable is lots 30 & 31 which have since been demolished and replaced with vegetation, as vegetation plays a key role in increasing ground stability.
Figure 4: Simplified Hallett Cove perspective sketch (Source: Travis Hart, 2016)
Figure 4 above, is a sketch of the landslide location and the associated geological components of the area and the landslide itself. Houses 30 and 31 shown on the sketch were demolished for safety reasons. The ‘slide line’ (red line) is where the fault between the quartzite and slate rock is located. This fault is observed in Figure 5 below.
Figure 5: Cross section of Hallett Cove Landslide (Source: Sanders and Moon, 2007)
Shown above, is the fault underneath the cliff where the quartzite and slate rock layers meet forming the base of the landslide. These two rock types are composed differently and as such, create a fault where the two meet. This feature was the most significant factor that permitted the occurrence of the landslide.9
Figure 6: Hallett Cove landslide boulder terrace (Source: Travis Hart, 2016)
Figure’s 6 and 7 show the layered sediments from the 1996 Hallett Cove landslide. The picture shows how the mass of rock and dirt that once resided within and on top of the cliff. Thousands of tons of material were displaced, showing the force and weight behind a landslide can be significantly destructive. Luckily there was no members of the public or infrastructure below the cliff.
Figure 7: Hallett Cove landslide wave cut platform (Source: Travis Hart, 2016)
No other severe landslides have been recorded within Hallett Cove since the 1996 landslide. Within close vicinity to Hallett Cove, smaller landslides have occurred. No other severe landslides have been recorded within Hallett Cove since the 1996 landslide. Within close vicinity to Hallett Cove, smaller landslides have occurred at a number of locations between O’Sullivan Beach and Aldinga Beach.9
Hallett Cove Landslides 