Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopAPPENDIX 1. CALCULATION OF LANDSLIDE RETURN INTERVALSThe return intervals of the Tumbledown and Carne Landslides were estimated using the volume ofthe incised section of the Nattai valley as an indicator of erosion over time by landsliding. In doingso, we are assuming that landslides are the dominant erosional process leading to valley wideningonce a knickpoint has passed, and other processes such as slope wash have a negligible effect onerosion rates. The volume of the incised Nattai valley was determined by GIS (ArcMap, ArcHydro)using a 25 m digital elevation model, clipped to the top of the cliff line. The timeframe of valleyerosion is indicated by Oligocene and Miocene basalts (Wellman and McDougall, 1974; van derBeek et al., 2001) situated on the plateau in the Nattai and Grose River valleys which show thatvalley incision post-dates their extrusion.Tumbledown LandslideVolume of Tumbledown Landslide 1.5 M m 3 = 0.0015 km 3Volume of the incised Nattai valley 21.3 km 3No. of landslides of this size required to erode the incised Nattai 14,200valleyTimeframe of erosion14 – 34 MaEstimated return period of 1.5 M m 3 landslide1,000 – 2,500 yearsCarne LandslideVolume of Carne Landslide 30 M m 3 or 0.03 km 3Volume of the incised Nattai valley 21.3 km 3No. of landslides of this size required to erode the incised Nattai 710valleyTimeframe of erosion14 – 34 MaEstimated return period of 30 M m 3 landslide20,000 – 50,000 years108