INFLUENCES ON ROCKFALL RUNOUT, A CANTERBURY EARTHQUAKE CASE STUDY

Topography and vegetation greatly influence rock fall runout trajectories. Recent rockfall investigations on Banks Peninsula suggest modern boulders have greater runout distances than those in paleo events. Goat Rock, Banks Peninsula, a basaltic dome, had significant failure during the 2010-2012 Canterbury Earthquake Sequence. Modern and paleo rockfall extends from source regions to the vegetated creek bed, classified into zones; 1) Talus with sporadic vegetation (<3 m height). 2) Track cut into slope with wire farm fence above and below. 3) Grassed hill paddock (average slope 27°) with Podocarp (Totara) tree stumps (average diameter 0.38 m) and felled trunks (average length 3.5 m) from late 1800’s forestry. 4) Densely vegetated boulder-rich creek bed.

Measurements of boulders, runouts, stumps, and trunks were compiled in ArcGIS map. Analysis indicates modern boulders concentrate in the talus and road sections, with 76% having runout distances <40 m. Paleo boulders of the grassy slope rest against and amongst Podocarp stumps and have varying upslope burial wedges. Paleo boulders in the talus are predominantly larger than the lower stream, with both populations varying in size, weathering and burial. Image analysis pre and post the 09/04/2010 earthquake identified two modern impact skid tracks which reached the base of slope. Bounce distances and bounce properties were calculated with heights of 0.7 m for a shallow bounce, 1 m for a normal bounce, and 1.3 m for a high bounce; impact velocities of 11 m/s; and rebound velocities of 8 m/s.

Concentration of modern boulders close to source reflects the influence of modern features. The track served as a bench in the slope, farm fencing (prior to breaking) acted similarly to dense vegetation, and in the talus zones dense vegetation and loose boulders absorbed impacts and arrested blocks. Paleo boulders are similarly arrested in the talus zone, more often among the remnants of the Podocarp forest. From these comparisons, we infer the pre-European Podocarp forest and dense native vegetation inhibited runout, arresting boulders in the upper-mid slope. Modern influences (tracks and fences) combined with regenerating bush can reduce runout distances; however, in the absence of impedance, boulders will have runout distances greater than paleo events.