Section B is located on a southeast-facing section of the cliff in an area where very large blocks (up to 250m3) of basaltic ignimbrite have come to rest along the talus slope (see Figure 4.3). The section line is ~40m from a building, but is at a different orientation such that the structure is not within the runout zone of the section when modelled in 2D by RocFall™.
6.4.1 Engineering Geology Model
Figure 6.5 shows the schematic engineering geology model for Section B at Redcliffs. The lower 10m of the talus slope was obscured from view of the TLS due to overhanging trees, so
this part of the slope profile was inferred from aerial photographs and field observations, as indicated by the dashed line on Figure 6.5.
Figure 6.4: RocFallTM modelling output showing the Bounce Height Envelope for Section A, Redcliffs. Upper plot shows the maximum bounce height of all modelled rocks as a line graph, while the lower plot represents the paths travelled by each of the 100 rocks sampled. Note that the vertical axis of the lower plot has been reduced post- modelling, so the slope of the profile appears flatter than actually measured.
The talus slope reaches ~25m above ground level (AGL) in this Section, obscuring the basal lava flows, the orange tuff, and the lower welded basaltic ignimbrite unit. Thicknesses of these units have been interpreted from limited exposures adjacent to the section line. Above the talus, the basaltic ignimbrite unit comprises the ≥15m exposed part of the cliff at Section A, with that showing brecciated ignimbrite apart from a ≤5m thick welded ignimbrite band near the top of the exposure. The overall gradient of the talus slope is ~33°, however the slope is considerably steeper in the upper 8m of the talus slope where a number of very large boulders have been stopped at the top of the talus slope.
6.4.2 Runout Analysis
As shown in Figure 6.6 and Table 6-5, modelled rockfall runout data provided results very similar to runout observed from field observations and aerial photograph interpretation. The maximum distance travelled by a modelled rock was ~44m. Therefore 100% of modelled runout stopped within the limits of the measured profile, consistent with field observations (Figure 6.5).
As shown by Figure 6.6, the distribution of rocks along the length of the modelled slope reflects rocks stopping along areas of the slope where the gradient was lower. Some 73% of modelled rocks came to rest less than 15m (horizontally) from the zero datum, and up to ~93% had stopped before 25m from the zero datum (horizontally). These two zones, particularly the area from 5-15m horizontally from the zero datum, represent lower gradient
areas of the slope, which obviously facilitates the entrapment of rock. This is consistent with field observations, as the area ~5m from the horizontal datum represents the top of the talus slope. The modelled results therefore reflect the distribution observed in the field.
Runout did not exceed the measured toe of the slope during modelling due to the large rock catch area created by the low-gradient upper talus surface. This is caused by the very large blocks that have fallen prior to March 2011, which act to contain a large volume of material near the top of the talus.
Table 6-5: Summary runout results for modelled Section B at Redcliffs. Mean distance
travelled (m)
Maximum distance travelled (m)
% of rocks passing measured toe of slope
13.5 43.7 0.0
Figure 6.6: RocFallTM modelling output showing Horizontal Location of Rock End-points for Section B at Redcliffs. Upper plot shows rock end-point distribution graphically as a bar graph, while the lower plot represents the paths travelled by each of the 100 rocks sampled. The green slope on lower graph shows the material has been modelled as vegetated soil cover using RocFall™ default parameters. Note that the vertical axis of the lower plot has been reduced post-modelling, so the slope of the profile appears flatter than actually measured.
6.4.3 Kinetic Energy and Bounce Height Analysis
Total kinetic energy at the toe of the slope for Section B ranged from ~1,300 – 50,000J (1.3- 50kJ) for the 500, 5 000 and 20 000kg rocks modelled (Table 6-6). As with Section A, the increase in kinetic energy relates to the increase in boulder mass.
Table 6-6: Total kinetic energy summary at the toe of the slope (~43m from the zero datum) for Section B, Redcliffs.
Modelled Boulder Mass (kg) Mean kinetic energy at x=43m (J) Maximum kinetic energy at x=43m (J) % of rocks passing measured toe of slope 500 1,300 1,300 1.01 5,000 12,500 12,500 1.01 20,000 50,200 50,200 1.01
1Location of data collector for kinetic energy is ~1m upslope of the measured toe of the slope.
Maximum bounce height data for Section B are presented in Figure 6.7 and Table 6-7. From these data, it can be seen that the maximum bounce height of modelled rocks is ~10.5m above the slope surface, and this occurs at a break in slope where rocks bounce and fall over a near-vertical section of the cliff face. At the toe of the slope the maximum bounce height is zero, suggesting any rocks reaching thins point are rolling at this point on the slope.
Table 6-7: Summary of bounce height envelope data for all modelled boulder masses for Section B, Redcliffs. Maximum bounce height
above slope (m)
Maximum bounce height at toe of slope (m)
10.5 (6m1) 0.0 (44m1)
1
Figure 6.7: RocFallTM modelling output showing the Bounce Height Envelope for Section B, Redcliffs. Upper plot shows the maximum bounce height of all modelled rocks as a line graph, while the lower plot represents the paths travelled by each of the 100 rocks sampled. Note that the vertical axis of the lower plot has been reduced post- modelling, so the slope of the profile appears flatter than actually measured.