6. Modelling of Sections A – D and Discussion
6.6 Section D
Section D is located near the centre of the field area at Redcliffs, approximately 25m northwest of, and parallel to, School Hall Section E (Figure 6.1). The location of Section D has been chosen as it provides a section similar to Section E in orientation and proximity to buildings, the base of the section being ~15m from two residential dwellings. It displays a steeper lower cliff face than Section E, caused by a much lower talus apron at the base of the slope and a correspondingly greater maximum drop height (H*=54m). Another significant feature of Section D is that the toe of the slope is unobstructed, in contrast to the rock bund, rockfall fence, and other structures that intercepted the path of rockfall at Section E.
6.6.1 Engineering Geology Model
The schematic engineering geology model of Section D is shown in Figure 6.11. From this figure, the exposed basal lava flows are prominent, comprising the lower ~27m of the cliff face. Overlying the lavas, the orange tuff unit shows the best evidence of the bench feature
that is visible in various places throughout the cliff face. The influence of this feature is discussed in Sections 6.6.2 and 6.6.3. Above the orange tuff, the welded ignimbrite unit is ~10m thick, with brecciated ignimbrite comprising the remaining ~20m above the welded unit. The main part of the talus slope is ~28°, with the key feature being the short horizontal distance this feature covers (<24m from the start of the talus apron), when compared to other sections.
6.6.2 Runout Analysis
From Figure 6.12 and Table 6-11, it can be clearly seen that by volume and distance, rockfall runout for Section D far exceeded the toe of the modelled slope. The maximum distance travelled by modelled rock was ~69m, with ~85% of all modelled rocks exceeding the toe of the slope. The distribution of rockfall runout is dominated by horizontal end-points between ~46 - 69m from the zero horizontal datum. This area represents the flat area at the base of the slope, modelled as grassed soil.
The high volume of rock exceeding the modelled toe of the slope shows the influence that high source area elevation (>57m AGL) and a very steep (~67°) cliff face have on rockfall runout. The results from 2D RocFall™ modelling for this section line do not correlate well with field observations, however as discussed in Section 6.7, this is considered to be the result of a high maximum drop height to talus over horizontal slope distance ratio, and is useful in predicting maximum runout potential at Redcliffs, given the cliff geometry.
Table 6-11: Summary runout results for modelled Section D at Redcliffs Mean distance
travelled (m)
Maximum distance travelled (m)
% of rocks passing measured toe of slope
54.0 69.0 85.0
Figure 6.12: RocFallTM modelling output showing Horizontal Location of Rock End-points for Section D 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.
6.6.3 Kinetic Energy and Bounce Height Analysis
The maximum kinetic energy recorded at the toe of the modelled slope for Section D was 8,500,000J (8,500kJ) (Table 6-12). This value represents the largest kinetic energy of any of the modelled sections at Redcliffs (including Section E), the implications of which are discussed in the following section.
Table 6-12: Total kinetic energy summary at the toe of the slope (~43m from the zero datum) for Section D, Redcliffs.
Modelled Boulder Mass (kg) Mean kinetic energy at x=47m (J) Maximum kinetic energy at x=47m (J) % of rocks passing measured toe of slope 500 51,000 210,000 85.0 5,000 510,000 2,100,000 85.0 20,000 2,100,000 8,500,000 85.0
Maximum bounce heights recorded from modelling Section D show an overall maximum height of 26m above the slope surface at ~24m from the horizontal zero datum (Figure 6.13 and Table 6-13). This location corresponds to the near vertical (~82°) lower part of the cliff face where the basal lava flows are exposed, and therefore is not strictly a bounce height. At the toe of the modelled slope, the maximum bounce height was ~8m. This value from the base of the slope is also far higher than from any other modelled section at Redcliffs, and the implications of this are discussed in the following section.
Table 6-13: Summary of bounce height envelope data for all modelled boulder masses for Section C, Redcliffs. Maximum bounce height
above slope (m)
Maximum bounce height at toe of slope (m)
26.0 (241) 8.0 (471)
Figure 6.13: RocFallTM modelling output showing the Bounce Height Envelope for Section D, 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.