Protection Implications

Implications for protection measures based on rockfall modelling at Redcliffs are primarily related to the influence that source area height has on the behaviour of rocks at the toe of the slope. Through the detailed analysis undertaken (Sections 6.7.1-6.7.3), the two key parameters to consider when designing protection measures at Redcliffs are:

• Total cliff height over horizontal slope distance (H/x)

• Maximum drop height to talus over horizontal slope distance (H*/x)

For the H*/x parameter, two threshold values have been identified to estimate the ratio of maximum drop height to talus (H*) to horizontal slope distance (x) at which the percentage of rocks passing the toe of the slope, and the maximum bounce height at the toe of the slope become markedly different from observed limits. The following statements define this:

• If H*/x ≥0.6, modelling suggests that the percentage of rocks passing the current toe of the talus slope will exceed 5%; and

• If H*/x ≥0.75, modelling suggests that this could be an indication of significantly greater bounce height, as shown by the maximum bounce height at the toe of the slope of 8.0m for Section D, but ≤1.0m for all four other modelled sections, but this requires more data and analysis

For the H/x parameter, a threshold value has been identified for the percentage of rocks passing the toe of the slope. The following statement defines this value:

• If H/x ≥1.04, modelling suggests that the percentage of rocks passing the toe of the talus slope will be ≥5%.

A threshold value pertaining to maximum bounce height at the toe of the slope has not been identified. This suggests that the H*/x parameter is the primary control of this variable.

Kinetic energy has not been considered in the detailed analysis in Section 6.7 as this parameter is controlled by the mass of the falling boulder, a component that does not alter the trajectory of modelled rocks in RocFall™. This variable has been estimated in analysis of each modelled section line (Sections 6.3-6.6 and 5.8) however, and reference to these sections should be made prior to considering protection measures in any detail.

Selection and design of protection measures is not within the scope of this thesis. However, considering the large quantity of rocks modelled as exceeding the base of the talus slope, and the high bounce heights and kinetic energy values produced at the toe of the slope, a reinforced, earthen bund seems to be an appropriate device to contain or stop debris at the base of the talus apron. If required, a rockfall fence could be constructed at the top of the bund to increase the height of the protection structure. It should be noted that RocFall™ does not allow consideration of fly-rock trajectory, as fragmentation is not considered in the programme. The addition of a rockfall fence with appropriately spaced linkages and anchors could provide extra protection from fly-rock.

Analysis of further section lines would be necessary to better define the likely runout extent for boulders impacting the present talus apron as part of any design programme. It is

suggested that the computed runout distance should be initially derived from the RocFall™ programme, and that protection measures be designed for placement at this location. A more detailed hazard analysis should therefore precede a full design recommendation for a bunding system at this site, including further evaluation of long-term cliff stability. Evaluation of the suitability to use fallen rock in the construction of the bunding system would also be beneficial.

6.8 Synthesis

• Modelling of Section Lines A-D proved that the RocFall™ model developed through calibration at Section E was generally accurate when applied to other areas of the cliff at Redcliffs.

• Section Line D produced significantly higher results for maximum runout distance, percentage of rocks passing the modelled toe of the slope, and maximum bounce height at the toe of the modelled slope.

• Rockfall source area elevation with respect to the top of the talus apron was known to be the primary control on rockfall runout behavior as identified through testing of Section E. Further analysis of a selection of cliff parameters (total cliff height, H; maximum drop height to talus, H*; horizontal slope distance, x; H/x; and H*/x), led to the identification of a number of key parameters:

o _{A maximum horizontal distance to cliff height (H:V) ratio of 1.4H:1V was}

identified. This provides a maximum runout envelope giving an angle from the top of the cliff to the toe of the talus slope of 35°.

o _{The “maximum drop height to talus” over “horizontal slope distance”}

parameter (H*/x) exhibits the greatest control in modelled sections over the percentage of rocks passing the existing toe of the slope, and the maximum bounce height at the base of the slope.

o _{A threshold value of H*/x ≥0.6 suggests that the percentage of rockfall runout}

o _{A threshold value of H*/x ≥0.75 suggests that this could be an indication of}

significantly greater bounce heigh at the toe of the slope, as shown by the maximum bounce height of 8.0m for Section D, but ≤1.0m for all four other modelled sections, however this value requires more data and analysis to be of greater application

• The 1.4H:1V ratio can be applied (with additional consideration for fly-rock) to future land-use planning decisions, as is commonly used in other cities throughout New Zealand for similar mass-movement failures

• H*/x threshold values can be used in conjunction with the 1.4H:1V ratio in the design of rockfall protection measures at Redcliffs.