Rockfall Runout Analysis – Primary Modelling Phase Variables

Rockfall runout was the primary output used to calibrate theoretical models with observed field data. This section presents results of model testing where rockfall runout was affected by varying the “primary phase” input parameters. Rockfall bounce height and kinetic energy analyses are presented in Section 5.8.

5.7.1 Slope profile input effect

As discussed in Section 5.6, modifications to the originally measured TLS-derived slope profile were required following initial modelling. This section presents the results of initial modelling, Figure 5.10: Simplified slope profile. Vertical and horizontal axes are equal scale: Vertical field of view =~70m, horizontal distance =~150m. Note vertex colour represents the material used to model each section. Blue = “bedrock”, Red = “talus cover”, Black = “asphalt”. Discussion of these materials is in Section 4.6

and the output following modifications. Comparison of the TLS-derived slope profile and the simplified slope profile is covered in the following section.

Modified TLS-derived slope profile

Figure 5.11 presents a comparison of horizontal end-point data from the original TLS-derived profile and the modified TLS-derived slope profile. The distribution of rockfall runout showed significant variation between slope profile input conditions. The red lines in the lower section of both diagrams represent the paths travelled by rocks in the model. Both models represent 100 rocks being released from the source area.

Altering the slope profile caused marked changes in the modelling results. As seen Figure 5.11a, the original slope profile produced a “spike” where ~45% of rocks came to rest within the first 10m of the slope. Figure 5.11b shows the modified slope profile, where rocks pass over the near-vertical Source Area 1 and continue onto the lower slope region.

Because of slope modification to smooth the upper portion of the slope profile, the distribution of rocks along the length of the slope, particularly in the Runout Zone, are more

Figure 5.11: Comparison of horizontal end-point location for TLS-derived slope profile (a) and modified TLS-derived slope profile (b; Profile modified by smoothing Source Area 1 slope). Note vertical axes of runout plot compressed post-modelling. Horizontal axes represent the location in metres from the zero datum; vertical axes represent the number of rocks. Note vertical scale has been reduced post-modelling producing a shrunken scale, therefore graphs are not to natural scale. Input Parameters: Talus: Rn=0.32, Rt=0.82, phi=30, Slope Roughness=5, Bedrock: Rn=0.35,

Rt=0.85, phi=30, Slope Roughness=0; Dynamic initial conditions.

consistent with field observations (Figure 5.2). The modified slope has therefore been adopted for analysis, even though it may remove some of the actual roughness. Note that the vertical axes have been shrunk post-modelling, so scales are not at natural scale.

5.7.2 Initial Velocity Conditions

Comparing rockfall runout distances under dynamic and static initial velocity conditions showed a consistently higher runout (both mean and maximum distance) when rocks had initial velocity. Table 5-6 summarises the results of testing from the primary modelling phase, including testing the influence that initial velocity conditions had on modelled rocks.

Figure 5.12 shows modelled rocks that had initial velocity (horizontal velocity=1.5ms-1_{, vertical} velocity=1.0ms-1_{) generally had greater mean and maximum runout distances than those} rocks which started under static initial conditions (horizontal velocity=0.0ms-1_{, vertical} velocity=0.0ms-1_{). As expected the mean ruonut distance reduces significantly as the height of} initial fall reduces (i.e. from about 75m in Source Area 1 to about 30m in Source Area 3), but the reduction in maximum runout distance (while greater than the mean) does not show this change (from 100m in Source Area 1 to about 90m in Source Area 3). There is relatively little difference between the initial static and dynamic profiles, however, which is again most likely an artifact of the computer programme.

5.7.3 Angular Velocity

When the RocFall™ model calculates rockfall paths the default setting is to consider the influence of angular velocity on the path of the falling rock. When this influence was not considered, mean rockfall runout was consistently higher than default conditions from all three source areas (Figure 5.13).

The difference between mean runout values within each source area became smaller as the elevation of the source area reduced. This was interpreted to show the influence that source area elevation had on the time the angular velocity had to affect the modelled rock path. This

Table 5-6: Summary table of primary modelling phase results. Blue shading indicates RocFall™ default settings. Full data sets are provided in Appendix 6

PRIMARY MODELLING PHASE

Model Code Seeder Zone Initial Velocity Angular Velocity Considered Boulder Mass (kg) Max. Runout (m) Mean Runout (m) Max. Kinetic Energy at x=71m Mean Kinetic Energy at x=71m Bounce Height at x=71m (m, above slope)

5.1 Area 1 Dynamic Yes 5,000 102 77 9.7E+05 3.9E+05 2.5 

5.2 Area 2 Dynamic Yes 5,000 93 70 5.9E+05 2.9E+05 2.5 

5.3 Area 3 Dynamic Yes 5,000 86 36 4.3E+05 2.1E+05 1.0 

5.4 Area 1 Static Yes 5,000 98 75 8.2E+05 3.8E+05 4.0 

5.5 Area 2 Static Yes 5,000 89 60 5.4E+05 2.5E+05 3.0 

5.6 Area 3 Static Yes 5,000 85 29 2.7E+05 2.0E+05 0.5 

5.7 Area 1 Dynamic Yes 2,000 102 77 3.9E+05 1.5E+05 2.5 

5.8 Area 2 Dynamic Yes 2,000 93 70 2.4E+05 1.2E+05 2.5 

5.9 Area 3 Dynamic Yes 2,000 86 36 1.7E+05 8.5E+04 1.0 

5.10 Area 1 Dynamic Yes 500 102 77 9.7E+04 3.9E+04 2.5 

5.11 Area 2 Dynamic Yes 500 93 70 5.9E+04 2.9E+04 2.5 

5.12 Area 3 Dynamic Yes 500 86 36 4.3E+04 2.1E+04 1.0 

5.13 Area 1 Dynamic Yes 10,000 102 77 2.0E+06 7.7E+05 2.5 

5.14 Area 2 Dynamic Yes 10,000 93 70 1.2E+06 5.9E+05 2.5 

5.15 Area 3 Dynamic Yes 10,000 86 36 8.5E+05 4.2E+05 1.0 

5.16 Area 1 Dynamic Yes 20,000 102 77 3.9E+06 1.5E+06 2.5 

5.17 Area 2 Dynamic Yes 20,000 93 70 2.4E+06 1.2E+06 2.5 

5.18 Area 3 Dynamic Yes 20,000 86 36 1.7E+06 8.5E+05 1.0 

5.19 Area 1 Dynamic No 5,000 122 85 1.5E+06 5.8E+05 7.5 

5.20 Area 2 Dynamic No 5,000 101 76 9.3E+05 3.8E+05 5.0 

suggests that rocks from higher elevation source areas are more affected by angular velocity compared to rocks from lower elevations, but the height of fall is still the dominant influence.

Figure 5.12: Plot of dynamic versus static initial velocity conditions against mean runout distance (from zero datum).

5.7.4 Boulder Mass

Intuitively, the mass of a rock is an important input parameter to investigate due to the potential for larger rocks to cause more damage at the toe of the slope than smaller rocks due to their larger momentum and kinetic energy. This is based on simple physics in that if the rocks can initially gain the momentum required to convert potential energy into kinetic energy, then rocks with greater mass will travel at greater speed. This would create larger bounce heights and allow rocks to travel further, in addition to impacting barriers with greater force. All of these factors contribute to a greater hazard at the toe of the slope.

The mass of the rock does not affect rock runout or bounce height in RocFall™, rather it is only considered in calculating total kinetic energy (Stevens 1998), Results from testing the influence of boulder mass on rock runout proved there to be no relationship between boulder mass and rockfall runout (Table 5-6).