ASSESSMENT OF ROCKFALL PROTECTION MEASURES AND RECOMMENDATIONS TO CONTAIN ROCKFALL IN URUMAU RESERVE, LYTTELTON

This report assesses the condition and efficacy of the rockfall protection fencing and the protective pine forest that have been implemented within Urumau Reserve, Lyttelton (NZ). Field observations suggest that these measures have been effective in mitigating rockfall hazard at the site, but improvements and additions to the existing system are required to accommodate future rockfall events. RAMMS (Rapid Mass Movement Simulation) numerical rockfall modeling simulations were conducted to understand the relative and combined influence of the protective fencing and the protective pine forest, determine optimal fence heights, and identify areas of potential future rockfall hazard. Simulations were run with varying vegetation cover and heights of fencing to understand the effects these two parameters have on rockfall dynamics. The model results highlight the primary importance of the protective pine forest in reducing rockfall runout and indicate that protective fencing is unable to accommodate the range of boulder bounce heights if the fencing height is ≤ 1.08 m. Based on our results, we recommend the following actions to contain future rockfall events within Urumau Reserve: (1) maintain or increase the current density of the pine forest, (2) replace sections of fencing with a height ≤ 1.08 m, (3) add an additional ~12 m of protective fencing in the southernmost section of the site, and (4) perform biannual site visits to inspect the condition of the fencing and the pine forest. Our study emphasizes the importance of revisiting rockfall sites to ensure protective measures remain resilient and in optimal condition.