Terrain Stability Assessment In The Context of Forestry Operations

Terrain Stability Assessment (TSA) as it relates to forest harvesting operations is a process wherein an upcoming harvest area is evaluated on its hazards pertaining to land sliding/mass movement. While the process for TSA has undergone multiple changes with the advances in knowledge and technology over time, it is necessary to continue to evaluate them with regard to forestry operations to allow for optimal management decisions. To understand contemporary practise, interviews with nine different management companies in New Zealand highlighted an experience driven approach to the identification and management of unstable terrain. Ambiguity remains once unstable areas are identified, with respondents making the case for site specific evaluation and recommendations over blanket, nationwide prescriptions. Various assessment techniques were considered, with a goal of assessing the efficacy of implementing them under NZ steep slope plantation forestry conditions. Relevant techniques pertaining to the landslide issues faced in NZ forestry were compiled into one TSA process and are demonstrated via six case studies across New Zealand. This method involves three stages. Data collection and validation (LiDAR, geological information), modelling (susceptibility via empirical regression, ‘Spiekermann Model’; morphometric ratios, ‘Melton Ratio’), and field assessment. Overall, the TSA methodology demonstrated in the case studies captured landslide hazards specific to each site. Limitations and assumptions of the process are discussed, with areas of future study identified.