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. This is relevant to current New Zealand conditions, where increasing frequency of weather events such as cyclone Gabrielle continue to put pressure on the forest industry to improve its practise, in order to maintain its social license to operate.

The Pacific Northwest (PNW) region (British Columbia, Washington, Oregon, and California) shares several similarities with New Zealand forestry in terms of landscape, harvesting methods, and growing crop. As such, they share many of the same issues - one being landform instability. Several of these PNW states have developed and tested TSA processes for new harvest plans. Within the scope of this project, these processes were reviewed. They generally involve input from a third party such as a geologist or geotechnical engineer, with prescriptions that include ‘leave areas’ of standing timber, buffer zones around waterways and alternate methods of road construction (such as minimising side-cast). New technology and methods are also affecting the way this region conducts TSAs. For example, Oregon now utilises modelling derived from LiDAR and ground information to prescribe their ‘leave areas’. Although having slightly different environment management goals/requirements to New Zealand, this study helps show that this process can be learned from.

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. While there may not exist a definitive ‘written approach’ to assessing terrain stability in New Zealand, many processes were similar across companies in terms of resources used and steps taken. Generally, this involves the utilisation of available LiDAR data (or contour) to generate maps in GIS showing slope, hill shade, as well as aerial imagery. Sites are then visited and ‘walked’, which involves looking for ‘problem’ features which may have been identified by previous mapping. 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 (structured similarly to an approach from the British Columbia Ministry of Forests and Environment). The process illustrates an approach that a forest management company within New Zealand would be able to replicate to help inform operational decision making. Overall, the TSA methodology demonstrated in the case studies did a fair job of capturing the landslide hazards specific to each site. Combining initial modelling and a subsequent field visit allowed for the identification and recording of shallow slip and debris flow behaviour – however, some sites presented different erosion considerations. Finally, these limitations are discussed as well as any assumptions made throughout the process, with areas of future study identified.