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Item Towards empirically validated models of soft-rock landslides' occurrence, activity, and sediment delivery : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Manawatū, New Zealand(Massey University, 2023) Williams, ForrestWithin New Zealand, soft-rock landslides present a severe hazard to infrastructure and contribute to the degradation of river systems by delivering large amounts of sediment to waterways. Updates to New Zealand’s national policy statement for freshwater management necessitate accurate accounting of freshwater sediment sources, but current sediment budget models do not account for the sediment inputs from soft-rock, and other large slow-moving landslides. To understand which factors lead to the occurrence and continued activity of these landslides and the role they play in New Zealand’s river sediment dynamics, I have completed the following objectives. (i) I have mapped large landslides within the Whanganui-Rangitikei soft-rock hill country in the North Island of New Zealand and conducted a geostatistical analysis to determine which factors control their occurrence. (ii) I have developed a novel remote sensing framework for monitoring large, slow-moving landslides that is based upon time-series Interferometric Synthetic Aperture Radar (InSAR) and time-series sub-Pixel Offset Tracking (sPOT) analyses. Furthermore, I have shown that this framework can identify large landslide activity with an accuracy of 91% and measure the movement of landslides moving with an average velocity of 2.05 m/yr with a mean absolute error of 0.74 m/yr. (iii) I have applied this framework to the landslides of the Whanganui-Rangitikei soft-rock hill country and used its results to perform a geostatistical analysis to determine which factors control a landslide’s current activity state and to estimate the total sediment mass delivered by soft-rock landslides to the rivers of this region. In total, I mapped 1057 large landslides in this region and identified 66 of them as currently active. I find that low slopes, river incision, alignment between bedding planes and slopes, and forest cover are predictive of landslide occurrence, but that low slopes and high annual precipitation rates best predict the current activity states of these landslides. I also find that soft-rock landslides contribute a 10±2% of the total sediment mass delivered to the river systems of this region. Overall, this thesis advances our understanding of why soft-rock landslides occur and provides a framework that will allow future studies to monitor these landslides at region to country-wide scales.Item Quantifying the performance of silvopastoralism for landslide erosion and sediment control in New Zealand’s hill country : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Physical Geography at Massey University, Palmerston North, New Zealand(Massey University, 2022) Spiekermann, RaphaelLandslide erosion results in loss of productive soils and pasture. Moreover, sediment delivered to streams from landslides can contribute to the degradation of freshwater and marine receiving environments by smothering benthic habitats and increasing turbidity, light attenuation, and sediment-bound contaminants. Silvopastoralism is an important land management practice used to combat landslide erosion and improve the health of downstream aquatic ecosystems. Yet, the effectiveness of widely spaced trees in reducing shallow landslide erosion and sediment delivery at hillslope to catchment scales remains largely unknown. Previous studies have been limited by scale (e.g., hillslope) or method (e.g., univariate analyses). This research aims to develop spatially explicit modelling to assess the impact of differing tree species, planting densities, and individual tree location, on rainfall-triggered landslides and sediment delivery while accounting for varying environmental conditions, such as slope gradient, lithology, or soil type. As such, this thesis combines geospatial methods and statistical models to address key challenges related to erosion and sediment control in New Zealand’s pastoral hill country. First, using a study area in the Wairarapa, located in the southeast of the North Island, New Zealand (840 km2), a method was developed using open-source remote sensing products to generate high-resolution individual tree influence models for the dominant tree species. The objective was to generate a spatial explicit representation of individual trees for landscape-scaled statistical slope stability modelling. The combined hydrological and mechanical influence of trees on slopes was inferred through the spatial relationship between trees and landslide erosion. These spatial distribution models for individual trees of different vegetation types represent the average contribution to slope stability as a function of distance from tree at 1-m spatial resolution. The normalised models (0-1) largely agree with the shape and distribution of force from existing physical root reinforcement models. Of exotic tree species that were planted for erosion and sediment control, poplars (Populus spp.) and willows (Salix spp.) make up 51% (109,000) of trees located on hillslopes at a mean density of 3 trees/ha. In line with previous studies, poplars and willows have the greatest contribution to slope stability with an average maximum effective distance of 20 m. Yet, native kānuka (Kunzea spp.) is the most abundant woody vegetation species on hillslopes within the study area, with an average of 24 trees/ha, providing an important soil conservation function. A large proportion (56% or 212.5 km2) of erosion-prone terrain in the study area remains untreated. In a world-first, this allowed the influence of individual trees to be included in a statistical landslide susceptibility model using binary logistic regression to quantify the effectiveness of silvopastoral systems at reducing landslide erosion and to support targeted erosion mitigation. Models were trained and tested using a landslide inventory consisting of 43,000 landslide scars mapped across the study area. Model performance was very good, with a median Area Under the Receiver Operating Characteristic curve (AUROC) of 0.95 in the final model used for predictions, which equates to an accuracy of 88.7% using a cut-off of 0.5. The effect of highly skewed continuous tree influence models on the maximum likelihood estimator was tested using different sampling strategies aimed at reducing positive skewness. With an adequate sample size, highly skewed continuous predictor variables do not result in an inflation of effect size. Application of the landslide susceptibility model was illustrated using two farms from within the study area (Site 1: 1,700-ha; Site 2: 462-ha) by quantifying the reduction in shallow landslide erosion due to trees. Compared to a pasture only baseline, landslide erosion was reduced by 17% at Site 1 and 43% at Site 2 due to all existing vegetation. The effectiveness of individual trees in reducing landslide erosion was shown to be less a function of species than that of targeting highly susceptible areas with adequate plant densities. The excellent model performance means spatial predictions are precise, which has implications for land management as the maps provide greater certainty and spatial refinement to inform landslide mitigation. The terrain occupied by the “high” susceptible class – defined as the terrain where 80% of mapped landslides were triggered in the past – occupies only 12% of Site 1 and 7% of Site 2. This suggests there is great potential for improved targeting of erosion mitigation to these areas of the farms where landsliding may be expected in the future. To enable biological mitigation to be targeted to critical source areas of sediment, determinants of sediment connectivity were investigated for a landslide-triggering storm event in 1977. In a first of its kind, a morphometric landslide connectivity model was developed using lasso logistic regression to predict the likelihood of sediment delivery to streams following landslide initiation. An experiment was undertaken to explore a range of connectivity scenarios by defining a set of sinks and simulating varying rates of sediment generation during runoff events of increasing magnitude. Sediment delivery ratios for the 1977 event ranged from 0.21 to 0.29, equating to an event sediment yield of 3548 t km-2 to 9033 t km-2. The likelihood of sediment delivery was greatly enhanced where debris tails coalesce. Besides scar size variables, overland flow distance and vertical distance to sink were the most important morphometric predictors of connectivity. When scar size variables were removed from the connectivity model, median AUROC was reduced from 0.88 to 0.75. By coupling landslide susceptibility and connectivity predictions in a modular form, we quantified the cost effectiveness of targeted versus non-targeted approaches to shallow landslide mitigation. Targeted mitigation of landslide-derived sediment was found to be approximately an order of magnitude more cost-effective than a non-targeted approach. Compared with a pasture-only baseline, a 34% reduction in sediment delivery can be achieved by increasing slope stability through spaced tree planting on 6.5% of the pastoral land. In contrast, the maximum reduction achievable through comprehensive coverage of widely spaced planting is 56%. The coupled landslide susceptibility and connectivity predictions (maps) provide an objective basis to not only target mitigation to areas where future shallow landslides are likely to occur, but – perhaps more importantly – target future tree planting to locations that are likely to be future sources of fine sediment. In this way, the research presented in this thesis is both methodologically novel and has immediate application to support land management decisions aimed at creating a more sustainable socio-ecological landscape.Item Quantifying hillslope response to glacier retreat : landslide mechanics, processes and impacts : a dissertation presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physical Geography at Massey University, Palmerston North, New Zealand(Massey University, 2022) Cody, EmmaLandslides are a natural hazard experienced in all countries around the world. Their formation is heavily influenced by internal and external processes including geology, earthworks, rainfall and in some instances glacier ice. This thesis investigates the response of alpine hillslopes to glacier retreat through the utilisation of movement monitoring techniques, subsurface investigations and geotechnical slope stability analysis. Three objectives are proposed and guide the research outcomes of this thesis. The first objective is to investigate the underlying preconditions, preparatory factors and triggers which control and enhance paraglacial rock slope failures through the study of the Mueller Rockslide in New Zealand’s South Island. Preconditioning and progressive weakening of the rockslide began over 8000 years prior to current glacier retreat, with the rockslide forming ~7500 years ago. Movement monitoring shows the rockslide to accelerate gradually over its history, culminating in maximum movement rates of ~6 m per year. Movement is constrained along structurally conditioned discontinuities, identified through a novel ground investigation combining geophysical and geotechnical techniques. Glacier debuttressing is considered a primary trigger of controlled slope failure, with increased displacement in spring attributed to snow melt and rainfall. Step-path failure of the rockslide is ongoing with tensile stress accumulating in the landslide body leading to segmentation of the rockslide into four zones. Ongoing progressive failure will likely lead to block sliding of the rockslide. The second objective of this thesis was to identify drivers of movement in paraglacial sediment slopes. A novel suite of monitoring techniques was used at Fox Glacier which showed clear correlation between hillslope failure and glacier retreat and downwasting. Unlike previous studies, this thesis found debuttressing to be the primary trigger of slope failure with rainfall acting as an accelerant. Debris flows, once thought to be a dominant process in sediment slope adjustment did not begin until debuttressing was completed. Finally, this thesis investigated the potential impacts of paraglacial landsliding on the broader environment. Both the rock and sediment slope failures in this study have deformed their supporting glaciers and a combination of monitoring techniques have uncovered to how sediment is delivered to the proglacial zone. Due to both sites being located within a high seismic hazard zone, catastrophic failure of both sites is possible. Continued failure of the Mueller Rockslide will likely result in a landslide dam and will also lead to continued retrogression of the headscarp and weakening of the surrounding hillslope. Continued failure of the sediment slopes at Fox Glacier will contribute large volumes of sediment to the proglacial zone, allowing for remobilisation of sediment in flood conditions. Both landslides have apparently created morphological change in their supporting glaciers. A unique combination of monitoring methodologies was used to provide insight into landslide movement and evolution, but their usage should not be limited only to studies of landslides. The methodologies used in this thesis provided context across a range of spatial and temporal scales.Item The influence of rainfall and river incision on the movement rate of a slow-moving, soft-rock landslide in the Rangitikei, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Physical Geography at Massey University, Manawatu Campus, Palmerston North, New Zealand(Massey University, 2018) Holdsworth, Charlotte NaomiThe Rangitikei Slide, a slow-moving landslide near Taihape, New Zealand, was monitored to determine the movement patterns and identify the primary movement drivers. The sediment delivery of landslide material to the Rangitikei River was also estimated to inform the sediment yield from slow-moving landslides connected to a fluvial system. RTK-dGPS monitoring, photogrammetry, and pixel tracking of time-lapse imagery was used to categorise movement patterns, and pixel tracking at different temporal resolutions (weekly and hourly) in conjunction with environmental data identified the drivers and classified the influence on movement. The findings aimed to improve the understanding of these landslide types in New Zealand in order to propose more effective management strategies both locally and around the world. It was found that the landslide comprised several blocks exhibiting different movement rates, and that movement was influenced by a seasonal trend likely from groundwater fluctuations increasing pore pressures in the landslide mass. River erosion by the Rangitikei was identified as a key movement driver and has likely influenced movement since landslide initiation. This was supported by historic aerial imagery and photogrammetry, which showed that the landslide has preserved historic movement phases and these indicate fluvial influence. The estimation of sediment contributions found that ~19,000 t/year of sediment is entering the Rangitikei River from the toe, which is considered a conservative estimate. This contribution is substantial; the Rangitikei Slide is producing almost 3,000 times more sediment per kilometre than the non-landsliding areas of the Rangitikei Catchment. Based on these findings, several management options were proposed for the Rangitikei Slide, with recommendations included for managing slow-moving landslides around the world. It was also evident that further research is needed to better understand slow-moving landslides due to the significant hazard they represent in regard to their sediment contribution to the surrounding environment.Item Is Queenstown a sanctuary? : a retrospective study of the preparation, mitigation and recovery of the Queenstown community from the effects of the Queenstown flood and hazard slip events of 1999 : a thesis presented in partial fulfilment of the requirements for the degree of Master of Arts in Psychology at Massey University, Palmerston North, New Zealand(Massey University, 2002) Richardson, VivienUsing a salutogenic paradigm, the present study examined the factors that contributed to hardiness and resilience, in the Queenstown community, following the flood and slip events of November 1999. It was hypothesised that sense of community, coping style, self-efficacy, and social support would predict stress, as measured by the Hopkins Symptom Checklist-21 (HSCL). It was also hypothesised that unique environmental qualities of Queenstown, i.e. the lakes, mountains, small community and being a visitor destination, would make a significant contribution to participants sense of community. Hazard knowledge, preparation and mitigation, with particular reference to flood hazards was also examined Additionally, in depth interviews with a high intensity sample of participants, were also carried out. This qualitative information was intended to examine the strengths within the community that had helped the community to deal with the flood and slip events. Additionally, the interviews were used as a forum, for participants to raise their own issues, relating to these events. The hypothesis that the selected variables would predict resilience, was not supported, but support was found for the importance of the unique physical and environmental qualities of Queenstown to residents. The interview results provided support for the view that the Queenstown community was able to withstand the effects of the flood and slip events, with recovery of businesses and tourism being almost complete at four months post event. Though some of those properties affected by the event could never be restored, resulting in irreplaceable losses and changes to individuals' lives, the community showed remarkable strengths and had done much to address inadequacies highlighted by the events.Item Shallow landsliding and catchment connectivity within the Houpoto Forest, New Zealand.(Massey University, 2013) McCabe, Michelle; Fuller, Ian C; McColl, Sam TActive landslides and their contribution to catchment connectivity have been investigated within the Houpoto Forest, North Island, New Zealand. The aim was to quantify the proportion of buffered versus coupled landslides and explore how specific physical conditions influenced differences in landslide connectivity. Landsliding and land use changes between 2007 and 2010 were identified and mapped from aerial photography, and the preliminary analyses and interpretations of these data are presented here. The data indicate that forest harvesting made some slopes more susceptible to failure, and consequently many landslides were triggered during subsequent heavy rainfall events. Failures were particularly widespread during two high magnitude (> 200 mm/day) rainfall events, as recorded in 2010 imagery. Connectivity was analysed by quantifying the relative areal extents of coupled and buffered landslides identified in the different images. Approximately 10 % of the landslides were identified as being coupled to the local stream network, and thus directly contributing to the sediment budget. Following liberation of landslides during high-magnitude events, low-magnitude events are thought to be capable of transferring more of this sediment to the channel. Subsequent re-planting of the slopes appears to have helped recovery by increasing the thresholds for failure, thus reducing the number of landslides during subsequent high-magnitude rainfall events. Associated with this is a reduction in slope-channel connectivity. These preliminary results highlight how site specific preconditioning, preparatory and triggering factors contribute to landslide distribution and connectivity, in addition to how efficient re-afforestation improves the rate of slope recovery.Item Revegetation of recent soil slips in Manawatu : a thesis presented in partial fulfilment of the requirements for the degree of Master in Applied Science at Massey University(Massey University, 2009) Prasad, Kamal KishorTrifolium repens, Lotus pedunculatus and Holcus lanatus were oversown on two recent soil slip surfaces at AgResearch’s Ballantrae pastoral hill‐country farm near Woodville. The two slip surfaces were located on (Manamahu steepland soil) sedimentary mudstone. One slip had a north aspect and the other had a south aspect. Both slips were located on a land class 6 with slope 28‐330. The pasture species were oversown during early spring and the percentage seedling emergence and early establishment from viable seeds oversown was analysed at early spring (Day 15), late spring (Day 45), early summer (Day 90), and late summer (Day 120). The slip surfaces showed micro‐climatic extremes in terms of both soil moisture and surface temperatures during the summer period. Significant differences (P < 0.05) were found in soil moisture between north and south facing slip surfaces. Higher soil moisture and lower soil mean temperature were recorded on the south aspect slip surface. Significant differences (P < 0.05) were found between the three pasture species in terms of seedling emergence and early establishment. Significant differences (P < 0.05) were also found with aspect. The south aspect slip surface had a higher percentage of seedling emergence and earlier establishment for all the species. Interaction between species by aspect became significantly different (P < 0.05) at Day 90 and Day 120. The main effects of time and species were also significantly different (P < 0.05) illustrating seedling emergence and establishment as a race against time. Trifolium repens was a more successful pasture specie, than L. pedunculatus and H. lanatus due to its higher consistency on both north and south slip surfaces. Oversowing T. repens during early spring is a viable option for rehabilitation of recent soil slips in Manawatu.