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Item Contrasting morphological responses to a singular flood event in neighbouring rivers and the implications for river management : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Geography at Massey University, Manawatū, New Zealand(Massey University, 2023) Coulston, Ethan JamesGeomorphic response to flood events is spatially and temporally variable and is influenced by many natural and anthropogenic processes. Research in recent times has shown the adverse geomorphic effects of rivers that have been managed by straightening, narrowing, and disconnecting them from their floodplain. This work attempts to evaluate the morphological response of small, rural gravel-bed rivers to discrete flood events and to put this response into the context of decadal-scale channel adjustments and river management practices. The Tauanui and Turanganui Rivers in South Wairarapa, New Zealand, were monitored and analysed to identify sediment dynamics within them and how they respond to discrete flood events and river management practices. This was achieved by analysing existing historic aerial and satellite imagery, cross-sectional survey data, and geomorphic change analyses using Structure from Motion (SFM) photogrammetry datasets collected in this project. Historical aerial imagery revealed that both rivers have significantly changed over time, with the area of active gravels reducing 38% in the Tauanui and 48% in the Turanganui River from the 1940s to 2013. A narrowing and straightening of both rivers and a proliferation of heavily vegetated banks was observed. It is suggested that these changes are linked to river management strategies, which have helped to develop floodplains for agriculture and occupation by people. Following a storm event on the 20th of June 2021, flooding caused significant geomorphic change. Geomorphic change analysis before and after suggested net aggradation of 1,564 m³ in the Tauanui River and 3,430 m³ in the Turanganui River. Although geomorphic change was significant in both study reaches, it contrasted. This contrast has been interpreted as a result of differences in river resilience and geomorphic thresholds. Similar to other studies, it is suggested that river management interventions have reduced resilience and brought both rivers closer to geomorphic thresholds. This has resulted in geomorphic change that is disproportionate to the flood magnitude. River management that homogenises river corridors is also detrimental to habitat diversity and increases the exposure of the surrounding land to flood risk.Item Multi-scale morphodynamics of unconfined coarse-bedded rivers in the Ruamāhanga catchment : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand(Massey University, 2023) Conley Jr, William C.Riverine floods are the most frequent global natural disaster and a fundamental behaviour of alluvial systems. Flood protection management tends to focus on constraining the spatial extent, frequency, and magnitude of inundation, but often neglects a second intrinsic alluvial behaviour: changes to the channels themselves. This omission can be particularly problematic for gravel-bed rivers, which are well-known for their change propensity across spatiotemporal scales due to complex sediment dynamics. Changes often vary asynchronously on timescales far greater than individual flow events with controls and processes that nest across spatial scales. Further, differential feedbacks mean that similar water discharge may result in two or more very different channel responses. This complex array of riverscape responses and biophysical feedbacks in space and time encapsulates riverscape dynamics, which lie at the heart of this thesis. The applied research of this thesis addresses important gaps regarding 1) suitability assessment of geospatial time-series data, 2) river avulsion hazard screening, 3) tectonic forcing of alluvial rivers, and 4) channel response to human forcing. The gravel-bed rivers of New Zealand’s Ruamāhanga catchment provide an excellent real-world setting to explore these gaps. The short, steep rivers draining the high-relief Tararua Range experience frequent, intense rain-driven hydrology and transit the low-relief of an active forearc basin after emerging from the mountain front. The rivers cross numerous active geologic structures including the Wairarapa Fault, known for the world’s largest single-event land-based horizontal displacement (18.7 m) in 1855. Rivers draining the Tararua Range are known to have filled and expanded into the late-1860s as landslide sediments introduced to rivers during the 1855 shaking gradually worked downstream. Associated changes in river channel and floodplain forms would have generated more severe flood responses given a comparable pre-quake precipitation event. While the rivers eventually adjusted to a more relaxed state of response, their high intrinsic dynamism continues to challenge human habitation. With extensive riparian agriculture and 26% of all buildings occurring on Holocene alluvium, humans have exerted considerable control over the past sixty years using frequent (annual- to sub-annual) earth-moving to incrementally train multithreaded, wandering rivers into the narrower, straighter, and steeper forms seen today. Confidence in localizing hazards and characterising river dynamics rests on certainty that the same locations is/are being compared through time and expressed as coregistration error for members of a time-series. I developed a numerical model to illustrate how root-mean-squared-error (RMSE) values, the customary form of error expression, from intercomparison of randomly-varied and systematically-biased datasets are substantially greater than RMSE values derived from comparison to a common reference. A case study from the Ruamāhanga catchment spanning a wide quality spectrum of archival aerial photomosaics (n = 5) indicates assumptions of residual normality are tenuous and RMSE consistently represents only 65-75% of error frequency and 30-36% of the maximum observed error magnitude. My results suggest that an empirically determined 95% confidence value is better suited to address simulated and real-world limitations. After reprocessing a subset of the historic data, some prior interpretations of channel movement were found erroneous. I propose a conceptual framework to aid fitness-for-purpose determination for different types of geospatial data quality, errors, and analyses to promote more suitable interpretations. Globally, the direct observational record of major river relocations (avulsions) is highly limited given their infrequent occurrence on timescales from decades to millennia. Field- and model-based investigations over the last 45 years have identified a diverse array of contributing factors. These include sedimentation rates, conveyance capacities, and erodibility of existing channels relative to potential receiving areas, although to varying degrees across landscapes. My review identifies topographic advantage as a common denominator across landscape settings and develop a relative digital elevation model (rDEM) approach for rapid, landscape-scale screening in GIS. I propose revision of the traditional two-phase avulsion model to be consistent with other threshold phenomena (e.g., landslides) that divides factors of the set-up phase into static and dynamic components. I present a simple, dichotomous conceptual framework along a gradient of sensitivity (threshold proximity) to aid resource prioritisation for follow-on investigation and/or mitigation. Explicit inclusion of coseismic displacement adds novelty, particularly as adjacent streams appear to be out-of-phase. Control of rivers by tectonic processes is traditionally investigated at millennial and orogen scales though some recent studies have explored annual and reach scales. Nonetheless, fluviotectonic dynamics operating between these timescales are relatively unexplored, especially regarding gravel bed rivers. I relate changes from a time-series of benchmarked cross-sections to surface deformation interpreted from a high-resolution DEM for a 16-kilometre study segment that crosses four active oblique strike-slip faults and several folds. Net and total bed change within and between cross-sections exhibit a high-degree of noise and lack reach-scale patterns. In contrast, patterns of total change accumulated over the time-series show strong spatial partitioning by intersecting geologic structures. The least dynamic cross-sections are generally in proximity to uplifted axes while the most dynamic cross-sections are generally downstream of such intersections and/or coincide with inferred back-tilting. This is the first analysis to suggest morphological forcing of an alluvial river by active geologic structures is detectable at decadal-scale and persistent during an interseismic period. Human river management is a critically important control, but seldom addressed by research across spatiotemporal scales. I address this gap by evaluating a mix of short- and long-term records to assess congruence between two common management aims: increased channel stability and reduced active footprint. Active belt width at the riverscape scale (~16 km) interpreted from a 69-year aerial photo record shows decreasing trend (-48% mean) and increasing uniformity (-62% SD) that converges on the width of the contemporary design corridor (fairway). By contrast, ultrahigh-resolution, reach-scale morphological budgeting over a series of sub-annual events finds roughly sixfold greater volumetric changes in sub-reaches with recent in-channel flood protection earthworks than adjacent untreated reaches. Treated subreaches experienced up to 16 metres of lateral bank erosion, had more instances of increased activity outside the fairway, and propagated changes into untreated areas upstream and downstream. While management actions appear collectively successful in long-term constraint of the riverscape’s active belt, the same actions amplify bed changes from common flow events. I call this anti-pattern the “fairway paradox”. Increased magnitude and frequency of reach-scale movements over short time periods creates maintenance dependencies and makes river responses to flooding less certain. Such scale-dependent responses mean action-effectiveness can only be assessed if aims explicitly identify spatiotemporal scale. Greater certainty regarding a river’s location at any point in time may be gained by less frequent mechanical interventions. This study is the first rigorous evaluation of the effectiveness of NZ river fairway management with broad implications given the widespread application of similar river management throughout NZ. Collectively, this thesis highlights importance of identifying different controls on river behaviour and the scales on which they operate. Substantive doubt is cast on broad application of theoretical or averaged design conditions over alluvial riverscapes with diverse and comingled controls. This is particularly important considering management has occurred under best-case conditions and a large sediment-generating earthquake is expected regionally every ~150 years. As the first work of its kind in NZ and possibly globally, this thesis provides a robust example for future multiscale investigations.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 Measuring short-term coastline changes from an active volcano (Ambae, Vanuatu) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science at Massey University, Palmerston North, New Zealand(Massey University, 2021) Reid, HannahAmbae is a small island in Vanuatu which in recent times has seen high levels of volcanic activity. Between August 2017 and December 2018, several volcanic eruptions occurred, some larger than others, but all with varying effects. A major eruption occurred in July 2018, causing sediment build-up at different locations along the western coastline of the island. The sediment build-up appeared to increase over months, and receded over continuing months. Analyses were completed to assess and evaluate satellite platforms and their ability to aid in gaining an understanding of coastal change over the time period between August 2017 and December 2018. Imagery from each platform was combined with the use of various water indices which aided in measuring the magnitude of coastal change over the time period. Sentinel-2, Landsat 8 and Planet satellite platforms were analysed for their effectiveness when used in conjunction with water indices, including the Normalised Difference Water Index (NDWI), Modified Normalised Difference Water Index (MNDWI), and Automated Water Index (AWEI). The Sentinel-2 platform with the NDWI water index was found to be the most effective for analysing the coastal change. The combination of this satellite platform with this index enabled a quantitative measurement of land change over a specific period of time (August 2017 to December 2018).Item Chasing the mud : analysing the geomorphic impact of the 19-21 June 2015 rainstorm and flood events in southern North Island, New Zealand : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Physical Geography at Massey University, Palmerston North, New Zealand(Massey University, 2021) Malloy, EricaHeavy and prolonged rainfall during the 19-21 June 2015 storm event, triggered the largest recorded flood in the Whanganui River and resulted in widespread flooding in adjacent Whangaehu and Turakina drainage basins. In addition, this high intensity rainstorm triggered many thousand landslides in the soft-rock hill country of these catchments, impacting productivity of vast areas of the landscape within the region. The principal objective of this research was to analyse and better understand the internal dynamics of the catchment sediment cascade and explain the manifestation of geomorphic change within the Whanganui, Whangaehu and Turakina catchments during the intense disturbance event of June 2015. To accomplish this, a geomorphic and sedimentological assessment of channel, floodplain and slope response to the 19-21 June rainstorm and flood events was undertaken to examine erosion, transportation and deposition zones that developed during this event. To facilitate a better understanding of the processes and mechanisms operating during this major storm event, this research provides an assessment of sediment sources and sinks that developed during the storm in the lower Whanganui, Whangaehu and Turakina catchments. It is surmised that the extreme nature of this rainstorm activated the landscape to a degree whereby slope-channel environments were highly connected. Using GeoEye satellite imagery, the distribution and extent of landsliding was mapped, and slope-channel connectivity quantified. Additionally, regions of overbank flood deposition within the flood corridors of these catchments were mapped and the areal extent quantified. Physical terrain attributes, such as slope aspect and angle, vegetation cover and rock type were analysed in relation to landsliding to assess the influence these factors had in determining slope instability. To evaluate the flow and sediment regime of these fluvial networks during the 19-21 June rainstorm event, field mapping and sampling of flood deposits was conducted, and an assessment of the thickness and character of these flood drapes was undertaken. Subsequent analysis of flood deposit samples incorporated grain size analysis so that a better understanding of the hydraulic processes that transported these eroded sediments could be formed. Additionally, sedimentary analysis facilitated the development of new and local knowledge of the processes that produced distinct vertical layering within the flood drape deposited during the 2015 flood event. A linkage is made between the developing flood hydrograph, event sediment flux hydrograph and the stratigraphic layering which was found at various sites within the June 2015 flood drape of the Whanganui River.