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Components and tidal modulation of the wave field in a semi-enclosed shallow bay
Get PDFThe wave fields of coastal bays are comprised of waves generated by far-off storms that enter the bay to combine with waves generated locally by winds inside the bay and regionally outside the bay. In any given location, the resultant wave field varies spatially and temporally, and affects coastal features, such as beaches in estuaries and bays (BEBs). However, wave fields in enclosed bays with tidal shoals are poorly studied, limiting the efficacy of coastal protection and restoration projects in these systems, a critical focus in light of ongoing sea level rise. Here we present observations of the wave field in Tomales Bay, a 20-km-long, narrow, semi-enclosed embayment on the wave-dominated coast of Northern California (USA) with a spring-tide range of 2.5 m. We deployed pressure sensors near several beaches along the linear axis of the bay. Low-frequency waves (4∗10−2−2.5∗10−1 Hz or 4–25-s period) were not observed further than 4 km of the mouth, delineating the “outer bay” region, where remotely generated swell and regionally generated wind waves could dominate. The wave spectrum of the landward “inner bay” was dominated by fetch-limited waves generated within the bay with frequency ≥2.5∗10−1Hz. The energy of both ocean waves and locally generated wind waves across all sites were controlled by the tide, but the former by changes in attenuation and the latter likely by modulation of wave generation. Wave energies were low at low tide and high at high tide, but high-frequency wind wave energy was increased during ebb tides while lower-frequency swell energy was reduced during ebb tides, suggesting different mechanisms of tidal influences. Thus, in addition to fluctuations in winds and the presence of ocean waves, tides exert a strong control on the wave energy spectra at coastal features in mesotidal regions. In general, events that may be impactful for BEB morphology are expected to occur when waves due to high winds or high-swell event arrive during high-tide periods. However, no such events were observed during our study and questions remain as to how rarely such events occur across the bay
New ternary powder metallurgy Ti alloys via eutectoid and isomorphous beta stabilisers additions
Get PDFA group of new ternary Ti alloys bearing eutectoid and isomorphous beta stabilising elements was created to be manufactured through the conventional powder metallurgy route. The effect of the simultaneous addition of the same amount of Mn and Nb on the manufacturability, properties, and hardening behaviour was investigated. The ternary alloys are composed of the α-Ti and β-Ti phases and have a lamellar microstructure resulting from the slow cooling upon sintering. However, the size of the equiaxed α grains and of the α + β lamellae is monotonically reduced, especially the interlamellar spacing, as the amount of alloying elements increases. Due to their physical properties, Mn enhances and Nb hinders densification during sintering resulting in a decreasing trend of the relative density with the alloying elements content. Consequently, the resistance to plastic deformation increases (UTS, 514–726 MPa), the ductility decreases (elongation, 13.2–2.6%), and the fracture mode changes from intergranular to transgranular. The new ternary alloys share the same hardening mechanism, but the amount of deformation after necking is, generally, higher for lower amounts of Mn and Nb
The interaction of buoyant river plumes with vegetation and consequences for sediment transport and deposition in coastal regions
Get PDFMangrove forests are one of the most prominent vegetated coastal habits in tropical and subtropical areas. These wetland ecosystems typically inhabit estuaries and tidally influenced river banks. The presence of trees helps to protect coastal regions through the dissipation of tidal currents and wave energy, forming an essential barrier against coastal erosion. River plumes are the primary mechanism of sediment delivery to these vegetated coastlines. As the buoyant freshwater merges with saline ocean waters, horizontal advection of the freshwater establishes the shape of the river plume and influences the distribution of sediments along the coast. In this thesis, I investigate the principal dynamics associated with the interaction of buoyant river plumes with vegetation. In particular, we investigate the interplay between mangrove vegetation, hydrodynamics, and sediment dynamics, specifically: (1) the principal drivers of sediment deposition within a mangrove-lined river delta, (2) the influence of river forcing and winds on the sediment transport, and (3) sediment dynamics of two adjacent and interacting coastal river plumes.
Understanding the hydro-morphodynamics of a river plume in a mangrove environment:
We developed a 3-D idealized Delft3D morphological model to explore the effects of vegetation on the river plumes and associated sediment transport patterns. Using an idealized model based on the Firth of Thames (FoT) mangrove forest located on the North Island of Aotearoa-New Zealand, we observed that, while sediment deposition occurred in the forest and tidal flats region of the model domain, the fringe region (between the vegetation and mudflat) experienced erosion. Compared to the eastern side, sediment deposition in the western side of the model domain was more prominent owing to the influence of Coriolis (Southern hemisphere). By examining the momentum balances on the surface and bottom layers in different regions of the river plume, we found that the principal balance between the bottom shear stress (enhanced by the presence of vegetation) and baroclinic pressure gradient largely controlled the sediment deposition in the riverine sections of the domain. Additionally, we found that vertical advection and diffusion during flood tide enhanced erosion in the fringe region of the mangrove forest. The advection of suspended sediment into the forest was controlled by factors including longer duration of high water slack at the forest fringe region, pressure gradients, and inertial acceleration. In the near-field region (close to the river mouth) of the river plume, during ebb tide, the barotropic and baroclinic pressure gradients coupled with Coriolis accelerations deliver sediments into the forest and mudflat regions. In the mid-field regions of the river plume, the magnitude of changes in the bed elevation was smaller due to larger Coriolis acceleration which arrested the spreading of the river plume. Furthermore, the far-field region (away from the river mouth) of the river plume experienced erosion due to more substantial tidal influence. Conversely, in the shallow forested regions, reduced movement of sediment in the offshore direction leads to an overall flood dominance.
Linking sediment transport within river plumes to varying river flows and winds:
Subsequent modeling experiments were carried out to investigate how the forcing factors of riverine discharge and wind velocities influence sediment transport within an idealized moderate-sized freshwater river plume. We assessed total sediment transport along with the relative contributions of riverine and bed-sourced sediment into a mangrove forest. Instantaneous and tidally averaged sediment fluxes were evaluated to investigate the critical linkages between the riverine flows, tidal influence, and winds in the presence of vegetation. In the near-field region of the river plume, riverine and bed-sourced sediment were dominant and found to be directed into the forest, indicative of an accretionary environment. In the mid- and far-field regions of the river plume, bed-sourced sediment was found to be the dominant contributor to total sediment transport and was directed out of the forest, indicative of sediment erosion. While the mass loads (directed into the mangrove forest) increased with an increase in riverine discharge, mass loads were relatively similar for large flow events. The river sediment delivered into the mangrove-forested regions was found to push into and through the forest front for large river discharges. The presence of 5 m/s wind velocities could alter the sediment deposition patterns in the mid- and far-field regions of the river plume. Despite the significant river momentum, strong winds (10 m/s) were also able to alter the sediment transport in the near-field region of the river plume. Moreover, results showed that in the far-field region of the river plume, strong winds combined with the effects of riverine discharge, Coriolis, and bed-shear stress affected the overall mass movement of sediment. In the case of easterly winds, a combination of tidal effects and wind stress controlled the sediment deposition into the forest. In the case of westerly winds, as the plume was pushed away from the forest, tidal effects controlled the total sediment transport.
Understanding the effects of coalescing buoyant river plumes on sediment transport:
We investigated how the interaction between plumes from two mangrove-lined rivers in close proximity affected coastal sediment transport patterns. In particular, I examined the links between the river plume coalescence and the trapping of sediment by mangroves, using an idealized 3-dimensional numerical. Sediment fluxes and relative contributions of riverine- and bed-sourced sediments were quantified along the western, central (located between the rivers), and eastern mangrove forests. The two river plumes coalesced into a single river plume that flowed along the edge of the central mangrove forest. As the river plumes were deflected to the left by Coriolis (Southern hemisphere), the total sediment fluxes were largest for the central and the western forest, with only modest sediment deposition along the eastern mangrove forest. Also, modeled transport fluxes along the central mangrove forest were the largest close to the rivers, consistent with the satellite imagery of the FoT field site. Flow variations through either river resulted in changes to the sediment transport patterns into the mangrove forests. In particular, an increase in flow discharge through the western river resulted in reduced contributions of the eastern river towards the total sediment transport fluxes. Conversely, an increase in flow discharge through the eastern river inhibited the contribution of western river-associated sediment towards the total sediment transport flux through the central, eastern, and western mangrove forests. Furthermore, this study revealed that 5 m/s wind velocities altered the expansion, coalescence, and overall sediment transport fluxes of the river plumes. The total sediment fluxes through the central and western mangrove forests were greatest for easterly winds. However, the total transport into the eastern mangrove forest was largest for the southerly winds, as the bulge of the eastern river plume was pushed into mangrove forests.
These modeling experiments help elucidate the interaction of river plumes with mangrove vegetation. The work presented in this thesis highlights the complex non-linear interactions of hydro-morphodynamics, sediment transport, the effects of river flows and winds within mangrove environments, and the fate of riverine sediments in the bay of the Firth of Thames. Finally, the results presented here provide insights into the sediment trapping capabilities of mangroves under variable conditions and thus may help to predict their ability to function as a coastal defense mechanism under future sea-level rise and sediment supply scenarios
A multifaceted approach to understanding unexpected sound change. The bilabial trills of Vanuatu’s Malekula Island
Get PDFThis paper demonstrates that unexpected sound changes are best explained by an approach that accounts for different motivations: phonetic, structural and social. Here, we focus on a multifaceted investigation of the crosslinguistically uncommon bilabial trills to show the complex interaction between different drivers of sound change. In this paper, we highlight and examine the prenasalized voiced bilabial trill mʙ and plain voiceless bilabial trill P [ʙ̥] found in a number of Oceanic languages spoken on Malekula Island in Vanuatu. We offer a comparative-historical analysis, and we identify the various forces that have led to the emergence and persistence of mʙ and P in Malekula languages: the historical articulatory environments, the particular make-up of the consonant inventories of these languages, complementary sound changes and phonological processes, contact with non-Austronesian languages, and in-group identity attachment. Furthermore, we offer a hypothesis for the relative timing of these factors on the historical pathway of Malekula’s bilabial trills
The natural frequency estimation for a system composed of a beam and a spring-mass system
No full textThis study presents an approximate method for determining the natural frequency of a system composed of a beam and a two-degree-of-freedom spring-mass system. The expression for estimating the natural frequency is derived following the standard procedures of the forced vibration analysis of a beam. The results obtained using the current method are in good agreement with those obtained through the dynamic stiffness method, especially when the spring stiffness is large. The innovativeness of the current method is that it reveals the relevance between the natural frequency of the system, the natural frequency of the beam, and the mode shape data at the positions where the spring-mass system is attached to the beam. The method is potentially useful in the dynamic wheel-track interaction analysis because the train wheel is normally simplified as a spring-mass system with high spring stiffness. It may also be applied to natural frequency-based damage detection where an auxiliary spring-mass system is used. When the spring-mass system roves on the beam, the curve of natural frequency versus spring-mass system location would be relevant to the mode shape square which is sensitive to local damage
Stemming the colonial environmental tide – Ecosystem-based management and shared Crown-Māori co-governance for sustaining our marine coastal estate in Aotearoa New Zealand – Summary report
No full textThis summary report was prepared by Dr Robert Joseph from Te Mata Hautū Taketake – the Māori and Indigenous Governance Centre, Te Piringa Faculty of Law at the University of Waikato for the National Science Challenge Sustainable Seas Ko Ngā Moana Whakauka Project
Natural frequency estimation for an Euler-Bernoulli beam carrying a mass with rotary inertia
No full textThis work presents an analytical approach to estimate the natural frequencies of an Euler-Bernoulli beam carrying a small mass with rotary inertia. By assuming unchanged mode shapes, a simplified frequency formula is derived and validated using the Dynamic Stiffness Method. The results show high accuracy for small mass and inertia values, and highlight how mass placement and rotary effects influence vibration behavior. The method also offers potential for structural health monitoring applications, such as crack detection
Implications for the Conservation of Deep-Water Corals in the Face of Multiple Stressors: A Case Study from the New Zealand Region
Get PDFThe waters around New Zealand are a global hotspot of biodiversity for deep-water corals; approximately one sixth of the known deep-water coral species of the world have been recorded in the region. Deep-water corals are vulnerable to climate-related stressors and from the damaging effects of commercial fisheries. Current protection measures do not account for the vulnerability of deep-water corals to future climatic conditions, which are predicted to alter the distribution of suitable habitat for them. Using recently developed habitat suitability models for 12 taxa of deep-water corals fitted to current and future seafloor environmental conditions (under different future climatic conditions: SSP2 – 4.5 and SSP3-7.0) we explore possible levels of spatial protection using the decision-support tool Zonation. Specifically, we assess the impact of bottom trawling on predictions of current distributions of deep-water corals, and then assess the effectiveness of possible protection for deep-water corals, while accounting for habitat refugia under future climatic conditions. The cumulative impact of bottom trawling was predicted to impact all taxa, but particularly the reef-forming corals. Core areas of suitable habitat were predicted to decrease under future climatic conditions for many taxa. We found that designing protection using current day predictions alone, having accounted for the impacts of historic fishing impacts, was unlikely to provide adequate conservation for deep water-corals under future climate change. Accounting for future distributions in spatial planning identified areas which may provide climate refugia whilst still providing efficient protection for current distributions. These gains in conservation value may be particularly important given the predicted reduction in suitable habitat for deep-water corals due to bottom fishing and climate change. Finally, the possible impact that protection measures may have on deep-water fisheries was assessed using a measure of current fishing value (kg km-2 fish) and future fishing value (predicted under future climate change scenarios)
Huri whakatau: The call for Māori software requirements elicitation
Get PDFIt is essential to consider specifications and design when examining the requirements for new software for stakeholders and software engineers. The Huri Whakatau project investigates current relevant requirements and design techniques to understand the Māori (Indigenous people of Aotearoa) demographic to identify any relevant gaps for examining requirements.
The process has been tested by investigating existing internationally accepted groups and their methodologies to attempt to adapt a new methodology for Māori groups. In the context of the research, an accepted group is defined as a group which is not from a developing country or Indigenous group. The research was able to identify three untested methodologies; this involved conducting an empirical study as a part of the experiment. The decision was to transform the internationally accepted soft systems methodology (SSM) with groups in a user study. Comparing previously recorded satisfaction using the technique and the use of Māori techniques determined that SSM would be the strongest candidate. When testing SSM with participants, overall, there was a higher level of satisfaction with Māori participants using the new proposed adapted Māori approach as opposed to the internationally accepted groups
Exclusion and inaction: Academic precariat experiences of union representation in Aotearoa New Zealand
Get PDFDuring the last 40 years, neoliberal reforms to the tertiary sector have led to the casualisation of academic labour and the emergence of an academic precariat in Aotearoa New Zealand. Despite the increasing size of the academic precariat, it does not appear that their voices, concerns, or interests have been adequately represented by the national tertiary union. By drawing on open-text responses from the Precarious Academic Work Survey (PAWS) about what unions could do to improve precarious academic working conditions, we discuss the issues created by the under-representation of precarious academics by the sector union. We communicate the results via four key themes of exclusion, participation, voice, and organising. Most participants articulated frustration and disaffection with the union, suggesting the need for a shift in strategy. This study adds to the growing body of employment relations research recognising that employee voices are multiple, diverse, and fragmented; indicating that unions must attend to the differential experiences of people working in the tertiary sector attributable to employment practices