title: The control of wind and waves on sediment transport asymmetry and the long-term morphological development of estuaries

authors: Hunt, Stephen

abstract: Most theories on the equilibrium shape of estuarine basins highlight the dominance of tidal asymmetry in controlling the hydrodynamic and morphodynamic feedbacks that ultimately lead to a stable morphological state.  This thesis uses field measurements, numerical modelling and sediment deposition records to investigate how waves interact with tidal processes and how these interactions influence sediment transport and the non-linear feedbacks between hydrodynamics and estuarine basin morphology.
Numerical modelling experiments were conducted over a series of idealised estuaries representing different stages of infilling, which show that wind and waves are far more efficient than tides at shaping intertidal areas, with the effect being subtly dependent on the depth distribution in the intertidal.  Moreover, the wind and wave climate can substantially alter the hydrodynamic regime over the entire estuary and in intertidal areas can exert a much greater control on asymmetry than tidal currents alone, which dominate in the deeper channels.  Under the effect of tides alone, currents over intertidal flats are to found to remain flood-dominant as the estuary infills, thus promoting continued accretion until tidal currents become too weak to entrain sediment.  Therefore, estuaries with only tidal currents are likely to evolve into in-filled areas of salt-marsh or mangrove with drainage channels, whereas fetch-aligned estuaries (in which wind-waves are common) have a greater probability of attaining deeper hydrodynamically-maintained stable intertidal areas.  Although waves have little direct influence on hydrodynamics within the subtidal channel, the tidal asymmetry is controlled by the height and volume of the intertidal areas and therefore is indirectly influenced by wave activity.
The higher-energy, infrequent storm-wave events can considerably modify estuarine morphology over short timescales whereas lower-energy but perpetual effects like the tides operate continuously and so the relative contribution of such events in shaping the long-term morphological evolution can be considered on a range of timescales.  Within this thesis a combination of hydrodynamic measurements, sediment deposition records and numerical modelling are used to determine the conditions under which observed waves are morphologically significant.  Morphological significance is defined as when waves influence tidal and suspended sediment flux asymmetry and subsequently infilling over geomorphological timescales.  By comparing a fetch-aligned and a non fetch-aligned mesotidal basin, it is shown that for a sufficiently large fetch, even small and frequently occurring wind events are able to create waves that are morphologically significant.  Conversely, in basins with a reduced fetch, wave events are less frequent and therefore of far less morphological significance.  
The role of tidal range in controlling sediment transport is investigated through consideration of its influence on bed shear stress, τmax. The decrease in bed shear stress associated with greater water depths is compared to increases owing to the larger fetch that accompanies the increased tidal range and the generally stronger currents associated with larger tidal range.  Here, it is shown that during neap tides the reduction in water depths around high water and tidal currents are not offset by the reduction in fetch.  Thus, it is shown that the basin-averaged τmax is similar during both spring and neap tides in addition to the shorter duration of slack water during neap tides.  Consequently, as τmax is lower in the subtidal channel during neaps, the sediment gradient (and hence sediment transport potential) between tidal flats and channels is greater. Thus it is concluded that sediment deposition potential may actually be increased during a neap tide.  This result is in sharp contrast to previous observations from microtidal wave-dominated environments, in which differences between spring and neap result in increased erosion during spring tides.  
Overall, this thesis shows that short period locally generated waves can be considered a morphologically significant hydrodynamic process within estuaries.  With increased fetch, waves become more morphologically significant relative to tides as even low, frequent wind speeds are able to generate waves that are capable of controlling patterns of sediment transport.

text: The control of wind and waves on sediment transport asymmetry and the long-term morphological development of estuaries Most theories on the equilibrium shape of estuarine basins highlight the dominance of tidal asymmetry in controlling the hydrodynamic and morphodynamic feedbacks that ultimately lead to a stable morphological state.  This thesis uses field measurements, numerical modelling and sediment deposition records to investigate how waves interact with tidal processes and how these interactions influence sediment transport and the non-linear feedbacks between hydrodynamics and estuarine basin morphology.
Numerical modelling experiments were conducted over a series of idealised estuaries representing different stages of infilling, which show that wind and waves are far more efficient than tides at shaping intertidal areas, with the effect being subtly dependent on the depth distribution in the intertidal.  Moreover, the wind and wave climate can substantially alter the hydrodynamic regime over the entire estuary and in intertidal areas can exert a much greater control on asymmetry than tidal currents alone, which dominate in the deeper channels.  Under the effect of tides alone, currents over intertidal flats are to found to remain flood-dominant as the estuary infills, thus promoting continued accretion until tidal currents become too weak to entrain sediment.  Therefore, estuaries with only tidal currents are likely to evolve into in-filled areas of salt-marsh or mangrove with drainage channels, whereas fetch-aligned estuaries (in which wind-waves are common) have a greater probability of attaining deeper hydrodynamically-maintained stable intertidal areas.  Although waves have little direct influence on hydrodynamics within the subtidal channel, the tidal asymmetry is controlled by the height and volume of the intertidal areas and therefore is indirectly influenced by wave activity.
The higher-energy, infrequent storm-wave events can considerably modify estuarine morphology over short timescales whereas lower-energy but perpetual effects like the tides operate continuously and so the relative contribution of such events in shaping the long-term morphological evolution can be considered on a range of timescales.  Within this thesis a combination of hydrodynamic measurements, sediment deposition records and numerical modelling are used to determine the conditions under which observed waves are morphologically significant.  Morphological significance is defined as when waves influence tidal and suspended sediment flux asymmetry and subsequently infilling over geomorphological timescales.  By comparing a fetch-aligned and a non fetch-aligned mesotidal basin, it is shown that for a sufficiently large fetch, even small and frequently occurring wind events are able to create waves that are morphologically significant.  Conversely, in basins with a reduced fetch, wave events are less frequent and therefore of far less morphological significance.  
The role of tidal range in controlling sediment transport is investigated through consideration of its influence on bed shear stress, τmax. The decrease in bed shear stress associated with greater water depths is compared to increases owing to the larger fetch that accompanies the increased tidal range and the generally stronger currents associated with larger tidal range.  Here, it is shown that during neap tides the reduction in water depths around high water and tidal currents are not offset by the reduction in fetch.  Thus, it is shown that the basin-averaged τmax is similar during both spring and neap tides in addition to the shorter duration of slack water during neap tides.  Consequently, as τmax is lower in the subtidal channel during neaps, the sediment gradient (and hence sediment transport potential) between tidal flats and channels is greater. Thus it is concluded that sediment deposition potential may actually be increased during a neap tide.  This result is in sharp contrast to previous observations from microtidal wave-dominated environments, in which differences between spring and neap result in increased erosion during spring tides.  
Overall, this thesis shows that short period locally generated waves can be considered a morphologically significant hydrodynamic process within estuaries.  With increased fetch, waves become more morphologically significant relative to tides as even low, frequent wind speeds are able to generate waves that are capable of controlling patterns of sediment transport.

year: 2016

title: Imagining Pākehātanga: how to be where you are

authors: Boyens, Aaron

subjects: Pākehātanga

abstract: Imagining Pākehātanga: how to be where you are is a creative practice thesis in which I explore the question: in what ways can I imagine Pākehātanga through personal experiences and shared family practices? In framing the contemporary context of Aotearoa / New Zealand I draw on literature relating to Decolonising Theory and Pākehātanga, leading to the development of a research design based on autoethnographic performance. The research was carried out in three phases using the following methods: individual interviews with family members; a session involving a thematic research conversation with a group of my family members; and creative practice throughout the project. The last phase culminated in an evening of autoethnographic performance incorporating creative writing, poetry and song. The performance script and an audiovisual recording of the performance are integrated with the written thesis text. Throughout the thesis, my creative writing weaves with my academic writing, revealing three clear responses to the research question: naming, listening, and longing. These responses function interdependently, together showing the potential for an infinite motion of imagination in action and possibilities for Pākehā practising a relational way of being. This is my quest of how to be, how to be where I am.

text: Imagining Pākehātanga: how to be where you are Imagining Pākehātanga: how to be where you are is a creative practice thesis in which I explore the question: in what ways can I imagine Pākehātanga through personal experiences and shared family practices? In framing the contemporary context of Aotearoa / New Zealand I draw on literature relating to Decolonising Theory and Pākehātanga, leading to the development of a research design based on autoethnographic performance. The research was carried out in three phases using the following methods: individual interviews with family members; a session involving a thematic research conversation with a group of my family members; and creative practice throughout the project. The last phase culminated in an evening of autoethnographic performance incorporating creative writing, poetry and song. The performance script and an audiovisual recording of the performance are integrated with the written thesis text. Throughout the thesis, my creative writing weaves with my academic writing, revealing three clear responses to the research question: naming, listening, and longing. These responses function interdependently, together showing the potential for an infinite motion of imagination in action and possibilities for Pākehā practising a relational way of being. This is my quest of how to be, how to be where I am.

year: 2024

title: Social consequences of Tūhoe migration: Voices from home in Te Urewera

authors: Nikora, Linda Waimarie

subjects: age effects

abstract: A previous study by Nikora, Guerin, Rua & Te Awekotuku (2004) of the social consequences of Tūhoe moving to the Waikato region found employment and a tertiary education to be the primary motivators. Tūhoe ‘movers’ remained in contact with those who remain in Te Urewera and retained a wish to return to their tribal homelands if presented with work opportunities or upon retirement. Through 29 intensive conversational interviews conducted by Tūhoe community researchers with Tūhoe people we explored the social consequences of migration upon those who had remained ‘at home’. Most participants recognised and strongly supported ‘movers’ to better themselves and find successful futures but hoped that they would one day return. An emerging theme was an apparent age-distribution gap of people between about 20 and 50 years of age in the Tuhoe homelands. This is concerning for a number of reasons.  The ‘age gap’ may potentially bring about a) a lack of good role-models for younger children; b) a lack of people to help out with heavy work; c) a gap in people to sustain traditional teaching models for children, and d) a lack of qualified people in trades and professions. The age gap also means that a whole middle-aged group that in other communities might be contributing funds for capital expenditures are not present in the community.

text: Social consequences of Tūhoe migration: Voices from home in Te Urewera A previous study by Nikora, Guerin, Rua & Te Awekotuku (2004) of the social consequences of Tūhoe moving to the Waikato region found employment and a tertiary education to be the primary motivators. Tūhoe ‘movers’ remained in contact with those who remain in Te Urewera and retained a wish to return to their tribal homelands if presented with work opportunities or upon retirement. Through 29 intensive conversational interviews conducted by Tūhoe community researchers with Tūhoe people we explored the social consequences of migration upon those who had remained ‘at home’. Most participants recognised and strongly supported ‘movers’ to better themselves and find successful futures but hoped that they would one day return. An emerging theme was an apparent age-distribution gap of people between about 20 and 50 years of age in the Tuhoe homelands. This is concerning for a number of reasons.  The ‘age gap’ may potentially bring about a) a lack of good role-models for younger children; b) a lack of people to help out with heavy work; c) a gap in people to sustain traditional teaching models for children, and d) a lack of qualified people in trades and professions. The age gap also means that a whole middle-aged group that in other communities might be contributing funds for capital expenditures are not present in the community.

year: 2008