title: Sediment transport and tidal inlet hydraulics

authors: Black, Kerry P.

abstract: An investigation into sediment transport and tidal inlet hydraulics was initiated to ascertain the impact of a proposed timber port development at Marsden Point in Whangarei Harbour. The program progressed along two parallel pathways, i.e. (i) an expository description of the relevant harbour characteristics using field measurements and numerical modelling and (ii) a detailed investigation into the physics of sediment transport under tidal flow.

The first of these provided an understanding of the hydraulic and sediment transport characteristics of the inlet while aiming to separate cause and effect in the present harbour configuration, The numerical modelling extended this to provide predictions of future behaviour after port construction. In the tidal environment, methods to isolate the effective component of the flow velocity responsible for the transport of sediment are often inadequately substantiated being mostly derived in rivers or flumes. Accordingly, the second aspect of the program necessarily approached and extended the current state of the art with respect to sediment transport under oscillating, non-steady tidal currents in an estuary.

Bathymetric and sea bed surveys revealed that the harbour is mostly stable in an inlet receiving low inputs of sandy sediment. The entrance is sheltered from most storm wave attack by land masses offshore. Due to preferential scouring of sandy bed material, the channels are typically lined with a predominantly-stable, shell-gravel lag. Zones of deposition primarily occur in areas of tidal current neutrality.

Outside the entrance, with the added wave influences, bathymetric changes are greater. Using the experimental results in conjunction with inferences from standard tidal deltas, the changes were seen to be occurring in a closed or partially-closed sediment loop in northern Bream Bay including ebb-tidal transport around the north and east of Mair Bank, wave-induced, shoreward sediment transport over the southern segments of the ebb-tidal delta and northward longshore transport along Bream Bay Beach and through the marginal flood channel off Marsden Point.

Similar controlling transport directions occur inside the entrance with the two most dominant being an ebb-directed route from One Tree Point along the shipping channel and out of the southern side of the entrance throat. Another passes up the Snake Bank flood ramp and around the north-east flank of the flood-tidal delta. 

More than 600 vertical velocity profiles were measured in the estuary. These were examined to derive bed friction coefficients and, over megaripples, the form drag and effective components of the bed shear stress were isolated. This was applied to the determination of the sediment threshold velocity and /the transport of sandy material as bedload. New methods to ascertain the threshold and to derive the bedload transport fluxes over megaripples were presented.

Wave height measurements and sediment trapping experiments revealed that the timber port would have a very limited influence on its environment due to changes of sediment transport patterns on the inter-tidal zone at the port site. 

One and two-dimensional numerical hydrodynamic models and a 2-dimensional numerical sediment transport model were produced for the study and applied to the proposed timber port construction and dredge zone. The sediment model provides erosion and sedimentation rates in a tidal inlet.

With the sediment transport model and the 2-dimensional hydrodynamic model in a coupled mode, numerical experiments were undertaken to produce a fully-developed estuary from an initially planar bathymetry. By alternately running each model to obtain water speeds and bathymetry changes, an estuary with inter-tidal flood-tidal deltas, ebb-tidal deltas, a deep main channel, marginal flood and ebb channels and a stable ebb shield developed. The method has considerable potential for the prediction of final equilibrium bathymetry after man-made changes. 

An iterative technique was designed which utilises the sediment transport model and the water velocities provided by the hydrodynamic model. The procedure determines the percentage of sand cover at all locations throughout the model region in a stable estuary and would allow an optimisation of the minimum quantities of dumped lag material necessary to prevent scour or accretion by establishing a dynamic equilibrium over large areas of the harbour bed.

After accounting for the shell cover which restricts the transport of sediment, the maintenance dredging requirements for the initial 2 berth port development (Case A7) were small and estimated to be in the range of 10,000 m³ to 30,000 m³ per year. The impact of the port in the vicinity of the NZRC wharf is expected to be minimal with some slow sedimentation of the order of a few centimetres per year possibly occurring. In Shoal  Bay, the port should have an extremely limited impact. In the shipping channel to One Tree Point, small amounts of erosion may occur in response to an increased flow velocity (≃5%) caused by deepening of the channel in the dredge zone.

text: Sediment transport and tidal inlet hydraulics An investigation into sediment transport and tidal inlet hydraulics was initiated to ascertain the impact of a proposed timber port development at Marsden Point in Whangarei Harbour. The program progressed along two parallel pathways, i.e. (i) an expository description of the relevant harbour characteristics using field measurements and numerical modelling and (ii) a detailed investigation into the physics of sediment transport under tidal flow.

The first of these provided an understanding of the hydraulic and sediment transport characteristics of the inlet while aiming to separate cause and effect in the present harbour configuration, The numerical modelling extended this to provide predictions of future behaviour after port construction. In the tidal environment, methods to isolate the effective component of the flow velocity responsible for the transport of sediment are often inadequately substantiated being mostly derived in rivers or flumes. Accordingly, the second aspect of the program necessarily approached and extended the current state of the art with respect to sediment transport under oscillating, non-steady tidal currents in an estuary.

Bathymetric and sea bed surveys revealed that the harbour is mostly stable in an inlet receiving low inputs of sandy sediment. The entrance is sheltered from most storm wave attack by land masses offshore. Due to preferential scouring of sandy bed material, the channels are typically lined with a predominantly-stable, shell-gravel lag. Zones of deposition primarily occur in areas of tidal current neutrality.

Outside the entrance, with the added wave influences, bathymetric changes are greater. Using the experimental results in conjunction with inferences from standard tidal deltas, the changes were seen to be occurring in a closed or partially-closed sediment loop in northern Bream Bay including ebb-tidal transport around the north and east of Mair Bank, wave-induced, shoreward sediment transport over the southern segments of the ebb-tidal delta and northward longshore transport along Bream Bay Beach and through the marginal flood channel off Marsden Point.

Similar controlling transport directions occur inside the entrance with the two most dominant being an ebb-directed route from One Tree Point along the shipping channel and out of the southern side of the entrance throat. Another passes up the Snake Bank flood ramp and around the north-east flank of the flood-tidal delta. 

More than 600 vertical velocity profiles were measured in the estuary. These were examined to derive bed friction coefficients and, over megaripples, the form drag and effective components of the bed shear stress were isolated. This was applied to the determination of the sediment threshold velocity and /the transport of sandy material as bedload. New methods to ascertain the threshold and to derive the bedload transport fluxes over megaripples were presented.

Wave height measurements and sediment trapping experiments revealed that the timber port would have a very limited influence on its environment due to changes of sediment transport patterns on the inter-tidal zone at the port site. 

One and two-dimensional numerical hydrodynamic models and a 2-dimensional numerical sediment transport model were produced for the study and applied to the proposed timber port construction and dredge zone. The sediment model provides erosion and sedimentation rates in a tidal inlet.

With the sediment transport model and the 2-dimensional hydrodynamic model in a coupled mode, numerical experiments were undertaken to produce a fully-developed estuary from an initially planar bathymetry. By alternately running each model to obtain water speeds and bathymetry changes, an estuary with inter-tidal flood-tidal deltas, ebb-tidal deltas, a deep main channel, marginal flood and ebb channels and a stable ebb shield developed. The method has considerable potential for the prediction of final equilibrium bathymetry after man-made changes. 

An iterative technique was designed which utilises the sediment transport model and the water velocities provided by the hydrodynamic model. The procedure determines the percentage of sand cover at all locations throughout the model region in a stable estuary and would allow an optimisation of the minimum quantities of dumped lag material necessary to prevent scour or accretion by establishing a dynamic equilibrium over large areas of the harbour bed.

After accounting for the shell cover which restricts the transport of sediment, the maintenance dredging requirements for the initial 2 berth port development (Case A7) were small and estimated to be in the range of 10,000 m³ to 30,000 m³ per year. The impact of the port in the vicinity of the NZRC wharf is expected to be minimal with some slow sedimentation of the order of a few centimetres per year possibly occurring. In Shoal  Bay, the port should have an extremely limited impact. In the shipping channel to One Tree Point, small amounts of erosion may occur in response to an increased flow velocity (≃5%) caused by deepening of the channel in the dredge zone.

year: 1983

title: Editorial: Striving for social justice: The power that picturebooks have to counter inequitable cultural hegemony

authors: Daly, Nicola

abstract: Children’s literature has a long association with education. It has transmitted cultural values to readers over many centuries, often reflecting dominant identities and cultures of the time. Picturebooks, a specific format in children’s literature, are unique in their marriage of image and text and can have a powerful influence on readers’ perceptions of themselves, others and the world around them. Our focus on picturebooks as a subset of children’s literature reflects our view that picturebooks have a particular power for a very wide age group, from those who cannot read independently, to those who are making choices on behalf of future readers. It is the fact that picturebooks are often chosen by adults, such as parents, librarians, teachers and publishers, for readers forming their views of the world that make them of particular interest when it comes to maintaining or disrupting existing power structures.

text: Editorial: Striving for social justice: The power that picturebooks have to counter inequitable cultural hegemony Children’s literature has a long association with education. It has transmitted cultural values to readers over many centuries, often reflecting dominant identities and cultures of the time. Picturebooks, a specific format in children’s literature, are unique in their marriage of image and text and can have a powerful influence on readers’ perceptions of themselves, others and the world around them. Our focus on picturebooks as a subset of children’s literature reflects our view that picturebooks have a particular power for a very wide age group, from those who cannot read independently, to those who are making choices on behalf of future readers. It is the fact that picturebooks are often chosen by adults, such as parents, librarians, teachers and publishers, for readers forming their views of the world that make them of particular interest when it comes to maintaining or disrupting existing power structures.

year: 2022

title: Iwi sex ratios in the New Zealand population census: Why are women so dominant?

authors: Kukutai, Tahu

abstract: Recent census-based studies of iwi (tribal) population growth have revealed a high degree of volatility that cannot be explained by demographic factors alone. Although focused on a small number of iwi, these studies have shown that changing patterns of identification are an important driver of iwi population growth, and that the propensity to identify with an iwi appears to be much stronger among Māori women than men. Thus, the vast majority of iwi in the census have far more females than males, and female domination has increased over time. This paper describes the key features of female-favoured iwi sex ratios in the census and explores possible explanations. Focusing on sex ratios for the ten largest iwi, we find that female domination is highest in the 25–44 age group, and that this pattern is consistent over time. Further analysis shows that Māori women aged 25–34 years are more likely than their male counterparts to know detailed aspects of their pepeha (tribal identity), to explore whakapapa (genealogy) and to speak te reo Māori. Our results underscore the importance of Māori women as cultural connectors within their whānau, as well as in a broader iwi context.

text: Iwi sex ratios in the New Zealand population census: Why are women so dominant? Recent census-based studies of iwi (tribal) population growth have revealed a high degree of volatility that cannot be explained by demographic factors alone. Although focused on a small number of iwi, these studies have shown that changing patterns of identification are an important driver of iwi population growth, and that the propensity to identify with an iwi appears to be much stronger among Māori women than men. Thus, the vast majority of iwi in the census have far more females than males, and female domination has increased over time. This paper describes the key features of female-favoured iwi sex ratios in the census and explores possible explanations. Focusing on sex ratios for the ten largest iwi, we find that female domination is highest in the 25–44 age group, and that this pattern is consistent over time. Further analysis shows that Māori women aged 25–34 years are more likely than their male counterparts to know detailed aspects of their pepeha (tribal identity), to explore whakapapa (genealogy) and to speak te reo Māori. Our results underscore the importance of Māori women as cultural connectors within their whānau, as well as in a broader iwi context.

year: 2017