title: Morphodynamics of Ebb-Tidal Deltas

authors: Harrison, Shawn Robert

subjects: Ebb-tidal delta

abstract: The purpose of this study was to learn more about the significant drivers of ebb-tidal delta morphology using observational methods and fundamental physical relationships between forcing conditions and morphological response. Two techniques were adapted in a novel way to study the dynamics of geomorphic features at an ebb-tidal delta. A 5-year long record of video imagery was used to observe a natural mixed-energy ebb-tidal delta in the field. A semi-analytical ebb-jet model, which was coupled to an exploratory morphological model, was developed and used to explore the interactions between tidal currents, waves, and morphology, and to test the sensitivity of morphological development and response to changes in forcing conditions.

A detailed video-based observational record was used to identify and track geomorphic features over 5 years at an ebb-tidal delta on the energetic west coast of New Zealand at Raglan by using depth-limited wave-breaking patterns as a proxy for the position of shallow sandbars. Oblique 20-minute averaged time-exposure images were geo-rectified to provide detailed spatial measurements of ebb-tidal delta features such as the terminal lobe, mouth bar, channel margin linear bars (or levees), and swash bars over the 5-year duration. Movements of these features were quantified and related to wave and tidal forcing, including seasonal and interannual trends in wave climate. In general, the low-energy restorative summer waves led to a more cuspate terminal lobe, while in the high-energy erosive winter waves straightened the terminal lobe and moved it further seaward than the long-term average. Movements throughout the delta were intermittent between less active periods, with the fastest swash bar migrations occurring during the transition between seasons, namely winter to spring and summer to autumn.

A semi-analytical model for ebbing tidal jet flow was developed based on the balance of momentum between inertia, bed friction, turbulent mixing, and wave effects. Previous analytical jet models (Özsoy and Ünlüata, 1982; Joshi, 1982) were extended to include the effects of directly opposing breaking waves. The model was calibrated and compared with scaled laboratory measurements (Ismail and Wiegel, 1983) and numerical simulations (Nardin et al., 2013) of river jets flowing over flat bathymetry with non-breaking waves, and with detailed field measurements of jet flow and wave dissipation at New River Inlet, North Carolina (e.g. Wargula et al., 2014). The jet model demonstrated the influence of opposing breaking waves on ebb-jet currents and jet width, along with the emergence of a point of flow convergence. The contribution of wave effects to the momentum balance were shown to impact the rapid slowing of jet flow, overall extent of an ebb-jet, and increase the jet width agreeing with previous studies (Nardin and Fagherazzi 2012; Nardin et al., 2013; Olabarrietta et al., 2014). Using a channelization parameter to emulate the ability of channel levees to constrain jet spreading, the model was calibrated over nine and validated over sixty-one ebb-events, respectively, to flow and wave measurements at New River Inlet. The model predicted the jet conditions well, receiving an overall excellent skill score. However, the model over-predicted the dissipative effects to jet flow of depth-limited wave breaking over the shallow ebb-shoal. The calibrated friction coefficient was roughly an order of magnitude higher than measured in the field, suggesting that the friction term was absorbing underrepresented processes. Pressure gradient was a neglected process identified as potentially significant, but was effectively included in the jet spreading term.

The jet model was coupled to sediment transport formulae to form an exploratory type morphological model for exploring the sensitivities of ebb-tidal shoal and channel morphology to changes in forcing conditions and sediment characteristics. Equilibrium ebb-shoal morphology was formed over many model iterations and shown to be morphologically dependent on forcing conditions. Wave-dominant conditions developed ebb-shoals in closer proximity to the tidal inlet, with wider channels at their seaward end, than jet-dominant conditions. Increases in jet velocity increased the rate of development of the delta more than did increases in wave height. Sediment characteristics had very little influence on the equilibrium morphology, but did influence the rate of development. Short-term morphological responses of established morphology were sensitive to the initial channel width, with wide channels being most susceptible to the effects of waves and jet interaction. A double-barred ebb-shoal was shown to develop under mixed-energy conditions, with a shoreward bar being influenced by jet-flow and the seaward bar being influenced by wave breaking.

The observational data and modelling tools are used to explore ebb-tidal delta morphodynamics. Themes include the dependence on environmental conditions, transitions from equilibrium, and the competing influence of jet currents and opposing waves on ebb-shoal and channel morphology. The work asserts that ebb-tidal delta morphology forms as a result of dynamic balance between the dominant forces responsible for sediment transport, namely the ebbing tidal jet currents and opposing waves.

text: Morphodynamics of Ebb-Tidal Deltas The purpose of this study was to learn more about the significant drivers of ebb-tidal delta morphology using observational methods and fundamental physical relationships between forcing conditions and morphological response. Two techniques were adapted in a novel way to study the dynamics of geomorphic features at an ebb-tidal delta. A 5-year long record of video imagery was used to observe a natural mixed-energy ebb-tidal delta in the field. A semi-analytical ebb-jet model, which was coupled to an exploratory morphological model, was developed and used to explore the interactions between tidal currents, waves, and morphology, and to test the sensitivity of morphological development and response to changes in forcing conditions.

A detailed video-based observational record was used to identify and track geomorphic features over 5 years at an ebb-tidal delta on the energetic west coast of New Zealand at Raglan by using depth-limited wave-breaking patterns as a proxy for the position of shallow sandbars. Oblique 20-minute averaged time-exposure images were geo-rectified to provide detailed spatial measurements of ebb-tidal delta features such as the terminal lobe, mouth bar, channel margin linear bars (or levees), and swash bars over the 5-year duration. Movements of these features were quantified and related to wave and tidal forcing, including seasonal and interannual trends in wave climate. In general, the low-energy restorative summer waves led to a more cuspate terminal lobe, while in the high-energy erosive winter waves straightened the terminal lobe and moved it further seaward than the long-term average. Movements throughout the delta were intermittent between less active periods, with the fastest swash bar migrations occurring during the transition between seasons, namely winter to spring and summer to autumn.

A semi-analytical model for ebbing tidal jet flow was developed based on the balance of momentum between inertia, bed friction, turbulent mixing, and wave effects. Previous analytical jet models (Özsoy and Ünlüata, 1982; Joshi, 1982) were extended to include the effects of directly opposing breaking waves. The model was calibrated and compared with scaled laboratory measurements (Ismail and Wiegel, 1983) and numerical simulations (Nardin et al., 2013) of river jets flowing over flat bathymetry with non-breaking waves, and with detailed field measurements of jet flow and wave dissipation at New River Inlet, North Carolina (e.g. Wargula et al., 2014). The jet model demonstrated the influence of opposing breaking waves on ebb-jet currents and jet width, along with the emergence of a point of flow convergence. The contribution of wave effects to the momentum balance were shown to impact the rapid slowing of jet flow, overall extent of an ebb-jet, and increase the jet width agreeing with previous studies (Nardin and Fagherazzi 2012; Nardin et al., 2013; Olabarrietta et al., 2014). Using a channelization parameter to emulate the ability of channel levees to constrain jet spreading, the model was calibrated over nine and validated over sixty-one ebb-events, respectively, to flow and wave measurements at New River Inlet. The model predicted the jet conditions well, receiving an overall excellent skill score. However, the model over-predicted the dissipative effects to jet flow of depth-limited wave breaking over the shallow ebb-shoal. The calibrated friction coefficient was roughly an order of magnitude higher than measured in the field, suggesting that the friction term was absorbing underrepresented processes. Pressure gradient was a neglected process identified as potentially significant, but was effectively included in the jet spreading term.

The jet model was coupled to sediment transport formulae to form an exploratory type morphological model for exploring the sensitivities of ebb-tidal shoal and channel morphology to changes in forcing conditions and sediment characteristics. Equilibrium ebb-shoal morphology was formed over many model iterations and shown to be morphologically dependent on forcing conditions. Wave-dominant conditions developed ebb-shoals in closer proximity to the tidal inlet, with wider channels at their seaward end, than jet-dominant conditions. Increases in jet velocity increased the rate of development of the delta more than did increases in wave height. Sediment characteristics had very little influence on the equilibrium morphology, but did influence the rate of development. Short-term morphological responses of established morphology were sensitive to the initial channel width, with wide channels being most susceptible to the effects of waves and jet interaction. A double-barred ebb-shoal was shown to develop under mixed-energy conditions, with a shoreward bar being influenced by jet-flow and the seaward bar being influenced by wave breaking.

The observational data and modelling tools are used to explore ebb-tidal delta morphodynamics. Themes include the dependence on environmental conditions, transitions from equilibrium, and the competing influence of jet currents and opposing waves on ebb-shoal and channel morphology. The work asserts that ebb-tidal delta morphology forms as a result of dynamic balance between the dominant forces responsible for sediment transport, namely the ebbing tidal jet currents and opposing waves.

year: 2015

title: Assessing the Diversity of Antarctic and New Zealand Arthropods through DNA Barcoding

authors: Beet, Clare Rose

subjects: DNA Barcoding

abstract: Diversity is the foundation of all biological and ecological studies. Globally however, biodiversity is under threat from the varied and cumulative impacts of humans on the environment. It is thus necessary to continually develop tools that are able to assess diversity at the scales now required. DNA barcoding has become an increasingly common approach for species identification as it is efficient and can facilitate high throughput analyses without the routine need of taxonomic experts. This thesis examines the genetic diversity of invertebrate groups from Antarctica and New Zealand to gain an understanding of current baseline levels of variability and to facilitate their use as indicators of environmental change.
The juvenile stages of Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies) (EPT taxa) are key components of aquatic food webs and are frequently used as bioindicators of water quality. However, challenges in identifying juveniles to species levels are one factor limiting their more routine use. New Zealand has over 244 caddisfly species 106 species of stonefly and 50 mayfly species all of which are endemic. Here, my primary aim was to obtain COI sequence coverage for the New Zealand EPT taxa using expertly identified collections of adult specimens. A second aim was to examine levels of sequence diversity within taxa and to test the endemicity of the New Zealand fauna through comparison with international records. Thus far, 225 caddisfly BINs, 48 stonefly and 37 mayfly BINs have been generated. Average intraspecific divergences were between 1.2-1.4% for all three orders while average interspecific distances ranged from 24-32%. The designation of all New Zealand EPT species as endemic was supported with interspecific divergences generally above 13%. These data can facilitate the rapid and accurate assessment of larval specimens and can furthermore be used to facilitate research into the phylogenetic and phylogeographic patterns that have shaped the EPT fauna worldwide.
I also examined mitochondrial DNA (COI) sequences for three Antarctic springtail (Collembola) species collected from sites in the vicinity, and to the north of, the Mackay Glacier (77oS) within the Ross Sea region. This area represents a transitional zone between two biogeographic regions (North and South Victoria Land). Here, I assessed levels of genetic variability within and among populations of the three putative springtail species. Each of the three recognised species had multiple highly divergent intraspecific populations (5-11.3% sequence divergence). Based on molecular clock estimates, these divergent lineages were likely to have been isolated for 3-5 million years, a time when the Western Antarctic Ice Sheet (WAIS) was thought to have completely collapsed. Given the current isolation of these genetically distinct populations, any future changes in species’ distributions can be easily tracked through the DNA barcoding of individual springtails across the Mackay Glacier ecotone. 
Collectively these two studies have established baseline levels of COI diversity for New Zealand and Antarctic invertebrates. Furthermore, they have revealed hidden (cryptic) diversity in both regions and presented opportunities to incorporate DNA barcoding into future studies of New Zealand aquatic and terrestrial Antarctic ecosystems.

text: Assessing the Diversity of Antarctic and New Zealand Arthropods through DNA Barcoding Diversity is the foundation of all biological and ecological studies. Globally however, biodiversity is under threat from the varied and cumulative impacts of humans on the environment. It is thus necessary to continually develop tools that are able to assess diversity at the scales now required. DNA barcoding has become an increasingly common approach for species identification as it is efficient and can facilitate high throughput analyses without the routine need of taxonomic experts. This thesis examines the genetic diversity of invertebrate groups from Antarctica and New Zealand to gain an understanding of current baseline levels of variability and to facilitate their use as indicators of environmental change.
The juvenile stages of Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies) (EPT taxa) are key components of aquatic food webs and are frequently used as bioindicators of water quality. However, challenges in identifying juveniles to species levels are one factor limiting their more routine use. New Zealand has over 244 caddisfly species 106 species of stonefly and 50 mayfly species all of which are endemic. Here, my primary aim was to obtain COI sequence coverage for the New Zealand EPT taxa using expertly identified collections of adult specimens. A second aim was to examine levels of sequence diversity within taxa and to test the endemicity of the New Zealand fauna through comparison with international records. Thus far, 225 caddisfly BINs, 48 stonefly and 37 mayfly BINs have been generated. Average intraspecific divergences were between 1.2-1.4% for all three orders while average interspecific distances ranged from 24-32%. The designation of all New Zealand EPT species as endemic was supported with interspecific divergences generally above 13%. These data can facilitate the rapid and accurate assessment of larval specimens and can furthermore be used to facilitate research into the phylogenetic and phylogeographic patterns that have shaped the EPT fauna worldwide.
I also examined mitochondrial DNA (COI) sequences for three Antarctic springtail (Collembola) species collected from sites in the vicinity, and to the north of, the Mackay Glacier (77oS) within the Ross Sea region. This area represents a transitional zone between two biogeographic regions (North and South Victoria Land). Here, I assessed levels of genetic variability within and among populations of the three putative springtail species. Each of the three recognised species had multiple highly divergent intraspecific populations (5-11.3% sequence divergence). Based on molecular clock estimates, these divergent lineages were likely to have been isolated for 3-5 million years, a time when the Western Antarctic Ice Sheet (WAIS) was thought to have completely collapsed. Given the current isolation of these genetically distinct populations, any future changes in species’ distributions can be easily tracked through the DNA barcoding of individual springtails across the Mackay Glacier ecotone. 
Collectively these two studies have established baseline levels of COI diversity for New Zealand and Antarctic invertebrates. Furthermore, they have revealed hidden (cryptic) diversity in both regions and presented opportunities to incorporate DNA barcoding into future studies of New Zealand aquatic and terrestrial Antarctic ecosystems.

year: 2016

title: “Landlords wouldn’t give my application a second look.” Discrimination exacerbates inequalities in access to private rental housing

authors: Terruhn, Jessica

abstract: This report and the key insights are based on findings from a housing survey that was conducted with 800 residents in neighbourhoods in Auckland, Hamilton and Christchurch. 
• Perceptions of unfairness in Aotearoa’s housing sector are widespread. Renters, Māori, younger people and low-income groups as well as residents in neighbourhoods with high levels of housing deprivation are most likely to think that people are treated unfairly when trying to rent or buy a home in Aotearoa. 
• Nearly one in three respondents reported having experienced discrimination when trying to rent or buy a home in Aotearoa. Renters, Māori, younger people and low-income groups as well as residents in neighbourhoods with high levels of housing deprivation are most likely to report experiences of discrimination. 
• Advantage and disadvantage in securing a home are determined by a combination of interlocking factors, including income and employment status, age, family status, and race/ethnicity or skin colour. These patterns suggest widespread experiences of potentially unlawful housing discrimination. 
• People strategically try to avoid and mitigate discrimination. Expectations of being discriminated against and of being advantaged influence where and how home seekers search for housing. This finding signals that experiences of rejection play a role in constraining people’s housing choices. 
• The survey findings suggest that discrimination, as part of tenant selection, contributes to housing precarity and inequalities in access to rental housing. Therefore, this research points to an urgent need to address housing discrimination, especially in the context of high levels of residential mobility among renters and intense competition for rental properties.

text: “Landlords wouldn’t give my application a second look.” Discrimination exacerbates inequalities in access to private rental housing This report and the key insights are based on findings from a housing survey that was conducted with 800 residents in neighbourhoods in Auckland, Hamilton and Christchurch. 
• Perceptions of unfairness in Aotearoa’s housing sector are widespread. Renters, Māori, younger people and low-income groups as well as residents in neighbourhoods with high levels of housing deprivation are most likely to think that people are treated unfairly when trying to rent or buy a home in Aotearoa. 
• Nearly one in three respondents reported having experienced discrimination when trying to rent or buy a home in Aotearoa. Renters, Māori, younger people and low-income groups as well as residents in neighbourhoods with high levels of housing deprivation are most likely to report experiences of discrimination. 
• Advantage and disadvantage in securing a home are determined by a combination of interlocking factors, including income and employment status, age, family status, and race/ethnicity or skin colour. These patterns suggest widespread experiences of potentially unlawful housing discrimination. 
• People strategically try to avoid and mitigate discrimination. Expectations of being discriminated against and of being advantaged influence where and how home seekers search for housing. This finding signals that experiences of rejection play a role in constraining people’s housing choices. 
• The survey findings suggest that discrimination, as part of tenant selection, contributes to housing precarity and inequalities in access to rental housing. Therefore, this research points to an urgent need to address housing discrimination, especially in the context of high levels of residential mobility among renters and intense competition for rental properties.

year: 2024-08