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: Building beyond formality: Navigating Jamaica’s informal construction sector through class, culture and economic resilience

authors: Bennett, Sharayne

subjects: informal economy

abstract: This doctoral thesis is an exploration of Jamaica’s informal construction sector, unpacking its labour distinctiveness, actors, and activities while examining its deep-seated connections to the island’s historical, cultural, and socio-economic context. Through nine months of ethnographic engagement with an informal building construction crew, the thesis provides insights into the motivations of workers, interactions in the workplace, the culture of informal construction, and contributions to the national economy.
At its core, the study questions the persistence of Jamaica’s informal construction industry within the hegemonic context of a Western market economy. In this context, I examine the dynamics of class culture, including the rituals and forms of exchange among workers in this industry. The blurred lines between formal and informal aspects of building construction as well as the reproduction of technical knowledge also hint at reasons for the tenacity of the informal economy.
I critically engage with the concept of precarity to examine the long history of precarious lives in Jamaica. I explore the dichotomy of alienation versus ownership, emphasising the informal sector’s capacity to foster a sense of ownership in contrast to the alienation often experienced in formal systems. I also shine a light on gender disparities, discuss performances of masculinity and unveil the roles and experiences of women in this male-dominated environment. Finally, the study addresses the significant impacts of the 2019 Sars Cov2 pandemic on the social and economic realities within informal construction.

text: Building beyond formality: Navigating Jamaica’s informal construction sector through class, culture and economic resilience This doctoral thesis is an exploration of Jamaica’s informal construction sector, unpacking its labour distinctiveness, actors, and activities while examining its deep-seated connections to the island’s historical, cultural, and socio-economic context. Through nine months of ethnographic engagement with an informal building construction crew, the thesis provides insights into the motivations of workers, interactions in the workplace, the culture of informal construction, and contributions to the national economy.
At its core, the study questions the persistence of Jamaica’s informal construction industry within the hegemonic context of a Western market economy. In this context, I examine the dynamics of class culture, including the rituals and forms of exchange among workers in this industry. The blurred lines between formal and informal aspects of building construction as well as the reproduction of technical knowledge also hint at reasons for the tenacity of the informal economy.
I critically engage with the concept of precarity to examine the long history of precarious lives in Jamaica. I explore the dichotomy of alienation versus ownership, emphasising the informal sector’s capacity to foster a sense of ownership in contrast to the alienation often experienced in formal systems. I also shine a light on gender disparities, discuss performances of masculinity and unveil the roles and experiences of women in this male-dominated environment. Finally, the study addresses the significant impacts of the 2019 Sars Cov2 pandemic on the social and economic realities within informal construction.

year: 2024-11-27

title: Epidemiological evidence that can help to improve timely diagnosis of colorectal cancer in New Zealand.

authors: Hirsz, Malgorzata

subjects: Colorectal cancer

abstract: Incidence rates of colorectal cancer (CRC) in New Zealand (NZ) are among the highest in the world. The long-term survival rates in NZ are poor, which has partially been attributed to late diagnosis. Considering that CRC is a curable disease if diagnosed early, conducting research targeted at the improvement of early diagnosis is therefore important. This thesis provides statistical models for the calculation of CRC incidence rates in the entire NZ population and population strata defined by age, gender, ethnicity and diabetes status, and a model for CRC risk in individual patients referred to the secondary care. The models presented here could assist health professionals in the selection of patients for further investigation to facilitate earlier diagnosis. The empirical part consists of three independent observational studies, briefly described below.

Sub-study 1. The objectives were, first, to describe trends in CRC incidence in the NZ population and, second, to investigate whether there are any strata defined by gender and ethnicity with especially increased incidence rates of CRC. To address these objectives, I analysed data from the New Zealand Cancer Registry (years 1994–2018) using an age- period-cohort (APC) model. The overall CRC incidence rates in NZ decreased between 1994 and 2018 by an average of 1.31% per year. However, the decrease was observed only in patients 50 years and older. In those 30-<50 years old, the incidence rates increased between 1994 and 2018 regardless of gender and ethnicity. The increase was similar for proximal, distal and rectal cancers. The APC analyses revealed very strong cohort effects that could explain nearly the entire trends in CRC incidence, pointing out generations born in the 1970s and 80s being affected by the increased incidence rates, rather than individuals 30-<50 years old. The cohort effects were different in Ma ̄ori and non-Ma ̄ori populations. In non-Ma ̄ori born between approximately 1939 and 1955, incidence rates decreased sharply. By contrast, those Ma ̄ori generations have not benefitted from the sharp decrease in rates. However, CRC incidence increased substantially in both M ̄aori and non-M ̄aori groups born in the 1970s and 80s.

Sub-study 2. The objective was to estimate the IRR for CRC in patients with diabetes compared to those without diabetes, with relation to diabetes duration and use of insulin for diabetes control. Registration in the Virtual Diabetes Register (VDR) in the years 2014–2018 was used as a marker of a diabetes diagnosis. Tables with counts of the entire NZ population stratified by age, gender, and ethnicity were obtained from Statistics NZ. In total, data from 310,710 patients with diabetes, corresponding to 1,277,284 person- years and 2512 incident CRC cases were analysed using a Poisson regression model. Diabetes was associated with an overall increased CRC incidence of 13% compared to non-diabetes [IRR=1.13 (95% CI: 1.08, 1.18)]. The IRR was especially high in the first three months after diabetes diagnosis [IRR=2.55 (95% CI 2.02, 3.21)], likely due to detection bias. The association was equally strong in males and females. However, in the analysis by ethnicity, the incidence of CRC was increased only in non-Ma ̄ori patients and restricted only to those younger than 75 years.

Sub-study 3. The objective was to develop a predictive model for CRC risk in individual patients referred to secondary care. To develop such a model, I extracted information from free text included in e-referrals from GPs’ to the Gastroenterology and General Surgery departments in the Waikato Hospital from 2015-2018, including: symptoms; test results; and family history of CRC. The reference test was a full colonoscopy with visualisation of the cecum. Data from 3015 patients, 20-<90 years old were analysed using a logistic regression model. The final model included the following predictors associated with increased CRC risk: anaemia, rectal bleeding, palpable mass in abdomen or rectum, weight loss, age and gender, and a decreased CRC risk: family history of CRC, abdominal pain, and inflammatory bowel disease. The model discriminates patients with low CRC risk well. According to the final cross-validated model, around 20% of patients from our cohort had performed colonoscopy despite a very low CRC risk (less than 1.5%).

In conclusion, the APC analysis revealed an alarming pattern. According to the fitted APC model, the combination of increasing age and cohort effect in generations born in the 1970s and 80s will bring a wave of CRC diagnoses in the near future when the young generations with high CRC incidence rates will replace the old generations with low CRC incidence. The results from this study could therefore help policy-makers to plan the needs for gastroenterology services.
Secondly, CRC incidence rates in diabetes have been found to be slightly increased com- pared to non-diabetes but only in non-Ma ̄ori individuals younger than 75 years.
Third, the results suggest that males underutilise health services. As shown in sub- study 3, males underwent fewer colonoscopies than females, despite having a higher risk of CRC. The higher detection bias in males than in females (sub-study 2) could also suggest underuse of health services by males, but the difference was not statistically significant.
Finally, based on the fitted models for CRC incidence in sub-studies 1 and 2, population- wide CRC screening for Ma ̄ori and patients with diabetes, based on the incidence, instead of age alone, would be proposed to start at age 57.5 years if the screening in the general population starts at age 60 years.

text: Epidemiological evidence that can help to improve timely diagnosis of colorectal cancer in New Zealand. Incidence rates of colorectal cancer (CRC) in New Zealand (NZ) are among the highest in the world. The long-term survival rates in NZ are poor, which has partially been attributed to late diagnosis. Considering that CRC is a curable disease if diagnosed early, conducting research targeted at the improvement of early diagnosis is therefore important. This thesis provides statistical models for the calculation of CRC incidence rates in the entire NZ population and population strata defined by age, gender, ethnicity and diabetes status, and a model for CRC risk in individual patients referred to the secondary care. The models presented here could assist health professionals in the selection of patients for further investigation to facilitate earlier diagnosis. The empirical part consists of three independent observational studies, briefly described below.

Sub-study 1. The objectives were, first, to describe trends in CRC incidence in the NZ population and, second, to investigate whether there are any strata defined by gender and ethnicity with especially increased incidence rates of CRC. To address these objectives, I analysed data from the New Zealand Cancer Registry (years 1994–2018) using an age- period-cohort (APC) model. The overall CRC incidence rates in NZ decreased between 1994 and 2018 by an average of 1.31% per year. However, the decrease was observed only in patients 50 years and older. In those 30-<50 years old, the incidence rates increased between 1994 and 2018 regardless of gender and ethnicity. The increase was similar for proximal, distal and rectal cancers. The APC analyses revealed very strong cohort effects that could explain nearly the entire trends in CRC incidence, pointing out generations born in the 1970s and 80s being affected by the increased incidence rates, rather than individuals 30-<50 years old. The cohort effects were different in Ma ̄ori and non-Ma ̄ori populations. In non-Ma ̄ori born between approximately 1939 and 1955, incidence rates decreased sharply. By contrast, those Ma ̄ori generations have not benefitted from the sharp decrease in rates. However, CRC incidence increased substantially in both M ̄aori and non-M ̄aori groups born in the 1970s and 80s.

Sub-study 2. The objective was to estimate the IRR for CRC in patients with diabetes compared to those without diabetes, with relation to diabetes duration and use of insulin for diabetes control. Registration in the Virtual Diabetes Register (VDR) in the years 2014–2018 was used as a marker of a diabetes diagnosis. Tables with counts of the entire NZ population stratified by age, gender, and ethnicity were obtained from Statistics NZ. In total, data from 310,710 patients with diabetes, corresponding to 1,277,284 person- years and 2512 incident CRC cases were analysed using a Poisson regression model. Diabetes was associated with an overall increased CRC incidence of 13% compared to non-diabetes [IRR=1.13 (95% CI: 1.08, 1.18)]. The IRR was especially high in the first three months after diabetes diagnosis [IRR=2.55 (95% CI 2.02, 3.21)], likely due to detection bias. The association was equally strong in males and females. However, in the analysis by ethnicity, the incidence of CRC was increased only in non-Ma ̄ori patients and restricted only to those younger than 75 years.

Sub-study 3. The objective was to develop a predictive model for CRC risk in individual patients referred to secondary care. To develop such a model, I extracted information from free text included in e-referrals from GPs’ to the Gastroenterology and General Surgery departments in the Waikato Hospital from 2015-2018, including: symptoms; test results; and family history of CRC. The reference test was a full colonoscopy with visualisation of the cecum. Data from 3015 patients, 20-<90 years old were analysed using a logistic regression model. The final model included the following predictors associated with increased CRC risk: anaemia, rectal bleeding, palpable mass in abdomen or rectum, weight loss, age and gender, and a decreased CRC risk: family history of CRC, abdominal pain, and inflammatory bowel disease. The model discriminates patients with low CRC risk well. According to the final cross-validated model, around 20% of patients from our cohort had performed colonoscopy despite a very low CRC risk (less than 1.5%).

In conclusion, the APC analysis revealed an alarming pattern. According to the fitted APC model, the combination of increasing age and cohort effect in generations born in the 1970s and 80s will bring a wave of CRC diagnoses in the near future when the young generations with high CRC incidence rates will replace the old generations with low CRC incidence. The results from this study could therefore help policy-makers to plan the needs for gastroenterology services.
Secondly, CRC incidence rates in diabetes have been found to be slightly increased com- pared to non-diabetes but only in non-Ma ̄ori individuals younger than 75 years.
Third, the results suggest that males underutilise health services. As shown in sub- study 3, males underwent fewer colonoscopies than females, despite having a higher risk of CRC. The higher detection bias in males than in females (sub-study 2) could also suggest underuse of health services by males, but the difference was not statistically significant.
Finally, based on the fitted models for CRC incidence in sub-studies 1 and 2, population- wide CRC screening for Ma ̄ori and patients with diabetes, based on the incidence, instead of age alone, would be proposed to start at age 57.5 years if the screening in the general population starts at age 60 years.

year: 2021