title: Effects of low tide rainfall on the erodibility of intertidal cohesive sediments

authors: Pilditch, Conrad A.

subjects: sediment transport

abstract: Low tide rainfall may represent an important but little studied process affecting sediment fluxes on intertidal mudflats. In this study, we simulated rainfall events on an intertidal mudflat (median grain size=18.4 μm) of low slope (1 in 300) then quantified effects on sediment erodibility. Treatments consisted of a high (4.1 mm min⁻¹ for 6 min) and low (0.36 mm min⁻¹ for 60 min) rain intensity, chosen to match naturally occurring events and experiments were conducted seasonally (May and August) to encompass variations in ambient sediment stability. Changes in bed elevation due to rainfall were estimated using marked rods and sediment erodibility parameters (mass of sediment eroded at a flow velocity of 0.3 m s⁻¹ (ME-30, g m⁻²) and critical erosion velocity (Ucrit, m s⁻¹)) were determined in annular flumes (bed area=0.17 m²). Ambient/control sediment erodibility in May (ME-30=211 g m⁻², Ucrit=0.18 m s⁻¹) was higher than in August (ME-30=30 g m⁻², Ucrit=0.26 m s⁻¹) and was correlated with changes in biological activity. In May, surface sediment was influenced by high densities of the bioturbating snail Hydrobia ulvae (1736 ind. m⁻²) and low biomass of the sediment stabilising microphytobenthos (5.7 μg chlorophyll a cm⁻²). In contrast, in August H. ulvae densities were low (52 ind. m⁻²) and microphytobenthic biomass higher (9.2 μg chlorophyll a cm⁻²). The high rain treatment caused a decrease in bed elevation of between 1.5 mm (May) and 4.4 mm (August) and significantly reduced sediment organic content and microphytobenthic biomass. Rainfall increased sediment erodibility; compared to ambient sediments ME-30 increased by a factor of 1.4× in May and 8.8× in August and caused a 10–30% decline in Ucrit. The seasonal difference in treatment effect was due to the change in ambient sediment stability. The low rain treatment in August had no effect on bed elevation, microphytobenthic biomass or sediment erodibility. In May, the same treatment caused a reduction in bed elevation (0.5 mm) and microphytobenthic biomass but counter-intuitively, a decrease in sediment erodibility (ME-30 was reduced by 40%, Ucrit increased by 5%) compared to controls. We attribute this result to removal by rainfall of easily eroded surface flocs and biogenic roughness which resulted in an underlying sediment with a smoother surface and greater resistant to erosion. Results suggest that high intensity rain events may destabilise intertidal sediments making them more susceptible to erosion by returning tidal currents and that the sediment eroded during such events may represent a considerable fraction (up to 25%) of the seasonal variation in shore elevation. The impact of natural rain events are likely to vary considerably due to variations in droplet size, intensity and duration and the interaction with ambient sediment stability

text: Effects of low tide rainfall on the erodibility of intertidal cohesive sediments Low tide rainfall may represent an important but little studied process affecting sediment fluxes on intertidal mudflats. In this study, we simulated rainfall events on an intertidal mudflat (median grain size=18.4 μm) of low slope (1 in 300) then quantified effects on sediment erodibility. Treatments consisted of a high (4.1 mm min⁻¹ for 6 min) and low (0.36 mm min⁻¹ for 60 min) rain intensity, chosen to match naturally occurring events and experiments were conducted seasonally (May and August) to encompass variations in ambient sediment stability. Changes in bed elevation due to rainfall were estimated using marked rods and sediment erodibility parameters (mass of sediment eroded at a flow velocity of 0.3 m s⁻¹ (ME-30, g m⁻²) and critical erosion velocity (Ucrit, m s⁻¹)) were determined in annular flumes (bed area=0.17 m²). Ambient/control sediment erodibility in May (ME-30=211 g m⁻², Ucrit=0.18 m s⁻¹) was higher than in August (ME-30=30 g m⁻², Ucrit=0.26 m s⁻¹) and was correlated with changes in biological activity. In May, surface sediment was influenced by high densities of the bioturbating snail Hydrobia ulvae (1736 ind. m⁻²) and low biomass of the sediment stabilising microphytobenthos (5.7 μg chlorophyll a cm⁻²). In contrast, in August H. ulvae densities were low (52 ind. m⁻²) and microphytobenthic biomass higher (9.2 μg chlorophyll a cm⁻²). The high rain treatment caused a decrease in bed elevation of between 1.5 mm (May) and 4.4 mm (August) and significantly reduced sediment organic content and microphytobenthic biomass. Rainfall increased sediment erodibility; compared to ambient sediments ME-30 increased by a factor of 1.4× in May and 8.8× in August and caused a 10–30% decline in Ucrit. The seasonal difference in treatment effect was due to the change in ambient sediment stability. The low rain treatment in August had no effect on bed elevation, microphytobenthic biomass or sediment erodibility. In May, the same treatment caused a reduction in bed elevation (0.5 mm) and microphytobenthic biomass but counter-intuitively, a decrease in sediment erodibility (ME-30 was reduced by 40%, Ucrit increased by 5%) compared to controls. We attribute this result to removal by rainfall of easily eroded surface flocs and biogenic roughness which resulted in an underlying sediment with a smoother surface and greater resistant to erosion. Results suggest that high intensity rain events may destabilise intertidal sediments making them more susceptible to erosion by returning tidal currents and that the sediment eroded during such events may represent a considerable fraction (up to 25%) of the seasonal variation in shore elevation. The impact of natural rain events are likely to vary considerably due to variations in droplet size, intensity and duration and the interaction with ambient sediment stability

year: 2008

title: Experiences of Tongan Women Migrants at Paid Work in New Zealand

authors: Fa'anunu, Sinama Tupou

subjects: Tongan women

abstract: The employment experiences of Tongan women migrants have received little attention in the literature.  This study therefore, sought to shed light on the dynamics of their social and economic experiences at paid work in New Zealand.  It was guided by the theories of population geographies, feminist geography and postcolonialism.  The inter-relationships of these theories provided insights into the influence of migration on these women's identities, ethnicity and gender relations and also how these influence these women's experiences at paid work in New Zealand.  The data were drawn from two major sources: i) the New Zealand 2006 population census and ii) in-depth interviews held in Tonga and New Zealand, with greater focus on the interviews.

This study revealed that the Tongan women's decisions for migrating to New Zealand were influenced by social rather than economic incentives.  Migration has challenged these women's traditional roles and reconstructed their gender relations.  Many are breadwinners yet Tongan born men in New Zealand still predominantly engage in the labour force and have higher personal income.  Their experiences at paid work also differ from the New Zealand born Tongan women in New Zealand.  These differences reflect the availability of their social networks and their familiarity with the socio-economic systems in New Zealand.  They experienced successes and failures at paid work on their way to improving their lives in New Zealand.

text: Experiences of Tongan Women Migrants at Paid Work in New Zealand The employment experiences of Tongan women migrants have received little attention in the literature.  This study therefore, sought to shed light on the dynamics of their social and economic experiences at paid work in New Zealand.  It was guided by the theories of population geographies, feminist geography and postcolonialism.  The inter-relationships of these theories provided insights into the influence of migration on these women's identities, ethnicity and gender relations and also how these influence these women's experiences at paid work in New Zealand.  The data were drawn from two major sources: i) the New Zealand 2006 population census and ii) in-depth interviews held in Tonga and New Zealand, with greater focus on the interviews.

This study revealed that the Tongan women's decisions for migrating to New Zealand were influenced by social rather than economic incentives.  Migration has challenged these women's traditional roles and reconstructed their gender relations.  Many are breadwinners yet Tongan born men in New Zealand still predominantly engage in the labour force and have higher personal income.  Their experiences at paid work also differ from the New Zealand born Tongan women in New Zealand.  These differences reflect the availability of their social networks and their familiarity with the socio-economic systems in New Zealand.  They experienced successes and failures at paid work on their way to improving their lives in New Zealand.

year: 2007

title: Opening Address to the National Māori Graduates of Psychology Symposium 2002: Making a difference

authors: Nikora, Linda Waimarie

subjects: Maori

abstract: The opening address to the National Māori Graduates of Psychology Symposium 2002.  This address introduces the themes of the conference: Kia matāra - negotiating the challenges in Māori development, kia mau – recruitment and retention, Tuhia mai, whiua atu – research and methodology, tinia mai – intervantions and treatment, taitaia i te ahi manuka – pride upon the skin.

text: Opening Address to the National Māori Graduates of Psychology Symposium 2002: Making a difference The opening address to the National Māori Graduates of Psychology Symposium 2002.  This address introduces the themes of the conference: Kia matāra - negotiating the challenges in Māori development, kia mau – recruitment and retention, Tuhia mai, whiua atu – research and methodology, tinia mai – intervantions and treatment, taitaia i te ahi manuka – pride upon the skin.

year: 2003