title: Coastal and sediment dynamics in a high-energy, rocky environment

authors: McComb, Peter

abstract: A study of the wave dynamics, hydrodynamics and sediment dynamics was conducted at an energetic open-coast site at New Plymouth, New Zealand. The location features large, shore-normal reefs of volcanic agglomerate extending from an irregular shoreline, with “black” volcanic sands intermittently overlying a rocky bed. The breakwaters of an artificial harbour (Port Taranaki) interrupt the longshore littoral transport, and dredging and offshore dumping causes “downstream” erosion along the city foreshore. The key study objectives were to investigate the local coastal dynamics, and to examine ways to mitigate the effects of the harbour on the adjacent coast. 

Two large experimental field programmes were conducted involving: i) wave, current and suspended sediment measurements, ii) side-scan sonar and hydrographic survey, iii) sediment tracing, and iv) experimental dumping of 47,000 m³ of dredged sand in a rocky nearshore zone (6-10 m depth). Sediment trap calibration experiments were also performed. 

Analysis of the measured wave data has identified wave-induced motion as a significant source of velocity error in p, u, v meters on taut-wire moorings, and wave directions resolved from the cross-spectra of pressure and velocity have frequency dependent errors due to the phase difference between the pressure and velocity data. Accordingly, the spectra of wave direction should be resolved solely from velocity data. Calibration of sediment traps against an automated water sampler has provided the first strong evidence that traps are effective in deriving the time-averaged suspended sediment concentration SSC in a wave-dominated coastal environment. Rigid cylindrical traps of high Aspect Ratio have a 75% sampling efficiency. 

Wave transformations across the complex nearshore bathymetry are dominated by seabed friction and refraction. This process was simulated using the WBEND wave refraction model, applying a constant friction coefficient for sandy beds, and a Nikuradse roughness for rocky beds. Wind vector was found to influence the spatial distribution of wave energy, with whitecapping conditions under onshore-directed winds inducing a longshore “smoothing” in the wave energy, which appears to act in opposition to wave height reinforcement from refraction. 

Sediment tracing using an artificial fluorescent material proved to be an effective method for observing the transport of littoral sediments over sandy and rocky beds, over distances of up to 5 km. The tracer exhibited a horizontal diffusivity of the order 0.1 m²s⁻¹, of which approximately 5% was expressed in the cross-shore direction. 
The time-averaged entrainment (C₀) of “black” volcanic sands under high-energy conditions may be effectively predicted using the excess non-dimensional skin friction (θ₂.₅) raised to the power of 1.5, with the mean grain size used in the sediment mobility term (Ψ) and the median grain size in the friction factor (fw). The scale of turbulent mixing (lₛ) increases linearly with elevation above the bed, with a gradient of 0.46. Suspended sediments show a systematic (and linear) reduction in mean grain size with increasing elevation above bed. 

SSCs over rocky beds are supply-limited, and inversely proportional to the distance from contiguous sandy beds. Sediments are principally transported in the suspended form, and a system of irregular shore-normal reefs does not significantly obstruct the longshore flux. Rocky beds appear to be resilient to sediment inundation and the sand/rock sediment facies have remained positionally stable over 16 years. Dumping sand in a rocky reef environment does not necessarily result in adjacent beds becoming inundated with sand. In this environment, a nearshore dump mound did not migrate as a contiguous body, but slowly dispersed as suspended sediments. 

Net sediment transport at New Plymouth is directed alongshore to the northeast, although the sediments respond to local and temporal perturbations, which give rise to reversing and circulating fluxes. Port Taranaki traps 174,000 m³ of coastal sediments per year, of which 142,000 m³ accumulates at the breakwater tip. The latter are derived from the shallow zone seaward of the breakwater wall (i.e. within ∼50 m). Sediments in depths > 6 m (i.e. > 100 m from the breakwater wall) bypass the harbour and traverse the rocky reefs toward the eastern beaches. The historical offshore dumping practice removes sediments from the nearshore littoral budget, while the orientation of the main port breakwater causes the longshore sediment flux to be directed away from the adjacent (Kawaroa) coast, further reducing the supply of littoral sediments to that region. 

A new dumping ground has been identified for the breakwater-tip sediments, which is projected to retain the dredged sediments within the nearshore littoral system, feeding sand to the central and eastern city foreshore. Sedimentation of the coast immediately adjacent to the port is not expected to result from the long-term use of this ground.

text: Coastal and sediment dynamics in a high-energy, rocky environment A study of the wave dynamics, hydrodynamics and sediment dynamics was conducted at an energetic open-coast site at New Plymouth, New Zealand. The location features large, shore-normal reefs of volcanic agglomerate extending from an irregular shoreline, with “black” volcanic sands intermittently overlying a rocky bed. The breakwaters of an artificial harbour (Port Taranaki) interrupt the longshore littoral transport, and dredging and offshore dumping causes “downstream” erosion along the city foreshore. The key study objectives were to investigate the local coastal dynamics, and to examine ways to mitigate the effects of the harbour on the adjacent coast. 

Two large experimental field programmes were conducted involving: i) wave, current and suspended sediment measurements, ii) side-scan sonar and hydrographic survey, iii) sediment tracing, and iv) experimental dumping of 47,000 m³ of dredged sand in a rocky nearshore zone (6-10 m depth). Sediment trap calibration experiments were also performed. 

Analysis of the measured wave data has identified wave-induced motion as a significant source of velocity error in p, u, v meters on taut-wire moorings, and wave directions resolved from the cross-spectra of pressure and velocity have frequency dependent errors due to the phase difference between the pressure and velocity data. Accordingly, the spectra of wave direction should be resolved solely from velocity data. Calibration of sediment traps against an automated water sampler has provided the first strong evidence that traps are effective in deriving the time-averaged suspended sediment concentration SSC in a wave-dominated coastal environment. Rigid cylindrical traps of high Aspect Ratio have a 75% sampling efficiency. 

Wave transformations across the complex nearshore bathymetry are dominated by seabed friction and refraction. This process was simulated using the WBEND wave refraction model, applying a constant friction coefficient for sandy beds, and a Nikuradse roughness for rocky beds. Wind vector was found to influence the spatial distribution of wave energy, with whitecapping conditions under onshore-directed winds inducing a longshore “smoothing” in the wave energy, which appears to act in opposition to wave height reinforcement from refraction. 

Sediment tracing using an artificial fluorescent material proved to be an effective method for observing the transport of littoral sediments over sandy and rocky beds, over distances of up to 5 km. The tracer exhibited a horizontal diffusivity of the order 0.1 m²s⁻¹, of which approximately 5% was expressed in the cross-shore direction. 
The time-averaged entrainment (C₀) of “black” volcanic sands under high-energy conditions may be effectively predicted using the excess non-dimensional skin friction (θ₂.₅) raised to the power of 1.5, with the mean grain size used in the sediment mobility term (Ψ) and the median grain size in the friction factor (fw). The scale of turbulent mixing (lₛ) increases linearly with elevation above the bed, with a gradient of 0.46. Suspended sediments show a systematic (and linear) reduction in mean grain size with increasing elevation above bed. 

SSCs over rocky beds are supply-limited, and inversely proportional to the distance from contiguous sandy beds. Sediments are principally transported in the suspended form, and a system of irregular shore-normal reefs does not significantly obstruct the longshore flux. Rocky beds appear to be resilient to sediment inundation and the sand/rock sediment facies have remained positionally stable over 16 years. Dumping sand in a rocky reef environment does not necessarily result in adjacent beds becoming inundated with sand. In this environment, a nearshore dump mound did not migrate as a contiguous body, but slowly dispersed as suspended sediments. 

Net sediment transport at New Plymouth is directed alongshore to the northeast, although the sediments respond to local and temporal perturbations, which give rise to reversing and circulating fluxes. Port Taranaki traps 174,000 m³ of coastal sediments per year, of which 142,000 m³ accumulates at the breakwater tip. The latter are derived from the shallow zone seaward of the breakwater wall (i.e. within ∼50 m). Sediments in depths > 6 m (i.e. > 100 m from the breakwater wall) bypass the harbour and traverse the rocky reefs toward the eastern beaches. The historical offshore dumping practice removes sediments from the nearshore littoral budget, while the orientation of the main port breakwater causes the longshore sediment flux to be directed away from the adjacent (Kawaroa) coast, further reducing the supply of littoral sediments to that region. 

A new dumping ground has been identified for the breakwater-tip sediments, which is projected to retain the dredged sediments within the nearshore littoral system, feeding sand to the central and eastern city foreshore. Sedimentation of the coast immediately adjacent to the port is not expected to result from the long-term use of this ground.

year: 2001

title: Evaluating soil and landscape models to predict liquefaction susceptibility in the Hinuera Formation, Hamilton Basin

authors: McKay, Aleesha

subjects: liquefaction

abstract: Cone Penetration Tests (CPT) derived from the Hamilton section of the Waikato Expressway were analysed within CLiqTM software. The derived Liquefaction Potential Index (LPI) from each CPT was then combined with LIDAR, pedological and geological maps for statistical analysis. A soil model that incorporates the conditions of modern soil development with these derived LPI values was developed as a preliminary assessment tool for liquefaction potential within Hamilton Basin soils. The model shows that liquefaction is more likely to occur on interfluvial areas where there is little topographical relief. Pedological soils with high organic component are also a likely indicator of high liquefaction susceptibility.

text: Evaluating soil and landscape models to predict liquefaction susceptibility in the Hinuera Formation, Hamilton Basin Cone Penetration Tests (CPT) derived from the Hamilton section of the Waikato Expressway were analysed within CLiqTM software. The derived Liquefaction Potential Index (LPI) from each CPT was then combined with LIDAR, pedological and geological maps for statistical analysis. A soil model that incorporates the conditions of modern soil development with these derived LPI values was developed as a preliminary assessment tool for liquefaction potential within Hamilton Basin soils. The model shows that liquefaction is more likely to occur on interfluvial areas where there is little topographical relief. Pedological soils with high organic component are also a likely indicator of high liquefaction susceptibility.

year: 2017

title: Civilisation, settlers and wanderers: Law, politics and mobility in nineteenth century New Zealand and Australia

authors: Seuffert, Nan

subjects: colonization

abstract: Mobility was constitutive of the 19th century British colonial period in the Pacific. The circulation of capital and commodities, technologies of transportation and communication, travelling ideologies and systems of governance and surveillance, as well as the movement of explorers, whalers, labourers, settlers, missionaries, colonial administrators, convicts, soldiers, sojourners, immigrants, and transnational and displaced indigenous peoples, all shaped the politics and the period (see Ballantyne 2009: 7-8; Seuffert 2006: 7-8; Arrighi 1994: 48-58). Highly mobile British and European immigrants with money or skills were termed 'settlers', with associated connotations of stabilising and civilising influences. Missionaries and colonial officials who were integral to colonisation were also often highly mobile, carrying policies and regulatory regimes with them, and their colonial roles as 'civilising' influences included 'settling' and advancing the position of indigenous and other colonised peoples. In contrast, the mobility of poor whites and racialised populations - such as 'Melanesian' indentured labourers (Banivanua-Mar 2007: 3-4)2, Indian workers, displaced indigenous peoples and 'sojourner' Chinese - attracted the attention of law (and policy) makers and institutional authorities and were subjected to various forms of surveillance, regulation and policing designed to constrain and contain them.

text: Civilisation, settlers and wanderers: Law, politics and mobility in nineteenth century New Zealand and Australia Mobility was constitutive of the 19th century British colonial period in the Pacific. The circulation of capital and commodities, technologies of transportation and communication, travelling ideologies and systems of governance and surveillance, as well as the movement of explorers, whalers, labourers, settlers, missionaries, colonial administrators, convicts, soldiers, sojourners, immigrants, and transnational and displaced indigenous peoples, all shaped the politics and the period (see Ballantyne 2009: 7-8; Seuffert 2006: 7-8; Arrighi 1994: 48-58). Highly mobile British and European immigrants with money or skills were termed 'settlers', with associated connotations of stabilising and civilising influences. Missionaries and colonial officials who were integral to colonisation were also often highly mobile, carrying policies and regulatory regimes with them, and their colonial roles as 'civilising' influences included 'settling' and advancing the position of indigenous and other colonised peoples. In contrast, the mobility of poor whites and racialised populations - such as 'Melanesian' indentured labourers (Banivanua-Mar 2007: 3-4)2, Indian workers, displaced indigenous peoples and 'sojourner' Chinese - attracted the attention of law (and policy) makers and institutional authorities and were subjected to various forms of surveillance, regulation and policing designed to constrain and contain them.

year: 2011