title: Use of high-intensity data to define large river management units: A case study on the lower Waikato River, New Zealand

authors: Pingram, Michael Alister

subjects: biofish

abstract: The importance of environmental heterogeneity in lotic ecosystems is well recognised in river management, and continues to underpin studies of hierarchical patch dynamics, geomorphology and landscape ecology. We evaluated how physical characteristics and water chemistry measurements at high spatiotemporal resolution define channel units of potential ecological importance along 134 km of the lower Waikato River in North Island, New Zealand. We used multivariate hierarchical clustering to classify river reaches in an a priori unstructured manner based on (i) high-frequency, along-river water quality measurements collected in four seasons and (ii) river channel morphology data resolved from aerial photos for 1-km long reaches. Patterns of channel character were shaped by the depth and lateral complexity of constituent river reaches, while water quality patterns were represented by differences in clarity, chlorophyll fluorescence and specific conductance driven by tributary inflows in the mid-section of the river and tidal cycles in the lower section. Management units defined by physical characteristics or water quality did not necessarily align with boundaries typically reflecting clinal processes (e.g. tidal influence) or geomorphic, network or anthropogenic discontinuities. The results highlight the dynamic spatial and temporal properties of large rivers and the need to define clear objectives when deriving spatial units for management and research. Given that actions and targets for physical channel and water quality management may differ, the spatial extent identified for each of these does not necessarily need to directly coincide, although both should be considered in decision making and experimental design.

text: Use of high-intensity data to define large river management units: A case study on the lower Waikato River, New Zealand The importance of environmental heterogeneity in lotic ecosystems is well recognised in river management, and continues to underpin studies of hierarchical patch dynamics, geomorphology and landscape ecology. We evaluated how physical characteristics and water chemistry measurements at high spatiotemporal resolution define channel units of potential ecological importance along 134 km of the lower Waikato River in North Island, New Zealand. We used multivariate hierarchical clustering to classify river reaches in an a priori unstructured manner based on (i) high-frequency, along-river water quality measurements collected in four seasons and (ii) river channel morphology data resolved from aerial photos for 1-km long reaches. Patterns of channel character were shaped by the depth and lateral complexity of constituent river reaches, while water quality patterns were represented by differences in clarity, chlorophyll fluorescence and specific conductance driven by tributary inflows in the mid-section of the river and tidal cycles in the lower section. Management units defined by physical characteristics or water quality did not necessarily align with boundaries typically reflecting clinal processes (e.g. tidal influence) or geomorphic, network or anthropogenic discontinuities. The results highlight the dynamic spatial and temporal properties of large rivers and the need to define clear objectives when deriving spatial units for management and research. Given that actions and targets for physical channel and water quality management may differ, the spatial extent identified for each of these does not necessarily need to directly coincide, although both should be considered in decision making and experimental design.

year: 2014

title: Wave-frequency flows within a near-bed vegetation canopy

authors: Henderson, Stephen M.

subjects: Science & Technology

abstract: We study water flows and wave dissipation within near-bed pneumatophore canopies at the wave-exposed fringe of a mangrove forest on Cù Lao Dung Island, in the Mekong Delta. To evaluate canopy drag, the three-dimensional geometry of pneumatophore stems growing upward from the buried lateral roots of Sonneratia caseolaris mangroves was reconstructed from photogrammetric surveys. In cases where hydrodynamic measurements were obtained, up to 84 stems per square meter were observed, with stem heights <  0.6 m, and basal diameters 0.01–0.02 m. The parameter α =(frontal area of pneumatophores blocking the flow)/(canopy volume) ranged from zero to 1.8m−¹. Within-canopy water velocity displayed a phase lead and slight attenuation relative to above-canopy flows. The phase lead was frequency-dependent, ranging from 0 to 30 degrees at the frequencies of energetic waves ( >0.1 Hz), and up to 90 degrees at lower frequencies. A model is developed for wave-induced flows within the vertically variable canopy. Scaling suggests that acceleration-induced forces and vertical mixing were negligible at wave frequencies. Consistent with theory, drag-induced vertical variability in velocity scaled with ꓥ = Tw/(2πTf) , where Tw=wave period, Tf=2/(CD₍ₐ|ᵤ|₎) is the frictional time scale, CD≈2 is the drag coefficient, and  |ᵤ| is a typical flow speed. For fixed wave conditions ( |ᵤ| and Tw), theory predicts increasing dissipation with increasing vegetation density (i.e. increasing a), until a maximum is reached for order-one Λ. For larger Λ, within-canopy flow is so inhibited by drag that further increases in a reduce within-canopy dissipation. For observed cases, Λ≤0.38 at energetic wave frequencies, so wave dissipation near the forest edge is expected to increase with increasing pneumatophore canopy density. However, under different wave conditions, the most dense canopies may occasionally approach the dissipation maximum (Λ≈1). Predicted dissipation by the pneumatophore canopy was sufficient to attenuate most wave energy over distances slightly less (more) than 100 m into the marsh in 1 m (2 m) water depth.

text: Wave-frequency flows within a near-bed vegetation canopy We study water flows and wave dissipation within near-bed pneumatophore canopies at the wave-exposed fringe of a mangrove forest on Cù Lao Dung Island, in the Mekong Delta. To evaluate canopy drag, the three-dimensional geometry of pneumatophore stems growing upward from the buried lateral roots of Sonneratia caseolaris mangroves was reconstructed from photogrammetric surveys. In cases where hydrodynamic measurements were obtained, up to 84 stems per square meter were observed, with stem heights <  0.6 m, and basal diameters 0.01–0.02 m. The parameter α =(frontal area of pneumatophores blocking the flow)/(canopy volume) ranged from zero to 1.8m−¹. Within-canopy water velocity displayed a phase lead and slight attenuation relative to above-canopy flows. The phase lead was frequency-dependent, ranging from 0 to 30 degrees at the frequencies of energetic waves ( >0.1 Hz), and up to 90 degrees at lower frequencies. A model is developed for wave-induced flows within the vertically variable canopy. Scaling suggests that acceleration-induced forces and vertical mixing were negligible at wave frequencies. Consistent with theory, drag-induced vertical variability in velocity scaled with ꓥ = Tw/(2πTf) , where Tw=wave period, Tf=2/(CD₍ₐ|ᵤ|₎) is the frictional time scale, CD≈2 is the drag coefficient, and  |ᵤ| is a typical flow speed. For fixed wave conditions ( |ᵤ| and Tw), theory predicts increasing dissipation with increasing vegetation density (i.e. increasing a), until a maximum is reached for order-one Λ. For larger Λ, within-canopy flow is so inhibited by drag that further increases in a reduce within-canopy dissipation. For observed cases, Λ≤0.38 at energetic wave frequencies, so wave dissipation near the forest edge is expected to increase with increasing pneumatophore canopy density. However, under different wave conditions, the most dense canopies may occasionally approach the dissipation maximum (Λ≈1). Predicted dissipation by the pneumatophore canopy was sufficient to attenuate most wave energy over distances slightly less (more) than 100 m into the marsh in 1 m (2 m) water depth.

year: 2017

title: Prototypical gang recruits in New Zealand prisons

authors: Lett, Daniel

subjects: Gangs

abstract: Gangs have long been associated with antisocial behaviour and crime. To maintain themselves, gangs require a steady flow of new recruits. A better understanding of who is being recruited into gangs is the first step in preventing recruitment and lowering the impact of gangs on society. We asked what the prototypical gang recruit in New Zealand prisons is? To answer this, we needed to establish which factors are most likely to make someone more prototypical. Using these factors as our focus, we completed a series of difference of proportion analyses on archival data acquired through Te Ara Poutama. We found that the most prototypical factors for a gang recruit in New Zealand prisons were being a Māori, repeat offender, with a medium RoC*RoI, convicted of violent crimes, being held in a high security unit, and being between the ages of 20-39. These results suggest that prototypicality for gang recruits varies across settings.

text: Prototypical gang recruits in New Zealand prisons Gangs have long been associated with antisocial behaviour and crime. To maintain themselves, gangs require a steady flow of new recruits. A better understanding of who is being recruited into gangs is the first step in preventing recruitment and lowering the impact of gangs on society. We asked what the prototypical gang recruit in New Zealand prisons is? To answer this, we needed to establish which factors are most likely to make someone more prototypical. Using these factors as our focus, we completed a series of difference of proportion analyses on archival data acquired through Te Ara Poutama. We found that the most prototypical factors for a gang recruit in New Zealand prisons were being a Māori, repeat offender, with a medium RoC*RoI, convicted of violent crimes, being held in a high security unit, and being between the ages of 20-39. These results suggest that prototypicality for gang recruits varies across settings.

year: 2021