Sediment Dispersion at the New Auckland Marine Disposal Ground, Northeast New Zealand

Abstract

A controversial history of near shore dredged material disposal east of Auckland, New Zealand, starting in the mid-1980s, resulted in the use of a temporary deep water site that did not satisfy the requirements of the London Dumping Convention. In New Zealand, since most maintenance dredged material contains a low level of contamination, it is a common requirement that open-sea disposal sites are retentive, so that impacts can be monitored. In 2007, preliminary investigations for a proposed site located on New Zealand’s northeast shelf were initiated. Indications that the site was suitable prompted Maritime New Zealand (MNZ) to grant a permit for a trial disposal of 5,000 m3 of muddy dredged material on the condition that disposal operations were monitored to assess the potential for dispersion of the material beyond the boundary of the site. The overall aim for this thesis research, based on the questions raised by MNZ, was to determine the potential for dispersion at the AMDG and classify the site based on its dispersive qualities. This aim was approached in three ways: (i) investigation of the hydrodynamic setting, (ii) measurement of the disposal process and the resultant plume, and (iii) development and implementation of a model designed to simulate the disposal characteristics under conditions not observed in the field. The first two approaches involved measurement campaigns, which were undertaken in 2008 and 2010. The 2008 campaign primarily focused on investigation of the hydrodynamic setting through the deployment of a long-term upward facing ADP, which was complemented by hydrological measurements (CTD), and nearby wind records. The 2010 campaign corresponded to the trial disposals at the AMDG, where 4 disposals were monitored using a range of techniques. Stationary water sampling and OBS turbidity measuring stations were positioned in the vicinity of the disposal location with the intention of recording data that could be used to calibrate backscatter data recorded with a vessel mounted ADCP. These data were supplemented by additional pre- and post-disposal measurements, such as sediment cores, MBES backscatter, dynamic penetrometer profiles, and under water video imagery, which provided information on the depositional fate of the disposed material. Therefore, measurements were collected during all stages of the disposal process, providing a unique dataset for a deep-water disposal site. Due to the low number of published studies on disposal plume dispersal and the site specific nature of the process, it was not known in advance what the most efficient and practical techniques for monitoring the plume were. The identification of optimal measuring methods was a secondary outcome of this work. It was found that, due to the transient nature of the plume, stationary sampling techniques were not able to satisfactorily record the plume because its position was difficult to predict. Taking sequential measurements along transects proved to be the optimal approach for tracking the plume. Specifically, backscatter data from the vessel mounted ADCP records provided the best perspective on the spatial and temporal characteristics of the disposal plume. MBES bathymetry data recorded after the completion of all disposals was ultimately inconclusive regarding depositional fate because the deposits were less than 20 cm thick and, therefore, unresolvable at the water depths of the AMDG and the frequency of the system employed. However, a backscatter map, developed from the same MBES dataset, corroborated some of the findings from the plume monitoring surveys by showing the impact locations of the disposed loads, which appeared as lighter gray patches (higher density substrate) amongst the darker gray natural site sediment areas. Analysis of ADCP backscatter data obtained during the trial disposals indicated that the extent of horizontal dispersion was greatest in the surface region (500 800 m) due to stronger current velocities that occur as a result of the decreased influence of friction from the seabed and the increased influence of wind-driven currents. However, in all cases, after the descent of the dredged material to the seabed during the first few minutes, the maximum concentrations were always located near the seabed where horizontal dispersion was low (~200 m). Based on these findings, it was concluded that the weak ambient forcing mechanisms have the potential for the greatest dispersion, rather than the dynamic forces associated with the disposal process. It was found that while generally producing low ambient current velocities, the dominant forcing mechanisms at the AMDG were temporally variable, which could lead to a range of different dispersion characteristics. Tides, wind, and the East Auckland Current (EAUC) were identified as the predominant drivers. Tidal currents were relatively slow (2 10 cm/s), but in general appeared to be more important than wind-driven currents in the surface zone. The influence of the EAUC varied during the field campaigns, where its influence appeared to be weak during the monitored trial disposals, but strong during the long-term deployment of 2008. This variability corresponds to the findings of other studies undertaken on the dynamics of the northeast coast region. The short-term mechanisms of the disposal process, additionally captured in the ADCP backscatter records, showed similar characteristics to those previously described in the literature (i.e., 1 Convective Descent, 2 Dynamic Collapse, and 3 Passive Dispersion). However, through analysis of the rate of dilution throughout the 3 phases, an additional transitional phase was identified. This phase, observed both spatially (with distance from the disposal location) and temporally (with time after disposal), was characterised by a decreased rate of dilution. From the findings, an alternative conceptual model for the disposal process was developed in which the transitional phase was described as a turbulent zone, where water at the interface of the dynamic zone is set in circular motion, therefore, preventing dilution at a particular location or time. After momentum is reduced enough, the turbulent forces give way to the diffusive forces and passive dispersion becomes the dominant mode for dispersion. The main finding of this research was that the potential for dispersion beyond the boundary of the AMDG is low indicating that the site behaves retentively. This finding is partly a result of the low velocities of the ambient currents, but also because of the operational limitations of the tug-towed disposal method employed for disposal. For minimising the dispersion potential of the AMDG for future operations at the site, it is recommended that the disposal method remain unchanged, that material types more susceptible to dispersion not be disposed there, and that disposal not be undertaken when tidal currents are aligned with the wind direction for winds greater than 20 knots.