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The Continental Shelf as a Site for Dredged Material Disposal, Northeast New Zealand

Disposal of dredged material has been an on-going problem in the Auckland Coastal Marine Area (CMA) since the early 1980s in New Zealand. Many disposal grounds have been established and used, but public concern over adverse effects resulted in their ultimate closure. Presently, dredged material is disposed off-shore at a site simultaneously accessed by the Royal New Zealand Navy for disposal of WWII munitions recovered from coastal areas. As early as the mid-1990s, parliamentary focus groups established the need for a more suitable disposal option for dredged material. Establishment of a disposal site north of Cuvier Island in waters deeper than 100 m was one of the key recommendations presented by these groups. The need for a new site was compounded after the establishment of the Hauraki Gulf Marine Park in 2000. Taking up the majority of the Auckland CMA this culturally significant Park makes the consent for open water disposal a complex process. A site east of Great Barrier Island in 140 m of water has been identified as a potential suitable site for disposal of dredged material. The main goal of the present study is to determine the suitability of this site and provide the necessary information required by enforcing authorities for permit submission. Investigations to determine the suitability of the site were undertaken in several ways. An extensive literature review of previous studies was carried out to gain insight into the physical and biological characteristics of the northeast coast and shelf. The main hydrodynamic features of the region and the observed behaviours were determined. Attentions were then directed at determining the more specific site characteristics. Analytical calculations were undertaken using known site parameters to estimate the potential for transport of sediment away from the site after disposal. Through analysis of known wave and current measurements it was estimated that only rarely would sediment be entrained off the seafloor. Samples were then collected from the site in November 2007, which were used for sediment textural analysis and benthic identification. It was determined that the main textural component of the site sediments is muddy/sand. Diversity of benthic species is relatively high, but abundance is low. Polychaetes were the most diverse and abundant taxon identified at sample locations across the site. Next, the 3DD model was used to numerically simulate 2-dimensional tidal currents. Depth-averaged spring tidal currents at the site were predicted to be less than 0.2 ms-1. The derived bottom velocity for such a current is 0.08 ms-1, which is much less than the velocity required for initiation of sediment movement in this case. The numerical simulation also showed that residual spring tidal flow is directed to the southeast. Finally, an assessment of potential impacts was done by reviewing previous studies of ecological impacts caused by disposal of dredged material. Based on the preliminary studies summarised above, the review of potential impacts indicates that there will only be minimal effects at and surrounding the proposed site. The result of this study is an encouraging step toward establishment of a new disposal option, but further research is required to confidently declare that the site is suitable for disposal operations.
Type of thesis
Flaim, B. K. (2008). The Continental Shelf as a Site for Dredged Material Disposal, Northeast New Zealand (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/2445
The University of Waikato
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