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Hauraki Gulf sedimentation: Unintended consequences of flood protection

The Universities of Waikato and Bremen have examined sedimentation in the Hauraki Gulf, focussing on the sediment discharges from the Coromandel Peninsula and the Waihou/Ohinemuri river systems, to assess whether anthropic land-use changes over the last millennium have resulted in increased sedimentation rates. Previous studies identified increased sedimentation due to burning of vegetation following Polynesian settlement, and Kauri logging, mining and forest clearance following European settlement. Most studies attribute the highest sedimentation rates to European settlement. This paper considers the results of investigations of depocenters in the central Hauraki Gulf, Southern Firth of Thames, and Coromandel Harbour, as well as the sediment pathway along the Ohinemuri and Waihou Rivers from mining areas of the southern Coromandel Peninsula to the Hauraki Gulf. Contrary to expectations, the central Hauraki Gulf was found to consist of palimpsest fluvial and biogenic marine deposits, with minor Holocene sedimentation lacking a clear anthropic signal. The shallower depocenters contain thicker sequences of Holocene sediments, with high spatial variability. Areas with clear anthropic signals were those proximal to potential sources. Commonly there was a significant time gap between the palynological indications of European settlement and changes in metal concentrations or inferred sedimentation rates, which was inconsistent with historical records suggesting land disturbance was contemporary with settlement. Gold mining in the southern Coromandel Peninsula generated tailings that were deposited into the Ohinemuri River. Initially the Ohinemuri River aggraded and annual floods deposited tailings on the floodplains, predominantly between the Waihou and Ohinemuri Rivers south of Paeroa. Since 1910, the floodplains have progressively been drained and protected by stopbanks constructed from sediment dredged from the beds of rivers and canals, and intertidal flats; with the final major scheme being completed in 1972. Metal concentrations in surficial sediments indicate most tailings have been deposited on floodplains upstream of Paeroa, with little evidence of transport downstream of the Waihou/Ohinemuri confluence. Since the completion of the stopbanks major floods have predominantly discharged sediment into the Firth of Thames, instead of onto the floodplains. The progressive removal of the floodplains as potential depositional sites coincides with the documented expansion of mudflats and mangrove in the southern Firth of Thames. This may also account for the delayed onset of a significant anthropic signal in the Firth of Thames depocenter.
Conference Contribution
Type of thesis
de Lange, W. P. (2018). Hauraki Gulf sedimentation: Unintended consequences of flood protection (pp. 79–79). Presented at the NZCS Conference 2018: Crossing the water, Whiti I Te Wai, Gisborne, New Zealand.
© 2018 copyright with the author.