Dredge Spoil and Inner Shelf Investigations off Tauranga Harbour, Bay of Plenty, New Zealand

Healy, Terry; Harms, C.; de Lange, W.; ,

Dredge Spoil and Inner Shelf Investigations off Tauranga Harbour, Bay of Plenty, New Zealand

by Terry Healy, Univ of Waikato, Hamilton, New Zealand,
C. Harms, Univ of Waikato, Hamilton, New Zealand,
W. de Lange, Univ of Waikato, Hamilton, New Zealand,

Document Type: Proceeding Paper

Part of: Coastal Sediments

Abstract:

Dispersion of dredge spoil from an inner shelf dump-mound in 11-17 m water depth, 3 km offshore from Tauranga Harbour, Bay of Plenty, New Zealand, was investigated as part of environmental monitoring. Graphic representation and volumetric calculations, based on hydrographic surveys for a 10 year period, were used to analyse bathymetric changes over the dump-mound. In addition, textural analyses of surficial samples in conjunction with sub-bottom seismic profiling, allowed interpretation concerning direction of sediment movement. These data were related to hydrodynamic data in order to develop a temporal and spatial conceptual model of sediment dispersion from the dump-mound. Volume reduction of the dump-mound, which consists of medium to coarse sand and pumice granules, was highest in the first 2 - 3 years. Material eroded from the dredge spoil accumulated in a zone 300 m shoreward of the dump-mound. Sediment textural parameters of sorting and skewness suggest that a lag surface had formed over the dredge spoil and that a net movement of fine sand and pumice granules occurs in an onshore direction. Current measurements during the period of monitoring showed a weak onshore directed current associated with light onshore as well as strong offshore winds, and a strong northwest directed alongshore flow during an easterly storm. It is likely that the medium-coarse sand accumulation zone shoreward of the dump-mound is caused by a number of processes including interaction of waves and onshore currents, storm induced upwelling effects, wave focussing, and a decrease in wave threshold velocity on the inshore spoil mound slope.

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