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

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.