Dredge spoil dispersion from an inner shelf dump-mound
A study was undertaken on the dispersion of dredge spoil from a dump-mound on the inner shelf in 11-17 m water depth, 3 km offshore from Tauranga Harbour, New Zealand. Graphic and volumetric analysis of bathymetric changes over the dump-mound and its near vicinity, between 1978 and 1988, showed that based on echo-sounding surveys a high reduction in dump-mound volume occurs in the first two to three years after the dumping. These changes can be explained by high compaction and erosion of the unconsolidated dredge spoil. Between 1980 and 1985 about two thirds of the eroded material accumulate in a semi-circle up to 400 m south of the dump-mound. From 1985 onwards, after about seven years, the dump-mound appears to be stable. A sea-floor survey was undertaken, including side-scan sonar, sub-bottom seismic profiling, visual observations by underwater-video and SCUBA diving, and remote sediment sampling. It showed the existence of a natural coarse sand facies with wave-generated megaripples (coarse-grained ripples) which is overlain by a lightly rippled to flat fine sand facies. Repeated surveys indicate that the bedforms react to storms. It is suggested that the fine sand blanket is stripped off the coarse sand at places causing a patchy coarse sand distribution. The natural sediment facies distribution at the dump-sites is masked by dumped medium sands from Tauranga Harbour and its ebb tidal delta. Sorting and skewness parameters and pumice content indicate that a lag surface has formed over the dump-site. Natural sediments in the onshore vicinity of the dumpsites have been altered by the deposition of fine sand and pumice after their erosion from the dredge spoil suggesting a general net onshore movement of sediment. The mineralogical similarity of the dredge spoil and the natural inner shelf sediments prevents any larger scale tracing of dredge spoil. Wave data collected outside of this study indicate that the threshold velocity for fine sand (2 ɸ) over the dump-mound is exceeded 30 - 50 % of the time. Current meter records over a period of 18 days in June 1988 show a weak southerly directed current associated with light onshore and strong offshore winds, and a strong northwest directed alongshore flow during an easterly storm. Threshold velocities of unidirectional and combined flow (unidirectional flow modified by waves) are rarely exceeded over a rippled bed of fine sand during peak flow. Total boundary shear stresses indicate that stresses induced by waves are increased by the additional effect of currents. The current also provides a directional component to the stress. It is suggested that low threshold velocities on a gentle slope increase erosion of coarse material from the dump-mound. This material is only transported over a small distance and deposited in a small zone south of the mound. Fine sand and pumice are temporary deposited in the with the fine sand blanket coarse sand facies but eventually mix entering the exchange zone between the inner shelf and beach systems. No serious environmental impacts of dredge spoil so far. Dumping can be assessed from the dumping of dredge spoil on the inner shelf are not very pumice do not effective for beach remain at the beach nourishment as fine sand and permanently because of their hydraulic difference to the medium sized beach sand.
Networked Digital Library of Theses and Dissertations