Nearshore and inner shelf sedimentation on the east Coromandel coast, New Zealand
The east Coromandel coastline on the northeast coast of New Zealand has been characterised by localised erosion problems since its commercial subdivision. Increasing pressures have also arisen in recent years for possible use of the adjacent continental shelf as a repository for dredge and mining tailings, and extraction of commercial aggregate and mineral deposits. This has raised the question of dynamic links between the continental shelf and local beach systems I and resulting impacts on the coastline from such activities. The east Coromandel shelf study was therefore undertaken to investigate the modern and past shelf sedimentation processes, and relate these findings to local resource management issues. The results also contribute to the present international understanding of inner shelf sedimentation processes in 'storm-dominated' shelf environments. Surficial sedimentation patterns have been examined by a variety of methods, including: the use of sea-bed drifters and Aanderaa current meters to determine the shelf hydrodynamics; the collection of 563 samples to determine surface sediment textural patterns; a detailed examination of morphological features on the sea floor through a series of 3 side-scan sonar surveys in which 620 km of sonograph and echosounding trace have been obtained; diver surveys using depth of activity rods to provide information on the mobility of surface sediments under different sea conditions; the collection, of 174 km of continuous sub-bottom seismic profiles which have been used to determine sub-surface stratigraphic patterns; and an examination of the provenance of shelf sediments through a detailed examination of their light and heavy mineralogy. The east Coromandel coast is located on an active plate margin, resulting in the presence of a steep and rocky coastline fronted by a relatively narrow continental shelf (20 to 30 km wide), and a local geology dominated by Tertiary volcanics covered by a thin veneer of more recent volcanic air fall deposits. It is also located on a lee shelf in a mid-latitude zone of dominant westerly winds, with these prevailing weather patterns disrupted by high speed east to northeasterly winds associated with the infrequent passage of subtropical low pressure systems, occluded fronts, or more infrequent decaying tropical cyclones. This physiographic setting results in spatially and temporally highly variable sedimentation patterns, with sediment transport primarily controlled by the interaction of wave oscillatory and wind-generated currents during storm conditions. However, characteristics of tide-. and oceanic current-dominated conditions also occur in some areas. And conditions generally vary along the coast from being higher energy (current-dominated) on the southern exposed coast, to lower energy (wave- or current-dominated) on the northern embayed coastline. Shelf sedimentation is inferred to occur under three different scale events of fair weather, storm, and extreme storm conditions. The model proposed involves extreme storm conditions transporting large quantities of fine nearshore sands seawards onto the inner shelf surface, where they are slowly reworked along the shelf during annual storm events and back onshore during fair weather conditions. This latter onshore return of fine sands is proposed to result from an enhanced shear stress effect of long-period swell waves over coarse megarippled inner shelf sands. Shelf sedimentation patterns within the study area are inferred to have been primarily established by processes occurring at about the time sea level stabitised 6500 years B.P. At this stage, transgressive shelf sediments were reworked into equilibrium with the regional oceanographic conditions by the winnowing of fine sand from coarser material during storms, with the fine sands subsequently transported onshore by wave-induced currents during calm periods. Depending on the volume of sediments incorporated into transgressive sand bodies and the local shelf gradients, each embayment then developed into either an integral part of the shelf transport system (usually off large estuary systems), with fine shelf sands moving onshore and forming wide barrier spits, or a closed sedimentary system unaffected by shelf sedimentation patterns (pocket embayments). Shelf equilibrium was evidently attained by the fine sands forming a steep seaward dipping concave-up nearshore profile, and coarse sands developing a flatter inner shelf profile. An autochthonous mode of shelf sedimentation characterised the east Coromandel coast in early Holocene times, and is still evident under present conditions in most areas by the cyclical reworking of fine sands between the lower nearshore and inner shelf surfaces. Most east Coromandel embayments are, however, presently characterised by varying degrees of an allochthonous mode of shelf sedimentation. This involves the deposition of very fine grained, volcanic glass-enriched terrigenous sands, which are derived from either local catchments or Bay of Plenty river systems. The east Coromandel coast generally differs from most allochthonous shelf environments because the supply of terrigenous sediments has not as yet been sufficient to blanket the lower nearshore and inner shelf surfaces on the open ocean coastlines. Two separate modes of shelf sedimentation occur at present, with a nearshore inner shelf recycling of fine allochthonous shelf sands, and an open exchange of terrestrial autochthonous deposits from the upper nearshore surface to the mid shelf plain. The results of the study suggest that coastal erosion problems can largely be resolved by establishing stable incipient and frontal dune systems along the coast. and encouraging accretion by renourishment from adjacent infilling harbour deposits. The study has also shown that the dispersion of dredged sediments provides potential benefits to the coast in terms of promoting beach accretion, and that sand and mineral extraction can proceed without adverse coastal erosion effects, provided the extraction does not interfere with the natural shelf sedimentation system.
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