Sedimentation on the East Coromandel Coast.

The east Coromandel coastline on the northeast coast of New Zealand has been characterised by localised erosion problems since its commercial subdivision. Increasing pressures also rose in the 1980s 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 and resulting impacts on the coastline from such activities. 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. 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 tha 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. Purpose: The east Coromandel shelf study was 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.

Current meter data, 563 sediment samples (shelf, barrier and rivermouth), sidescan sonar, sub-bottom profile data. Much of the data was gathered by Dell and Colby (Colby, 1984; samples between Otama and Whangapoua, 0-30 m water depth. Dell, 1985; samples between Waihi and Whangamata, 5-50 m depth). Sediment samples: Whiritoa (>70 samples, 0-50 m); Otahu/Whangamata (50-60 samples, 0-30 m); Onemana (18 samples, mostly <30 m); Opoutere (14 samples, most <30 m); Hotwater Beach (9 samples, 0-50 m); Hahei (5 samples, 10-30 m); Mercury Bay (8 samples, most >10 m); Otama (~67 samples, <30 m); Matarangi/Whangapoua (93 samples, 0-30 m). Sediment data given in appendices: Mean grain size, mode, sorting, kurtosis. %Fe and %carbonate (across shore transects), angularity, colour. Observations of bedforms. Box core descriptions. Detailed description of mineralogy. Seabed photographs (7) Chapters: 1 INTRODUCTION 2 REGIONAL SETTING 3 REVIEW OF SHELF SEDIMENTATION PROCESSES 4 REGIONAL OCEANOGRAPHIC PATTERNS 5 SURFICIAL SEDIMENT TEXTURAL PATIERNS 6 SHELF MORPHOLOGICAL SURVEYS 7 SHELF MORPHO DYNAMICS 8 SHALLOW SUB-BOTTOM SHELF STRATIGRAPHY 9 SEDIMENT COMPOSITION 10 CONCEPTUAL MODELS FOR SHELF SEDIMENTATION AND EVOLUTION 11 IMPLICATIONS AND CONCLUSIONS OF STUDY Appendices: 1 Current measurement information 2 Sediment sample locations 3 Sed~ment texture results 4 Side-scan sonar and sub-bottom fix positions 5 Bedform surveys 6 Box core data 7 Radiocarbon dates 8 Mineralogy results 9 Entropy Program Results

Offshore, eastern Coromandel Peninsula, NZ

1984-1990

Current information gathered with seabed drifters and Aandera current meters. 563 seabed surfact sediment samples. 620 km of sidescan sonar. 174 km of sub bottom profiles. Frequency of collection: Irregular. Samples collected over two studies, and several sampling dates.

Gaps in collection: N/a Attribute accuracy: All sample locations recorded using ship radar and/or sextants. Locations shown on maps with little detail. No coordinates given. Completeness: There are lots of sediment samples collected between Waihi and Opoutere, and lots in the Otama/Whangapoua area, relatively few between these areas.

All sediment textural details recorded in hard copy within thesis. Digital Format: n/a

Was undertaken for the purposes of PhD Thesis, University of Waikato.

None Sensitivity/Confidentiality: None, usual copyright applies.

Study complete.