Sedimentation Rates in Whangamata Estuary, Coromandel

The East Coromandel, New Zealand, possesses many natural harbours of which Whangamata is one of the most densely populated. In 1991, the Whangamata Community Council was planning long-term development in the Whangamata Harbour, and required a baseline study to be carried out in order to identify existing conditions and avoid possible problems in the future. Abstract from Thesis: The surficial sediment patterns in the tidal inlet and estuary were investigated by measuring textural parameters using the RSA, and both petrographic and cathodoluminescent observation of the mineralogical fraction. The predominantly sandy estuarine sediments were modified by gravels and muds around the harbour margins, and by shell lag at the entrance. The sediments were inferred to be a combination of: a) fluvial sands, gravels and muds derived from the breakdown of volcanic rocks and the erosion of tephras in the hinterland; b) marine-derived sediments swept in off the inner shelf during the post-glacial marine transgression; and c) modern biogenic material. Sedimentation rates were assessed from dating three cores, utilising 14C dating of shell beds to obtain actual dates, palynology to observe vegetation changes in the catchment in the last 6000 years, and 210Pb to provide an insight into sedimentation rates in the last 100 years. Sedimentation rates were found to have increased from 0.1 mm yr-¹ in pre-Polynesian times, to 0.3 mm yr-¹ owing to Polynesian agricultural practises, to 11 mm yr-¹ since 1880, owing to European land clearance, and more recently, the development and felling of forestry. Tidal inlet stability was assessed by completing a tidal gauging. According to the area/prism relationship established by Hume & Herdendorf (1988), the inlet gorge fell within the 95% confidence interval for stability for a single spit barrier estuary. All equations used to predict an equilibrium cross-sectional area calculated a substantially smaller area than that measured in the field, which indicated that the inlet cross-section has the potential to accrete if sediment availablity allows. Application of Bruun's depth/width ratio indicated that the inlet was hydraulically efficient, probably as the result of tidal forcing of currents against the rocky headland. The Whangamata inlet has maintained locational stability for the last 700 years based on the presence of Maori artifacts discovered on the beach adjacent to the entrance. Monitoring inlet cross-sectional changes by echo-sounded survey transects over fourteen months, and diver bedform observations, indicated that the inlet was undergoing an accretory phase during 1989 and 1990. Numerical wave simulation was undertaken to indicate points of wave focusing and consequent areas of dune susceptibility to erosion, and to investigate the effect of sea level rise from possible global warming. Numerical wave analysis indicated Whangamata beach was most susceptible to wave focusing and consequent erosion to winds from the northeast and southeast. The entrance becomes unuseable in strong southeasterly winds, and the inlet location may have been influenced by the lack of wave focusing in this area. Historical photographic analysis since 1944 revealed a sizeable increase in the density and colonisation by mangroves adjacent to residential areas. Dune width along the main beach decreased from 180 m of blow-out dunes in 1944 to a maximum of 40 m today, owing to residential construction on the dunes. Recommendations were offered regarding future development, tourism, the increase of mooring facilities, and comments on the siting of a marina. Dredging of the Moanaanuanu channel was discussed, as was the problem of flooding of the Tikoti golf course upstream of a causeway constructed in 1976.

Study of estuarine sedimentation in Whangamata estuary. Sediment samples taken and analysed for texture, and both petrographic and cathodoluminescent observation of the mineralogical fraction. Sedimentation rates from cores - radiocarbon 14C and 210Pb dating, pollen analysis. Tidal gauging undertaken. Echo sounded survey across channel Diver bedform observations. Numerical wave simulation. Photographic analysis of estuary and beach.

Whangamata Harbour and Beach

1989-1990

Textural parameters of sediment determined using Rapid Sediment Analyser (RSA) Other methods as described above. Frequency of collection: Mostly one-off data collection. Some repeated surveys of estuary entrance.

Data quality: Variable

Reported in hard copy thesis.

Master of Science thesis. Assessment of Whangamata marina viability and impacts. Assessment of infilling rates.

Thesis available in University of Waikato library. No confidentiality.

Study completed.

Related information: Whangamata marina application.