Bathymetry and sediments of Ngatangiia Harbour and Muri Lagoon, Rarotonga, Cook Islands

Collins, William T. (1995) Bathymetry and sediments of Ngatangiia Harbour and Muri Lagoon, Rarotonga, Cook Islands. Masters thesis, Memorial University of Newfoundland.

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Abstract

Ngatangiia Harbour and Muri Lagoon are situated on the East side of Rarotonga, Cook Islands. They consist of a reef passage, a harbour up to 3.5 m deep, an inner channel and a lagoon. A shallow reef flat with three islets or motus separates the inner channel from the reef edge. A fourth motu exists on the southern edge of the lagoon. The lagoon is a wide moat 1.25 m below mean sea level at its deepest point. -- Analysis of bathymetric surveys dating from 1948 has provided information on the historical development of the area. The data over the recording period showed an infilling of the harbour from 1948 which accelerated in the 1980's. The channel had little net change in sediment deposition where bathymetry is controlled by reef flat topography. The lagoon had a net loss of sediment over the period for which records are available. -- Hydrodynamics in the lagoon are controlled by two principal forcing mechanisms: deep water waves and tides. A 90% loss of wave height is predicted as an ocean wave crosses the reef at Muri Lagoon. Waves within the lagoon are primarily influenced by deep ocean waves and water level over the reef. Currents in the lagoon are primarily influenced by the amount of water transgressing the reef which closely correlates with the square of the deep water wave height multiplied by the deep water wave period. Tide becomes the dominant forcing mechanism during times of smaller, low frequency deep water waves. The variability of currents observed in the channel and harbour on a daily scale are related to tides and, on a larger scale, to deep water wave conditions. -- Seven sedimentary units have been identified in the study area. These are outer, inner and altered reef flat, lagoon, shoreline, river gravel and Porities boulder fields. Textural analysis of 53 samples shows variable grain size distribution, primarily dependent on transport mechanism. Average grain size for the lagoon and harbour area are coarse sand and, for the channel area, very coarse sand. -- Sediment stability in the lagoon is depth dependent. Sediments in water depths greater than 1 m are in motion about 10% of the time, between 0.75 m and 1 m are in motion about 12% and in depths less than 0.5 m are in motion about 25% of the time. Average transport rates of sediments in the channel were approximately twice that of the reef passage at Ngatangiia. -- The results indicate that the depositional pattern is the result of hydrodynamic processes active during extreme storm events. Subsequent modification of storm deposits is ongoing and accelerated during less-than-extreme storm conditions.

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