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Evolution of the Tauranga Harbour Entrance: Influences of tsunami, geology and dredging

The eastern entrance to Tauranga Harbour has been modified by capital and maintenance dredging since 1968, raising concerns that there have been, or will be, adverse impacts. This paper reviews the effects that extreme events, such as tsunami, and the influence that the underlying geological structures have had on the evolution of the present day entrance; and assesses the possible effects of historic dredging. A regional tsunami around the start of the 15th Century removed ~3 km from the eastern end of Matakana Island. Seismic data indicate the harbour entrance was probably located ~2 km west of the present position. Panepane Pt accreted eastward between the 15th and 19th Centuries displacing the harbour entrance until further migration was prevented by the rhyolite dome of Mauao. Seismic and core data indicate that the present-day ebb tidal delta consists of a thin veneer of Holocene sediments over a relict topography of Pleistocene volcaniclastic sediments. These form subsurface ridges parallel to the coast and one is connected at the eastern end to Mauao. The Entrance Channel was excavated through this ridge in 1968. Previously, the Pleistocene ridge restricted the depth to which the ebb jet could excavate a channel through the delta to ~10 m. The first complete bathymetric survey of the harbour entrance was in 1852. At this time the entrance was wide and shallow. By 1954, Panepane Pt had accreted 300 m eastward, and the harbour entrance had deepened in response. Over this period the ebb jet orientation became increasingly oblique to the shoreline, and the width of the jet decreased with an increase in velocity and offshore extent. Dredging since 1968 has had two main direct effects on the morphology of the entrance: a shallow shelf of boulders has been largely removed, deepening the channel along the flanks of Mauao; and creating a deep channel through the Pleistocene ridge. Indirect effects have included further accretion of Panepane Pt, an increase in the offshore extent of the ebb tidal delta, and the development of multiple lines of swash bars on the swash platform.
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Type of thesis
de Lange, W. P., Moon, V. G., & Johnstone, R. (2015). Evolution of the Tauranga Harbour Entrance: Influences of tsunami, geology and dredging. In Australasian Coasts and Ports 2015. Wellington, New Zealand: IPENZ.
© 2015 the authors.