Distribution of silty sediments in the shallow subsurface of the shipping channels of Tauranga Harbour

Abstract

Capital dredging in 1992 encountered some areas of siltier sediments that resulted in the formation of a highly visible plume. Subsequently multibeam and diver observations indicated that there are further areas of sediments within the shipping channel whose silt content exceeds the 5% threshold specified in the consent conditions of resource consents 65806 and 65807. A trial seismic reflection survey was conducted through the entire shipping channels from Stella Passage to A Beacon using a Knudsen Pinger Sub-bottom Profiler. This survey indicated that the harbour sediments consist of Holocene sediments (<7200 years old) overlying a complex topography of Pleistocene deposits. The underlying sediments have been eroded into a ridge and valley morphology that appears to be orientated with the valleys running sub-parallel to the present day open coast shoreline. This older deposit probably occurs underneath the entire length of the shipping channels, but the thickness of overlying Holocene sediment varies: being thicker over the valleys and thinner over the ridges. The proposed capital dredging will further reduce the Holocene sediment cover, and expose more of the older siltier sediments at the seabed. The existing exposures within the shipping channels do not appear to be susceptible to scour and form a resistant surface with a thin veneer of modern sand, and it is probable that any material exposed during the proposed capital dredging will also be stable. The proposed capital dredging anticipated the extraction of ~2.1 Mm³ of silty sediment from Stella Passage, the new Turning Basin, and Tanea Shelf. The seismic survey indicated that additional areas of potentially silty sediment occurred within the Maunganui Roads, Cutter Channel and the Entrance Channel. The Port of Tauranga undertook vibrocoring in the areas identified, and the resulting cores were described and sampled by the University of Waikato. The core results were used to create a three-dimensional model of the subsurface stratigraphy of the shipping channels that was used to identify areas of silt content exceeding 5%. The likely silt content is indicated by a colour coding: green = less than 5% silt; red = more than 5% silt, which requires management to minimise turbidity during dredging and involves restrictions on disposal options.