Investigation of the Sedimentological Impact of Dredged Material, Purakau Channel, Manukau Harbour, NZ

Regular Dredging of the Port of Onehunga, Manukau Harbour, Auckland, is necessary in order to allow shipping vessels to berth. This dredged material is dumped in a designated disposal ground in the Purakau Channel. This study investigated the sediment character at the Purakau Channel Disposal site, and the sedimentation processes which affect the dredge material, in order to provide a basis for ongoing monitoring of the site. Investigation into the sedimentary pattern in the Purakau Channel was undertaken with side-scan sonar, seismic reflection profiling, and sediment textural analyses. Analysis of bathymetric surveys of the disposal ground, tidal gauging, and calculation of wave parameters using local wind data were also undertaken. There is evidence of accumulation of dredge material within the northern part of the ground, while much of the dredge material is transported from the ground. Tidal gauging of the disposal ground indicates the potential for a slight net transport of sediment in the ebb direction. Disposal of dredge material in the channel has clearly altered the natural sediment character, which has the potential to have a detrimental impact on the distribution of benthic organisms in the local environment.