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Littoral sediment budget and beach morphodynamics, Pukehina Beach to Matata, Bay of Plenty

The Pukehina-Matata coastal sector is one of the least studied parts of the Bay of Plenty coastline. Currently this section of coast is in a stable, tending towards erosive, condition, with historical erosion of 0-0.2 m/year. Sediment mineralogy reflects the high input of quartzo-feldspathic material into the beach-dune-nearshore system. For the Otamarakau-Matata sector much of the sediment is provided from fluvial sources, predominantly the Waitahanui, Pikowai, and Herepuru streams, although the total stream input in this area is only 3,000 to 7,000 m³ per year. The sources of beach sand from Town Point to Otamarakau includes some erosion of catchment material, supplemented by littoral drift, erosion of submarine rock outcrops in the Town Point-Otamarakau region, and possible onshore reworking of pre-Holocene sediments. The greywacke gravels present within the littoral system, especially between Rodgers Road and Pukehina, are relict deposits, which are presently active within the beach-dune-nearshore system due to the small volume transfers of sandy sediments. Their original source, is suggested as from marine erosion of Castlecliffian sediments, such as exposed in the coastal cliffs at Matata. Net littoral drift is suggested as bi-directional from a centre-point near Otamarakau, to both the north-east along Pukehina Spit, and to the south-west towards Matata. Some counter-drift occurs between the Tarawera River mouth and Matata, and along the tip of Pukehina Spit, with nourishment of this area by the Waihi estuary. Nearshore sedimentary-morphodynamic units show that the nearshore and inner-shelf at Town Point, and from southern Pukehina Beach to Otamarakau, is characterised by the presence of numerous rock outcrops, which are responsible for the coarse sands and relatively higher carbonate abundances in this area. Sediment volumes within the beach-nearshore system, and alongshore transfers between sectors of the coast are small, with annual net littoral drift estimated as 15,000 m³ at Matata. Diabathic processes are considered to dominate, with the limit of significant onshore-offshore sediment transport no more than 12 m, and a parabathic limit of less than 6 m. The net change in sediment volume for the entire beach system within the Pukehina-Matata coastal sector between 1989 and 1993, produced a calculated deficit of sediment of 90,570 m³. In comparison a longer-term change, between 1978 and 1993, showed a sediment surplus of 218, 560 m³. Over the Pukehina-Matata coastal sector these volume changes are reasonably small and their variability reflects both the dynamic nature, and the delicate state of equilibrium, of the beach-dune-nearshore system. The derived littoral sediment budget shows that in order to balance the inputs and outputs within the system approximately 27,400 m³ of annual onshore sediment transport must occur. Current sand extraction at Otamarakau has resulted in a decline in the beach sediment volumes between Otamarakau and Pikowai, with this sector in a sediment deficit. Although natural processes mask the true impacts, the increased sand extraction rate of 36,000 m³ per year is liable to further deplete the beach-dunenearshore system. However, in the short-term these effects are unlikely to be immediately noticeable.
Type of thesis
Phizacklea, D. J. D. (1993). Littoral sediment budget and beach morphodynamics, Pukehina Beach to Matata, Bay of Plenty (Thesis, Master of Science (Technology) (MSc(Tech))). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/10711
University of Waikato
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