Using fluvial terraces to determine Holocene coastal erosion and Late Pleistocene uplift rates: An example from northwestern Hawke Bay, New Zealand

In order to make robust predictions of future coastal processes and hazards, historical rates of coastal processes such as coastal erosion need to be put into a long-term (Holocene) context. In this study a methodology is proposed that uses fluvial terraces to construct longitudinal profiles which can be projected offshore to infer paleo-coastline positions. From these positions, an average Holocene coastal erosion rate can be calculated. This study also shows how constraints can be placed on sea level changes and Late Pleistocene uplift rates using fluvial terraces, and by assuming the latter has been constant since similar to 55-37 ka, these constraints feedback into the coastal erosion rate calculations. For the northwestern Hawke Bay (North Island, New Zealand) coastline, Late Pleistocene uplift rates of 0.6 +/- 0.2, 0.6 +/- 0.2, and -0.1 +/- 0.1 (i.e., stable or subsiding) mm/yr have been determined for the Waikari, Mohaka, and Waihua River mouths, respectively. These rates are consistent with previous interpretations of subsidence to the northeast and uplift being the result of regional, subduction-related processes. A Holocene coastal erosion rate of 0.5 +/- 0.1 m/yr was determined for the Waikari River mouth, which is at the higher end of the calculated historical (similar to 1880-1980) rates (0.02-0.5 m/yr). If this difference is significant, then two possible reasons for this difference are: (i) the historical rate is affected by events such as the 1931 Napier earthquake, and (ii) the Holocene rate is the average of a steadily declining rate over the last 7.3 ka. (C) 2007 Elsevier B.V. All rights reserved.