Detailed geomorphological mapping of the coastal plain of the Kapiti Coast District

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Townsend, D.B.; Dellow, G.D. 2016 Detailed geomorphological mapping of the coastal plain of the Kapiti Coast District. Lower Hutt, N.Z.: GNS Science. GNS Science report 2016/37. 31 p. 

Abstract: This report and the accompanying Geographic Information Systems (GIS) data outline the surface geomorphology of the coastal plain of the Kapiti Coast District, with emphasis on the area west of State Highway One or the Main Trunk Railway. Geomorphological units were mapped directly from detailed aerial photograph mosaics and regional light detection and ranging (LiDAR) derived topographic base maps. The high resolution (1 m ground resolution) provide enough detail to enable the boundaries of geomorphic units to be located to within 10 metres in most cases, and to within 5 metres in some instances. The mapping of discrete geomorphic units of the Kapiti Coast District has highlighted the diversity of geomorphic features on the coastal platform. The range of geomorphic units mapped includes dune systems of five different ages, intervening dune deposits including beach ridges and peat swamps, and a variety of alluvial features such as fans and stream and river terraces. All geomorphic units have been mapped remotely as part of this desk-based study, but have been correlated with published and unpublished information where possible. The geomorphology of the coastal plain of the district reflects the competing geological processes between the uplifting and eroding Tararua Range to the east, and coastal-dominated processes to the west. Eroded sediment forms fans along the range front or is delivered to the coast mainly via the Otaki and Waikanae rivers. A slow, but steady regional uplift has allowed preservation of Last Interglacial marine terraces in some places. The sequence described above has been truncated by the sea when mean sea levels reached their maximum post-glacial height about seven thousand years ago, forming a cliff now exposed mainly between Waikanae and Otaki. Sand dunes have been forming since at least the Last Interglacial time, but most of the dunes in the area are Holocene in age (10,000 years or less). Dune formation has impeded the courses of many of the smaller streams, resulting in extensive peat swamps accumulating in the poorly-drained areas between and inland from the dunes. The coastal plain of the Kapiti Coast District is a major corridor for transport, including State Highway One and the North Island Main Trunk Railway, and also the gas pipeline supplying Wellington. It is also a growth area for residential development, with new subdivisions underway or planned. It is intended that the information in this report is used to inform about the potential foundation conditions and locations of materials that may be expected to be encountered. The data, in future, may prove useful in additional applications such as assisting in the determination of earthquake ground shaking subsoil class and liquefaction potential. The scale of the mapping, however (at 1:25,000 or better in some areas), is not intended to be site-specific. (auth)