Barrell, D.J.A.; Townsend, D.B.; FitzGerald, T.M. 2021 A geomorphological characterisation of the coastal environment of the West Coast Region, South Island. Lower Hutt, N.Z.: GNS Science. GNS Science report 2021/46. 31 p.; doi: 10.21420/XCBA-DK53
Abstract:
A methodology has been developed for a geomorphological assessment of coastal environments in the West Coast Region. The aim is to provide technical input for the West Coast Regional Council in their work to define the nature and extent of the West Coast coastal environment. In this desktop assessment, 1:250,000-scale geological and topographic map digital datasets were used to derive geomorphological maps as a foundation for interpretations. The resources developed were a nominally 7-km wide geomorphological ‘strip map’ along the landward side of the coast, and three narrow strip maps (0.5-km-wide) landward of the coastline. The narrow maps, approximating a ‘thick line’ along the coast, highlight single-factor geomorphic considerations; (i) the nature of the shoreface sediments, (ii) the general type of shoreline landform, and (iii) the long-term tectonic environment (uplift versus stability). We also produced an interpretive map of former shoreline positions, based on topographic and geomorphological information. From this information, we developed two perspectives of the coastal environment. One perspective delineates the inland extent of preserved young coastal landforms, including dune fields, beach plains, estuaries, and near-coastal swamps and lakes. Another perspective takes a broader view of the geomorphological coastal environment, that delineates the general area that has been subject to coast-related processes since present sea level was attained, ~6,500 years ago. Wider geological and geomorphological considerations bear upon understanding the modern coastal environment of the West Coast. Global glacial-interglacial climate cycles of ~100,000 year average duration produced ~100 m amplitude natural sea level changes. The geometry of the continental shelf seaward of the present coast has a marked influence on river and glacier behaviour that may be manifested in various ways, along with the role of advance and retreat of ice-age glaciers sourced from the Southern Alps. There is a strong imprint of tectonic movements on coastal geomorphology, with the position of the modern, and previous, interglacial shorelines closely related to the positions of major active faults in the coastal area, both onshore and offshore. Some sections of the coast are influenced more by vertical tectonic land movements, others more by glacial and interglacial cycles and sediment supply. The geomorphological evidence points to considerable natural changes in the coastline over the past ~6,500 years. In many places, there has been a consistent story of erosion and cliff formation followed by progradation/building out of the coastal plain. The geomorphological coastal environment has a predictable array of ongoing natural processes, including erosion and sedimentation. These factors also bear upon the local hazardscape in relation to questions such as earthquake-induced liquefaction, alluvial fan flooding and debris flows, and sediment build-up. (auth)
