Decadal shoreline change on the New Zealand coast

The study of shoreline change in New Zealand is very important, particularly now given the threat of climate change and its associated sea-level rise. Yet, there has not been a comprehensive study on shoreline change around New Zealand for over three decades, since Gibb’s (1979) nationwide synoptic overview. Because Gibb’s (1979) study was synoptic, the reasons behind the measured shoreline changes were unable to be determined. The role of vegetation change, such as the introduction of Ammophila arenaria (marram grass), is just one of the potential causes of shoreline change Gibb (1979) was unable to consider. In addition, Gibb (1979) did not have the benefit of modern shoreline mapping technology, such as Geographic Information Systems (GIS) and Global Positioning Systems (GPS). This study examined four study sites: Mason Bay, Stewart Island; Kaitorete Barrier, Canterbury; Big Bay, Fiordland; and Tahakopa Bay, the Catlins. These four study sites are located in some of the most isolated areas of New Zealand, and are among the least affected by human occupation. In examining the shoreline change at these sites, modern shoreline mapping technology was used. Aerial photographs dating from 1943 were added to GIS software (ArcGIS) and orthorectified. The location of the shoreline, as indicated by the seaward extent of the dune vegetation, was mapped for each aerial photograph. GPS data taken at the shoreline of the sites during 2011 (except Big Bay) were added to the maps. Environment Canterbury (ECan) beach profiles were plotted from eight locations along the length of Kaitorete Barrier. The temporal and spatial decadal shoreline changes were then analysed. This study reached three main conclusions. (1) The introduction of Ammophila is able to force shoreline progradation, as shown at Mason Bay. There was spatial variability in progradation alongshore at Mason Bay, as with all four of the study sites. (2) A dwindling modern sediment supply may be significant on coastlines with strong alongshore littoral sand transport systems. It is considered that a dwindling modern sediment supply to Kaitorete Barrier has encouraged the naturally-occurring process of shoreline reorientation to continue; this is likely to continue into the future. (3) The two most remote, and least affected by human influence, of the four study sites – Big Bay and Tahakopa Bay – experienced virtually no shoreline change over the last 54 and 63 years respectively