80 Years of Shoreline Change in Northland, New Zealand

Abstract

Understanding long-term shoreline change patterns is crucial for effective management of coastlines. Decadal-scale records of shoreline change are typically determined by mapping shorelines using series of historical aerial imagery. In New Zealand, such shoreline change records are limited and highly fragmented. A national assessment of coastal erosion has not been conducted since benchmark work completed in 1978. This paper reports on the first step in an ambitious project to map national-scale shoreline change in New Zealand over the past century. We present a dataset of shoreline change since the 1930s for unconsolidated beaches in Te Tai Tokerau (Northland). Over 3000 km of shoreline imagery was georeferenced and digitised for historical imagery in the national archive. Shoreline data were analysed for nearly 50,000 transects spaced at 10m intervals using the Digital Shoreline Analysis System. Coastal change hotspots are noted at several locations on both west and east coasts, particularly at the distil edges of sand spits, river mouths and harbours. The highest rate of shoreline accretion and erosion (90th percentile rates ~ +10 and -8 m/y respectively) occur near North Head Kaipara Harbour. Localised areas of rapid coastal change occur along 90-Mile Beach and at Kokota Spit (Parengarenga) and Otiaia Point (Rangaunu Bay), and in these areas accretion is dominant, with 90th percentile rates of ~+2.5 m/y and total shoreline accretion of 200-300m. The west coast shows overall accretion over the ~80-yr observation period at a median rate of ~+0.4 m/y whereas the east coast shows effectively no change: it has been remarkably stable despite interdecadal climatic fluctuations and observed sea-level rise over the past century. Sediment supply and vegetation appear to be key drivers of the observed coastal change patterns.