Abstract
An in-depth understanding of shoreline processes and historical rates of change is crucial for effective coastal management because it provides the foundational knowledge needed to predict future coastal behaviour and respond to dynamic environmental challenges. By analysing historical data, patterns and trends become evident—such as rates of shoreline retreat or progradation—which inform risk assessments and help prioritise areas for protection or restoration. This understanding enables the development of evidence-based strategies, such as setback zones or beach nourishment programs, that balance ecological preservation, infrastructure protection, and community needs, ensuring sustainable and resilient coastal management decisions. It also informs where monitoring resources should be focused.
The sandy coastline of Golden and Tasman Bays, located on the northern part of New Zealand’s South Island, is the focus of this study. It differs from other regions in New Zealand in that it is macrotidal, a northeast-lee coast, and characterised by a low modal energy wave climate that is periodically subjected to high-energy onshore winds and waves during cyclonic activity. There has been limited research on this coastline, with previous studies focusing only on sections within each bay. This is the first study to analyse both Golden and Tasman Bays as a whole.
This study seeks to identify shoreline change patterns within the research area by undertaking Digital Shoreline Change Analysis (DSAS) on the sandy shorelines between Puponga in western Golden Bay and Rabbit Island in Tasman Bay. It then examines the key drivers and trends of shoreline change within that area. DSAS is the process of using geospatial and remote sensing tools to measure, compare, and quantify changes in shoreline position over time from digital datasets such as aerial imagery, satellite data, or coastal surveys.
The analysis was divided into sections of coastline that shared similar characteristics such as orientation, beach material, and coastal length. It used mapped shorelines dating back to 1843 in some locations through to 2022. This information was compared with coastal cross-section data for the region, which was updated as part of this study. Volumetric Change Analysis (VCA) was conducted at two locations to assess whether this method provides a more accurate understanding of shoreline change within the study area. VCA compares at least two elevation surfaces to assess volume change between them.
Some parts of the coast in Golden and Tasman Bay showed a high level of dynamism. The greatest rates of shoreline change occurred in proximity to river mouths and estuary outlets. Some parts of the coast in Golden and Tasman Bays showed a high level of dynamism. The greatest rates of shoreline change occurred near river mouths and estuary outlets. The sand spits occupying these locations exhibited large-scale change, both in terms of progradation and erosion. The Motupipi sand spit, for example, had progradation rates of up to 2.4 m/year and has advanced seaward by approximately 400 m since the earliest records. Conversely, a large spit at Rangihaeata fully eroded away during the study period. In contrast, parts of the coastline—particularly from Puponga through to Pakawau Inlet and along the Abel Tasman coastline—demonstrated a degree of stability. The analysis also showed that erosion rates have increased since 2003 for most of the coastline.
Sediment supply appears to be a key driver of coastal change in Golden and Tasman Bays, primarily influenced by river catchments that contribute to dynamic shoreline shifts near estuaries and river mouths. Human activities, such as causeway construction and land-use changes, have altered sediment flows, leading to persistent erosion in some areas (e.g. Marahau), while features like Motueka Spit benefit from sediment supplied by the Motueka River. Vegetation change has also played a role: historical deforestation increased sediment delivery, whereas recent revegetation has stabilised soils and reduced runoff. Aeolian processes are minimal due to the coarse sand and sheltered conditions, making wave activity the dominant force shaping the coast. Coastal changes are most substantial during rare, high-energy storm events, which can cause severe erosion and geomorphic alteration.
This study demonstrates that despite the generally low-energy wind and wave conditions in the research area, the coastline remains highly dynamic, with certain beaches, spits, and other features experiencing erosion and progradation leading to notable landform changes. While the current impact of relative sea-level rise (RSLR) is limited, increased storm intensity is likely to have greater short- to medium-term effects, especially given the slow post-storm recovery associated with low-energy conditions.
Advances in shoreline monitoring now allow for district-wide assessments, enabling more targeted future monitoring and better-informed coastal management. With the advent of LiDAR and the ability to survey specific areas using UAVs, it is recommended that DSAS be used to identify areas of change, followed by VCA to understand volumetric trends at specific locations.