Coastal change details revealed by new maps: City beaches are growing into the sea year on year

Tūranganui-a-Kiwa/Poverty Bay has been consistently building seaward by an average of more than a metre a year over the past 80 years.  

The largest changes to the Tairāwhiti coastline over this time have been around the Waiapu River mouth near Ruatōria, where both erosion and accretion of up to 5m or 6m a year has been happening.  

That’s what new maps showing coastal changes have revealed about Tairāwhiti. 

The Coastal Change project sits within the Resilience to Nature’s Challenges National Science Challenge. The work focuses on improving the understanding of coastal change by completing the first nationwide assessment of the changing coast over the past 80 years. 

Along with the bay consistently accreting, or building seaward, for the past 80 years at a rate of 1.3m a year, the Wherowhero Lagoon barrier — an important taonga for iwi — has been eroding at a rate of 1.2m a year. 

Further up the East Coast, Port Awanui on the East Cape has an average accretion rate of 2.4m a year. 

Coastal scientist Dr Emma Ryan said there was a lot of variability around Tairāwhiti but the dominant mode of coastal change was accretion. 

“That may be a function of the sediment supply, with high cliffs and the rivers that supply sediment to the coast. There are a lot of factors going on that influence coastal change,” she said. 

Research on the factors causing coastal erosion and accretion was yet to be done but was something they were working on, Ryan said. 

Around the Waiapu River mouth both erosion and accretion were happening in different areas of the coast, Ryan said. 

The maximum erosion rate was 5m a year, and the accretion was up to 6m a year. 

“Coastal change is very complex around river mouths — they are super dynamic, as everyone knows who lives in the area,” she said. 

The data was useful as it showed the variability in detail never done before. It had been surveyed in 10-metre sections, meaning the data was localised. 

“Decision-makers and communities can use this coastal change data to begin to understand those changes in detail through time, and the drivers for coastal change, and to plan for what the future may look like — particularly under sea-level rise. 

“Coastal erosion is a problem for many Māori communities around Aotearoa and on the East Coast. There is a lot of cultural heritage situated in coastal areas. 

“There has been an erosion of urupa. Many Māori communities on the East Coast are located in low-lying areas near the coast, or upon cliffed coasts that are continually eroding, and that will only continue in the future” she said. 

Buildings, homes, roads, marae and cultural heritage were situated on or close to the coast, and this data could be used for planning. 

“It doesn’t project future changes but it provides data on the past that can be used in conjunction with Matauranga Māori and Māori communities’ understanding of their coasts to help in understanding what’s happened to the coast in the past and what might happen in the future. 

“This dataset will help Māori communities to plan for that. 

“I know many Māori communities are already conducting climate change adaptation planning and considering coastal hazards — this data set provides another source of information to help in those decision-making processes,” she said. 

Opening a map and trying to understand how to use the data can be challenging, but the Coastal Change team has created guidance documents. One focuses on general advice on how to use the data, and another is tailored to help tangata whenua. 

Dr Shari Gallop has almost nearly completed a tangata whenua-specific guidance document, that will be available soon. 

The document includes case studies of other hapū from around the country that have already used the data in decision-making and trying to understand their coast a little more. 

● Access the maps at coastalchange.nz