Aerial view of Kai Iwi Beach (Mowhanau)
Kai Iwi Beach is a naturally eroding coastline, and this process is expected to accelerate due to climate change.
Understanding sea cliff erosion
Coastal erosion is a natural process that shapes landforms such as beaches and cliffs. Over time, waves, wind, and other environmental factors wear down the coastline, carrying material away. This process is influenced by factors such as sea level rise, wave energy, tidal action, coastal slope, rock strength, and the width and height of the beach. These elements interact to impact cliff stability, erosion rates, and the long-term retreat of the coastline.
How does sea cliff erosion happen?
Waves continuously crash against the base of cliffs, gradually weakening their foundation. As the lower part of the cliff erodes, the top becomes unstable and is more likely to collapse. This cycle repeats over time, causing the cliffs to retreat further inland.
Tokin + Taylor Hazard Assessment Report(PDF, 27MB) - defining sketch for coastal erosion on cliffs
Managing coastal erosion at Kai Iwi Beach
In the short to medium term, increasing coastal storm events may threaten Archer’s Bridge and the beach access it provides. Over the longer term, spanning the next 100 years, erosion poses a risk to council assets, including the car park and toilet block, as well as a small number of residential properties and roads in Mowhanau. The extent of this threat will depend on the management approach taken.
In early 2024, we commissioned an updated erosion hazard and risk assessment report from Tonkin + Taylor. Building on previous coastal assessments dating back to 1999. The report examines the coastal processes at work at Kai Iwi and assesses the potential risks.
Next steps for Kai Iwi Beach
The council is developing an adaptation plan for Kai Iwi Beach in line with Ministry for the Environment guidelines.
In the short term, the council will continue maintaining the existing rock revetment, a structure made of large rocks placed along the shoreline to protect the beach from erosion. This revetment absorbs and deflects wave energy, helping to prevent further erosion. It has been effective in managing erosion so far. Any new or additional erosion management initiatives would require external funding or would need to go through the business case process for consideration in the council’s Long-Term Plan.