Coastal Erosion Risks Surge on Hibiscus Coast Following Heavy Rain

Heavy rains and strong winds earlier this year triggered a series of coastal slips on the Hibiscus Coast, leading to the closure of several public access points, including the Pacific Parade stairs in Army Bay and parts of the Arkles Bay coastal walkway. Additionally, fencing controls were implemented at the Red Beach headland near Chelverton Terrace. In response to these incidents, the Auckland Council has undertaken geotechnical reviews of the affected locations, maintaining closures while stability assessments continue.

To better understand the reasons behind the frequent slips along the peninsula’s coastline, Hibiscus Matters spoke with Professor Mark Dickson, a coastal geomorphologist at the University of Auckland. He explained that the coastal erosion affecting the cliffs involves a combination of natural processes. “There is an interaction between marine processes, like wave action and intertidal weathering, and the inherent geotechnical weaknesses of the soft rocks in the area,” he stated.

The local geology consists of alternating layers of sandstone and siltstone, which are particularly vulnerable to erosion. The persistent action of waves and weathering gradually erodes the base of the cliffs, maintaining a steep face that is prone to landslides. Prolonged periods of rainfall can exacerbate these conditions by increasing pressure on the rock layers, ultimately leading to various types of landsliding failures.

Professor Dickson noted that specific geological characteristics, such as the dip of bedding planes and existing defects within the rock mass, can influence the nature of these failures. He emphasized the challenges of preventing landslides, stating, “Coastal cliff erosion is a global problem,” and highlighted that while engineered seawalls have been used to protect cliff bases, they can sometimes lead to unforeseen adverse effects. One such issue is the phenomenon of “end effects,” where erosion is concentrated in areas adjacent to the protected zone.

Cliff erosion can also play a role in sediment replenishment for nearby beaches, meaning that interventions to stop erosion in one location may unintentionally increase it in another. Professor Dickson suggested that establishing cliff-top buffer zones could be beneficial by limiting the encroachment of development toward the cliff edges.

As erosion and landslides are natural phenomena, the risks they pose are being increasingly recognized as residential areas, walkways, and public infrastructure encroach upon unstable coastal margins. Notably, slips can occur even after heavy rain has ceased, as residual water continues to infiltrate the cliffs, weakening the already fragile layers. This was tragically illustrated by a recent landslide in Mt Maunganui that resulted in fatalities.

In light of these hazards, the Auckland Council is adopting a more robust, risk-based approach to managing coastal hazards. The Unitary Plan’s coastal hazards framework is undergoing revisions through Proposed Plan Change 120. This initiative introduces stricter regulations for development in high-risk areas and is currently in effect while public submissions and hearings are ongoing.

To enhance public awareness, the Council has made information about coastal hazards and landslide risks more accessible online and in property information reports. For residents and potential buyers on the Hibiscus Coast, consulting hazard maps prior to making design or purchasing decisions is becoming increasingly critical. As the landscape shifts due to natural processes, understanding these risks will be essential for ensuring safety and sustainability in the region.