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A series of coastal slips early this year, triggered by heavy rain and strong winds, forced the closure of the Pacific Parade stairs in Army Bay and sections of the Arkles Bay coastal walkway, as well as fencing controls at Red Beach headland near Chelverton Terrace.
Auckland Council has carried out geotechnical reviews at the affected sites and has kept the areas closed while stability assessments continue.
To help explain why slips occur so frequently along the peninsula’s coastline, Hibiscus Matters spoke with Professor Mark Dickson, a coastal geomorphologist at the University of Auckland.
“Coastal erosion of cliffs in the area involves several natural processes,” he says. “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, which consist of alternating, weak layers of sandstone and siltstone. Wave action and weathering processes slowly erode the toe of the cliff, which keeps the cliff face steep and makes it inherently vulnerable to landsliding processes. Prolonged or intense rainfall can increase pressures that reduce the strength of the rock leading to a range of landsliding failures.
“Particular types of failure can be controlled by local geological characteristics, like the dip of bedding planes within the cliffs or pre-existing rock mass defects (for example, fractures).
“Preventing landslides is a difficult issue. Coastal cliff erosion is a global problem and there are many examples globally of efforts to prevent erosion with engineered seawalls to protect the cliff toe and a range of geotechnical methods have been attempted to reduce internal hydrological pressure and stabilise steep slopes. The engineering aspects of these methods are outside of my expertise, but it is important to note that some engineering efforts have been associated with unforeseen adverse impacts. For instance, a localised sea wall might have ‘end effects’ where erosion is then concentrated toward adjacent properties. Similarly, in some areas cliff erosion acts as a natural recharge of sediments to nearby beaches, so preventing erosion in one area might exacerbate erosion somewhere else. Where possible, cliff-top buffer zones are useful, restricting encroachment of houses toward the cliff edge.”
While erosion and landslides are natural processes, the risks are becoming more visible as homes, walkways and public infrastructure sit closer to unstable coastal margins. Slips can also occur some time after heavy rain has stopped, as water continues to move through cliff faces and weaken already fragile layers, which happened recently with tragic consequences in the Mt Maunganui landslide.
What this means for people living on the coast – From a planning perspective, Council is moving toward a stronger, risk-based approach to coastal hazards. The Unitary Plan’s coastal hazards framework is being updated through Proposed Plan Change 120, which introduces tighter rules for development in high-risk areas and has immediate legal effect while submissions and hearings are under way.
Council has also made coastal hazard and landslide information easier to access online and on property information reports. For residents and prospective buyers on the peninsula, this means checking hazard maps early, before design or purchase decisions, is increasingly important.
