Abstract:
The focus of this study is the ongoing slope failures in soft sediment coastal cliffs in the Ōrere Point
bay, east Auckland. Some of the weak materials exposed in the cliffs appear to be sensitive to changes
in moisture content, and therefore, extreme weather events can pose a threat to slope stability in this
area. For example, on 8 March 2017, a 1-in-100-yr rainfall event called the ‘Tasman Tempest’ brought
~210 mm of rain to the area in 24 hours, followed soon after by 157 mm of rain from ex-tropical cyclone
Debbie. This led to significant slope failures at Ōrere Point in the months following the 2017 rainfall
events, that were preceded, and followed, by periods of quiescence. In the present study, particular
attention is paid to the morphology, failure history, triggering factors, and failure mechanisms of the
Ōrere Point cliffs. The cliffs are formed of Plio-Pleistocene alluvial materials, deposited in a
palaeovalley formed within Mesozoic greywacke. Field investigations included shear vane,
penetrometer, and sedimentary logging, complemented by a ground penetrating radar (GPR) survey and
unmanned aerial vehicle (UAV) surveys. Soils sampled in the field were also analysed using laboratory
testing for soil index properties and microtexture, as well as a limited programme of shear strength
testing. At least one weathered tephra layer of indeterminate age was identified during this study in the
cliffs at Ōrere Point, which might be significant for stability. Indeed, elsewhere on the North Island
(Bay of Plenty), such tephra layers exhibit a fundamental control on cliff instability. Characteristic
failures at Ōrere Point include rotational slumps, which transition to earthflows. The conceptual model
of cliff recession here is thus controlled by the rotational failures. These form a series of embayments,
causing adjacent sections of exposed cliff to gradually lose lateral support and fail. In addition to the
prevailing porewater conditions, the material properties probably part-control the exact volume and
morphology of the cliff failures. Reducing porewater pressure in the cliffs through drainage at the Ōrere
Point township would be advantageous for cliff stability.