Engineering geomorphology of coastal landslides at Limestone Downs, North Island, New Zealand

Coastal landslides are widespread geomorphological features on New Zealand's North Island, representing a key coastal erosion process. Often, New Zealand's coastal landsliding is rainfall-induced, with young, weak, permeable sediments, failing along a shear surface coincident with an unconformable contact with underlying older, sedimentary rock. An example is the Plio-Pleistocene Awhitu Group sands in the west of the North Island, which in recent years have failed via extremely rapid flows, with little warning, leading to fatalities at built-up coastlines. Understanding the nature of the hazard posed by coastal landslides is often precluded by the heavily vegetated nature of the coast, and site access. We present a case study of two coastal landslides at Limestone Downs, on the Waikato coast, ∼70 km south of Auckland, using field investigation, laboratory testing, remote sensing and modeling. The northern landslide appears to be a relict, low-angle bedding-plane failure in sedimentary rock, possibly between the Miocene Waikawau Sandstone and the underlying Te Kuiti Group. In contrast, the adjacent southern landslide appears active, failing by retrogressive upslope extension, and is limited to Plio-Pleistocene Awhitu Group sediments. While both landslides may have been initiated by marine erosion, triggering from near-field earthquakes and tsunami effects cannot be ruled out.