Geomorphology and triggering mechanism of a river-damming block slide: February 2018 Mangapoike landslide, New Zealand

Abstract

Landslide dams can be very dangerous, with inundation occurring via rising waters upstream, and flooding downstream via dam breaching. Here, we report on a landslide that dammed the Mangapoike River in eastern North Island, New Zealand. The landslide is a low-angle wedge failure in the Miocene weak rock sandstones and mudstones of the Tolaga Group, forming a landslide dam (volume c. 8 million m3) and a lake 50 m deep with a surface area of 0.35 km2, before explosives were used to form a dam spillway to decrease lake level. The landslide formed along an escarpment in northwest-dipping sandstones, and is characterised by a linear lateral scarp, a headscarp, and a bedding-plane rupture surface, which controlled the landslide block geometry. The headscarp and lateral scarp have developed along propagating vertical fractures. The slide surface is a smoothed, northwest-dipping bedding plane, and intersects the vertical fractures in the lateral scarp, forming a wedge. While the principal failure mechanism was sliding involving a single large wedge-shaped block, the rapid movement led to disintegration of most of the block. Part of the detached slide block remained intact, but most of the displaced mass forming the landslide dam is disaggregated blocks in a sandy-silty matrix. Rainfall and meteoric groundwater probably did not initiate failure. Instead, river incision of the dip slope toe, and overpressurisation of fluids that are known to accumulate in sandstones overlain by impermeable mudstones in the region, probably decreased the effective stress along the existing bedding plane, initiating failure.