Geomorphology and triggering mechanism of a river-damming block slide: February 2018 Mangapoike landslide, New Zealand
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Sam McGovern I Martin S. Brook I Murry Cave
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, northwestdipping 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. Keywords Landslide dam . New Zealand . Wedge failure . Weak rock Introduction On the night of 24 February 2018, a large landslide occurred in the northeast of New Zealand’s North Island, 35 km southwest of Gisborne (Fig. 1; 38.853° S, 177.662° E). This landslide, a low-angle wedge slide (Hungr et al. 2014), involved the detachment of c. 8 million m3 of rock, with a maximum width across the toe of the detachment of 230 m. The landslide dammed the Mangapoike River, the headwaters of Gisborne city’s water supply, forming a lake c. 50 m deep with a surface area of 0.35 km2. Lake level progressively rose by 60 cm/day, and if the local authorities not intervened with explosives to construct a spillway through the landslide dam on 29 March 2018, the impact on transport and local infrastructure could have been far greater (Sharpe 2018). Landslide dams in New Zealand are widely distributed because of active tectonics, fluvial incision, rock, and soil materials on unstable slopes, narrow valleys, and a humid climate, and can be damaging (e.g. Nash et al. 2008). Thus, a dam burst would have seen a pulse of water flow 15 km down the Mangapoike Valley and into the Wairoa River, past the township of Wairoa and into the Pacific Ocean. Pre-2018 aerial imagery indicates
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