"Mangawhio drainage system response to a deep-seated landslide, Wanganui region"

Abstract

The Mangawhio Stream, Waitotara catchment, South Taranaki, New Zealand, is part way through a process of long profile restoration, following blockage by the Mangawhio landslide, 1400 years B.P. A large rotational slump deposited 32 hectares of landslide debris, blocking the valley of the Mangawhio Stream to a depth of 50 metres. Lake Mangawhio has formed upstream of the landslide debris, slowly infilling with sediment, with tributary streams grading their long profiles by constructing depositional wedges to meet lake level. Although its discharge regime has been down-rated by the attenuation of flood peaks, the Mangawhio Stream has degraded through the landslide debris to a maximum depth of thirty metres. There is evidence of a lake level two metres higher than the present, and the erosion taking place at and downstream of the lake outlet suggests that lake level fall will continue. The negative feedback mechanisms induced by the deposition of landslide debris are controlled by the magnitude and direction of change required in order to restore an equilibrium long profile. Lake level fall alters local hydraulic conditions which in turn affects the erosion regime degrading through the landslide debris. The intertwining of formative and removal factors associated with the landslide-dammed lake system is typical of the complex response instigated by the interruption of a fluvial system. The change to the Mangawhio fluvial system is not irreversible, but the re-establishment of a new graded long profile will most likely be at a higher overall elevation than was the case prior to the landslide occurrence.