Erosional and Depositional Processes of the 18 March 2007 Lahar at Mt. Ruapehu, New Zealand

Spatiotemporal variability in the deposits of lahars offers clues into the characteristics and fluid dynamics of sediment-laden flows. The 18 March 2007 Crater Lake break-out lahar at Mount Ruapehu, New Zealand, emplaced 1.4 million m3 of boulder-bearing, matrix-rich, massively and weakly bedded deposits over the first 47.4 km of its flow-path. Traditionally these would be classified as debris flow and hyperconcentrated flow deposits, respectively. Grain size and componentry analyses were performed on samples collected over the first 11 km of flow path, for both the 2007 lahar and existing deposits that contributed sediment to the lahar. Heavily altered landslide material contributed a major proportion of sediment to the flow 0.6 km from source, and was used as a marker to establish downstream evolution of flow characteristics. Variations in the proportions of altered material with grain size suggest that abrasion and cataclasis occurred during transport. Furthermore, oxidized components are more rounded than all other sediment contributions, demonstrating greater vulnerability of the former to mechanical breakdown. Ten of sixteen samples of the 2007 lahar have distinctive sand-sized (1-2 Φ) secondary modes that become more pronounced with depth in the deposit, while primary modes coarsen. Similar but primary modes of 1-2 Φ exist 7 km from source, in deposits near the head of a side channel that captured the upper portion of the lahar after it overtopped a drainage divide. The grain size data suggest that sand may percolate downwards from a dilute upper region of the flow into a lower depositional region. We put forth a model for deposition in the first 11 km reach by the waning hyperconcentrated phase of a lahar with a concentrated basal flow but a strong vertical concentration gradient. During peak sediment concentration, the laminar basal region underwent en masse deposition, containing an abundance of sand. As the sediment concentration decreased over time, turbulence increased in the basal flow, causing incremental deposition of sediment depleted in sand, and ultimately finer-grained, stratified deposits. Our results support the concept that deposit characteristics are highly dependent on the stratification of the mobile flow and the depositional regime, both of which are controlled by the sediment concentration and evolve over time.