Erosional and depositional processes of the 18 March 2007 lahar at Mt. Ruapehu, New Zealand

Abstract

Spatiotemporal variability in the deposits of lahars offers insights into the characteristics and fluid dynamics of these sediment-laden flows. The 18 March 2007 Crater Lake break-out lahar at Mount Ruapehu, New Zealand, emplaced 1.4 million m3 of both massive and 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 the flow path, for both the 2007 lahar itself and pre-existing deposits that contributed sediment to the lahar. Altered landslide material contributed a major proportion of sediment to the flow 400--800 m from the source of the lahar, and was used as a marker to understand downstream evolution of flow characteristics. Variations in the proportions of this altered landslide material with grain size and distance suggest that abrasion and cataclasis occurred during transport. Furthermore, altered landslide clasts are more rounded than all other sediment types, demonstrating greater susceptibility of the former to mechanical breakdown, which influences flow rheology. Ten of sixteen samples of the 2007 lahar deposit exhibit bimodal grainsize distributions. Primary modes coarsen with depth at locations where samples could be collected at multiple depths, while distinctive weak sand-sized (1--2Φ) secondary modes become more pronounced with depth in the deposit. Sand-sized primary modes 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. We put forth a model for deposition in the first 11 km reach by the waning phase of a lahar with a concentrated basal flow and a strong vertical sediment concentration gradient. As the sediment concentration of the flow reached its peak, the basal region generated sand grains through abrasion and cataclasis during intergranular collisions. This sand was also transported in the upper dilute transport region of the current and is preserved as secondary modes in deposits produced by rapid vertical accretion. As the sediment concentration of the flow decreased over time, turbulence increased in the basal flow, causing selective, incremental deposition of sediment now 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 flow and the depositional regime, both of which evolve over time and are controlled by the sediment concentration and flow competence.