Anatomy of a basin break-out flood: The 2007 Crater Lake break-out lahar, Mt. Ruapehu, New Zealand (Invited)

Break-out floods from natural or artificial impoundments are significant hazards in many environments and regions around the world, resulting in loss of life, damage to infrastructure, and dramatic geomorphic changes due to the very high rate of energy expenditure associated with high flow velocities and depths in newly created or underfit pre-existing channels. At Mt. Ruapehu, New Zealand, a forecast break-out flood from a summit Crater Lake enabled characterisation of the evolution and impact of a discrete floodwave through: (i) multi-parameter measurement of time-series hydraulic parameters at key locations along the channel by automatic equipment and human observers; and (ii) pre- and post-event high resolution topographic surveys and vertical aerial and oblique imagery of the flowpath. Failure of the tephra dam released c. 1.0 Mm3 of warm, highly mineralised water into the steep gorge of the upper Whangaehu River in <2 hours. LiDAR data indicate 4.5 and 1.4 Mm3 of net erosion and deposition respectively along the first 47 km of channel, with most material (65 %) eroded from the steep bedrock/step-pool upper gorge, comprising inputs from various intra- and extra-channel sources. Net erosion continued across the low-gradient Whangaehu Fan, where the flood anastomosed into multiple distributary channels, and the single-thread meandering river valley further downstream. Overall, patterns of erosion and deposition were a complex function of channel gradient, width and expansion ratio. Stage, discharge, sediment-load, geochemical, and frontal and flow velocity data show that the flood wave propagated as a number of compositionally distinct but overlapping kinematic waveforms. A complex series of interactions with the flowpath produced spatially and temporarily varying sediment loads that affected its density, viscosity, mobility, peak discharge and absolute volume. Peak volumetric bulking factor (3.6) and sediment concentrations (63 vol.%) were achieved early, and then maintained at almost constant values for c. 50 km, suggesting that feedbacks between the evolving flood and the flowpath stabilised key flow parameters at the rheological transition between hyperconcentrated and debris flow behaviour. This maximised sediment transport capacity, despite downstream changes in channel geometry that resulted in lowering flow competence and distal fining of the load. These findings have implications for the study of other impoundment break-out floods, in particular for the relationship between the volume and discharge of floods and the dimensions of their outflow channels.