Coupled fluid dynamics-sediment transport modelling of a Crater Lake break-out lahar: Mt. Ruapehu, New Zealand

SummaryLahars, and other sudden onset floods, are highly dynamic with temporally and spatially evolving sediment loads. Numerical models of sudden onset flood behaviour, particularly those applied to real world events, have tended to neglect sediment transport processes. This is a serious though perhaps understandable shortcoming given a lack of field measurements on flow-bed interactions, the extra computational complexity required, and the problem of iteratively updating ‘plastic’ channel boundaries. This paper applies a fluid dynamics model with suspended sediment and bedload transport to calculate flow conditions and rapid landscape change due to an initially dilute lahar from Mt. Ruapehu, New Zealand. We take advantage of an unprecedented dataset of net topographic changes determined from pre- and post-lahar airborne LiDAR surveys and time-series of hydrological measurements to run and validate the model. A sensitivity analysis of key controls on water surface elevation and final sediment thickness illustrates how the flow was supply-limited in the upper reaches. It was capable of transporting a greater volume and calibre of material than that available. Modelled patterns of net erosion and deposition can replicate those observed (r2 = 0.79) and are dominantly controlled by channel gradient and topographic confinement. Peak bed shear stress exceeded 9000 N m−2 in the Whangaehu Gorge, whilst rates of change of bed shear stress >600 N m−2 min−1 were modelled in the flow front. Of the total modelled in-channel erosion 3.36 × 106 m3, or 75%, was achieved between 10 and 40 min of the event initiation. Most erosion occurred during the rising limb and peak of the lahar hydrograph, with 45% of total deposition occurring on the waning limb. Overall, the model offers new insights towards a mechanistic understanding of longitudinal, lateral and temporal evolution of hydrodynamics and volumetric changes within a lahar and therefore has considerable advantages for physically linking processes and products and for hazard management.