Toward a model for the bulking and debulking of lahars

[1] The incorporation and loss of material (sediments or water) during the advance of a lahar can significantly influence the advance rate, mobility, rheology, and inundation limits of the flow, and therefore these processes have consequences for destructive potential. We have developed a theoretical treatment based on the conservation of flow volume and mass to describe the influence of such processes on the depth, discharge, and density during transit. We apply our model to a common scenario of lahar generation and evolution: the formation of an initially dilute flow by catastrophic breaching or ejection of a volcanic summit crater lake, followed by bulking due to incorporation of substrate materials, and later debulking due to sediment deposition. The key features of our model are time dependence and the inclusion of two terms describing the influx or efflux of sediments to or from the flow. For lahars triggered by crater lake water, we derive the time evolution of flow behavior for comparison with the well-documented 1953 lahar of Mount Ruapehu, New Zealand. Our model demonstrates the importance of these processes on observable quantities such as flow depth, density, and deposit profiles, hydrograph evolution, and transformations between rheologic/ density regimes.