A Probabilistic Analysis of Lahar Bulking and Debulking in Channelized Proximal Volcanic Settings

Lahars are gravitationally driven mudflows that occur on volcanic slopes and pose a significant threat to communities located downslope. The initial stages of their evolution, particularly bulking and debulking, within the proximal hazard zone are understudied and poorly understood. The goal of this study is to investigate the relationship between certain geometric channel characteristics and field observations of bulking and debulking from a 2012 lahar along a section of the Jamapa watershed, located within the proximal zone of the Pico de Orizaba. Drone acquired Digital Elevation Models (DEMs) were used to extract the geometric channel characteristics. The relationships were then investigated by developing logistic regression models with bulking and debulking as dependent variables and the 14 independent variables. When used in a multivariate model, these models show that lithology, channel width, and channel shapes are significantly related to observations of bulking and debulking, whereas slope, sinuosity, average catchment slope, and watershed contributing area are not. Although the models developed here are based on variables related to channel geometry and a single lahar event, this author recommends additional variables related to the physical characteristics of lahar flow itself be incorporated into future models. This could help improve its predictive capability and resolve issues related to abnormally small parameter estimates. Even though the models' results are far from perfect, they provide a foundation for developing more complex models that could advance our understanding of lahar evolution during the early stages that could be used to improve lahar flow models in the future.