DEBRIS-FLOW DEPOSITIONAL PROCESSES ON ALLUVIAL FANS REVEALED BY LIDAR POINT CLOUD ANALYSIS

Debris flows, a common mode of sediment transport and deposition on alluvial fans, represent a significant geologic hazard to the built environment. We use lidar point cloud data to quantify and visualize the distribution of debris-flow channels on active alluvial fans, and to quantify the volume of sediment deposited as channels migrate across the fan surface. Lidar point cloud data are detrended with respect to an idealized, radially symmetric fan morphology. The resulting residual maps show regions on alluvial fans where active deposition is occurring and where local accommodation space exists. Areas of a fan that lie below its average surface are interpreted to be regions of accommodation space, where the fan is likely to avulse and inundate next. Conversely, the areas of a fan that lie above the average surface are unlikely to be inundated as the accommodation space in these regions has been filled. These maps illustrate the relative probability of inundation by future debris flows and therefore spatial patterns of hazard. In addition to showing patterns of fan deposition, the residual maps can be used to estimate the volume of sediment required to resurface the fan. Using cosmogenically-derived catchment-average denudation rates to estimate the volumetric sediment flux from the catchment, we estimate both the average vertical deposition rates and resurfacing timescales of alluvial fans. Alluvial fans with shorter resurfacing timescales are interpreted to be, on average, more hazardous because they experience avulsion events more frequently.