Alluvial Fan Retreat: Tank Experiments

Conventional interpretations of alluvial fan margins attribute their changes to environmental factors such as tectonic activity or climate variations. Under steady dynamic conditions, fan margin (s) is expected to grow continuously, following the time (t) dependence of s~t^(1/3), based on the mass conservation. However, this study aims to propose a new concept that challenges this conventional understanding. A key finding of this research is that the alluvial fan margin can retreat even under constant upstream boundary conditions, a phenomenon significantly influenced by ‘groundwater infiltration’. This study focuses on investigating the role of infiltration process in alluvial fan evolution. Seven tank experiments with varying sediment and water discharge rates were conducted, enabling analysis of fan retreat under constant upstream boundary conditions. Fans typically exhibited continuous progradation, but a critical point was observed where runoff water no longer reached the fan margin, resulting in fan retreat. At this stage, all runoff water infiltrated into the sediment deposit. Applying Darcy’s Law, we found a strong correlation between deposit thickness (dh) and infiltration rate, assuming constant hydraulic conductivity (K_s). Based on these experimental results, a computational model was developed to simulate the alluvial fan trajectories under similar conditions. The findings provide insights into field-scale applications by accounting for infiltration processes on alluvial fans.