Channel Dynamics in an Experimental Alluvial Fan Under Constant Boundary Conditions: A Classification of Avulsion and Lateral Migration Events

Abstract. Alluvial fans exhibit complex channel dynamics, spanning gradual lateral migration to sudden avulsions. Although allogenic processes are recognized as key drivers of these behaviors, the autogenic mechanisms regulating channel change remain poorly understood. In this study, we present a quantitative analysis of the main channel kinematics on a widely graded, non-cohesive experimental alluvial fan, utilizing high-temporal-resolution RGB imagery and main channel centerline tracking. By employing two key metrics – displacement magnitude (normalized by channel width) and flow continuity, defined as the degree of overlap in the active channel footprint from one image to the next – we move beyond qualitative assessments, which are often subject to researcher bias, and establish a clear framework for distinguishing between migratory (continuous) and avulsive (discrete) channel behaviours. Our findings reveal that the fan alternates between supply-limited phases, when a small number of efficient channels route sediment to the toe with only localized reworking, and transport-limited phases, when a more complex, inefficient channel network traps sediment mid-fan, favoring abrupt reorganizations (i.e., avulsions). Contrary to the conventional assumption that systematic aggradation from toe to apex triggers large-scale channel abandonment, we demonstrate that lateral sediment redistribution often prevents fan-wide sediment buildup, thereby delaying or even preventing major avulsions. These results highlight the critical role of self-regulating autogenic processes, particularly the lateral reworking of coarse sediment, in controlling both the timing and scale of channel adjustments, emphasizing the need to incorporate localized feedback mechanisms into predictive models to improve our understanding of alluvial fan dynamics.